An Introduction to C# Generics
Juval Lowy
August 2003
Summary: This article discusses the problem space generics address, how they are implemented, the
benefits of the programming model, and unique innovations, such as constrains, generic methods and
delegates, and generic inheritance. (41 pages)
Download the GenericsInCSharp.msi sample file.
Note This article assumes you are familiar with C# 1.1. For more information on the C# language, visit
http://msdn.microsoft.com/vcsharp/language.
Contents
Introduction
Generics Problem Statement
What Are Generics
Applying Generics
Generic Constraints
Generics and Casting
Inheritance and Generics
Generic Methods
Generic Delegates
Generics and Reflection
Generics and the .NET Framework
Conclusion
Introduction
Generics are the most powerful and anticipated feature of C# 2.0. Generics allow you to define type-safe
data structures, without committing to actual data types. This results in a significant performance boost
and higher quality code, because you get to reuse data processing algorithms without duplicating
type-specific code. In concept, generics are similar to C++ templates, but are drastically different in
implementation and capabilities. This article discusses the problem space generics address, how they are
implemented, the benefits of the programming model, and unique innovations, such as constrains,
generic methods and delegates, and generic inheritance. You will also see how generics are utilized in
other areas of the .NET Framework such as reflection, collections, serialization, and remoting, and how
to improve on the basic offering.
Generics Problem Statement
Consider an everyday data structure such as a stack, providing the classic Push() and Pop() methods.
When developing a general-purpose stack, you would like to use it to store instances of various types.
Under C# 1.1, you have to use an Object-based stack, meaning that the internal data type used in the
stack is an amorphous Object, and the stack methods interact with Objects:
public class Stack
{
object[] m_Items;
public void Push(object item)
{...}
public object Pop()
{...}
}
Code block 1 shows the full implementation of the Object-based stack. Because Object is the
canonical .NET base type, you can use the Object-based stack to hold any type of items, such as integers:
Stack stack = new Stack();
stack.Push(1);
stack.Push(2);
int number = (int)stack.Pop();
Code block 1. An Object-based stack
public class Stack
{
readonly int m_Size;
int m_StackPointer = 0;
object[] m_Items;
public Stack():this(100)
{}
public Stack(int size)
{
m_Size = size;
m_Items = new object[m_Size];
}
public void Push(object item)
{
if(m_StackPointer >= m_Size)
throw new StackOverflowException();
m_Items[m_StackPointer] = item;
m_StackPointer++;
}
public object Pop()
{
m_StackPointer--;
if(m_StackPointer >= 0)
{
return m_Items[m_StackPointer];
}
else
{
m_StackPointer = 0;
throw new InvalidOperationException("Cannot pop an empty
stack");
}
}
}
However, there are two problems with Object-based solutions. The first issue is performance. When
using value types, you have to box them in order to push and store them, and unbox the value types
when popping them off the stack. Boxing and unboxing incurs a significant performance penalty in their
own right, but it also increases the pressure on the managed heap, resulting in more garbage collections,
which is not great for performance either. Even when using reference types instead of value types, there
is still a performance penalty because you have to cast from an Object to the actual type you interact with
and incur the casting cost:
Stack stack = new Stack();
stack.Push("1");
string number = (string)stack.Pop();
The second (and often more severe) problem with the Object-based solution is type safety. Because the
compiler lets you cast anything to and from Object, you lose compile-time type safety. For example, the
following code compiles fine, but raises an invalid cast exception at run time:
Stack stack = new Stack();
stack.Push(1);
//This compiles, but is not type safe, and will throw an exception:
string number = (string)stack.Pop();
You can overcome these two problems by providing a type-specific (and hence, type-safe) performant
stack. For integers you can implement and use the IntStack:
public class IntStack
{
int[] m_Items;
public void Push(int item){...}
public int Pop(){...}
}
IntStack stack = new IntStack();
stack.Push(1);
int number = stack.Pop();
For strings you would implement the StringStack:
public class StringStack
{
string[] m_Items;
public void Push(string item){...}
public string Pop(){...}
}
StringStack stack = new StringStack();
stack.Push("1");
string number = stack.Pop();
And so on. Unfortunately, solving the performance and type-safety problems this way introduces a third,
and just as serious problem—productivity impact. Writing type-specific data structures is a tedious,
repetitive, and error-prone task. When fixing a defect in the data structure, you have to fix it not just in
one place, but in as many places as there are type-specific duplicates of what is essentially the same data
structure. In addition, there is no way to foresee the use of unknown or yet-undefined future types, so
you have to keep an Object-based data structure as well. As a result, most developers find type-specific
data structures to be impractical and opt for using Object-based data structures, in spite of their
deficiencies.
What Are Generics
Generics allow you to define type-safe classes without compromising type safety, performance, or
productivity. You implement the server only once as a generic server, while at the same time you can
declare and use it with any type. To do that, use the < and > brackets, enclosing a generic type
parameter. For example, here is how you define and use a generic stack:
public class Stack<T>
{
T[] m_Items;
public void Push(T item)
{...}
public T Pop()
{...}
}
Stack<int> stack = new Stack<int>();
stack.Push(1);
stack.Push(2);
int number = stack.Pop();
Code block 2 shows the full implementation of the generic stack. Compare Code block 1 to Code block
2 and see that it is as if every use of object in Code block 1 is replaced with T in Code block 2, except
that the Stack is defined using the generic type parameter T:
public class Stack<T>
{...}
When using a generic stack, you have to instruct the compiler which type to use instead of the generic
type parameter T, both when declaring the variable and when instantiating it:
Stack<int> stack = new Stack<int>();
The compiler and the runtime do the rest. All the methods (or properties) that accept or return a T will
instead use the specified type, an integer in the example above.
Code block 2. The generic stack
public class Stack<T>
{
readonly int m_Size;
int m_StackPointer = 0;
T[] m_Items;
public Stack():this(100)
{}
public Stack(int size)
{
m_Size = size;
m_Items = new T[m_Size];
}
public void Push(T item)
{
if(m_StackPointer >= m_Size)
throw new StackOverflowException();
m_Items[m_StackPointer] = item;
m_StackPointer++;
}
public T Pop()
{
m_StackPointer--;
if(m_StackPointer >= 0)
{
return m_Items[m_StackPointer];
}
else
{
m_StackPointer = 0;
throw new InvalidOperationException("Cannot pop an empty
stack");
}
}
}
Note T is the generic type parameter (or type parameter) while the generic type is the Stack<T>.
The advantage of this programming model is that the internal algorithms and data manipulation remain
the same while the actual data type can change based on the way the client uses your server code.
Generics Implementation
On the surface C# generics look syntactically similar to C++ templates, but there are important
differences in the way they are implemented and supported by the compiler. As you will see later in this
article, this has significant implications on the manner in which you use generics.
Note In the context of this article, when referring to C++ it means classic C++, not Microsoft C++ with
the managed extensions.
Compared to C++ templates, C# generics can provide enhanced safety but are also somewhat limited in
capabilities.
In some C++ compilers, until you use a template class with a specific type, the compiler does not even
compile the template code. When you do specify a type, the compiler inserts the code inline, replacing
every occurrence of the generic type parameter with the specified type.. In addition, every time you use
a specific type, the compiler inserts the type-specific code, regardless of whether you have already
specified that type for the template class somewhere else in the application. It is up to the C++ linker to
resolve this, and it is not always possible to do. This may results in code bloating, increasing both the load
time and the memory footprint.
In .NET 2.0, generics have native support in IL (intermediate language) and the CLR itself. When you
compile generic C# server-side code, the compiler compiles it into IL, just like any other type. However,
the IL only contains parameters or place holders for the actual specific types. In addition, the metadata
of the generic server contains generic information.
The client-side compiler uses that generic metadata to support type safety. When the client provides a
specific type instead of a generic type parameter, the client's compiler substitutes the generic type
parameter in the server metadata with the specified type. This provides the client's compiler with
type-specific definition of the server, as if generics were never involved. This way the client compiler can
enforce correct method parameters, type-safety checks, and even type-specific IntelliSense®.
The interesting question is how does .NET compile the generic IL of the server to machine code. It turns
out that the actual machine code produced depends on whether the specified types are value or reference
type. If the client specifies a value type, then the JIT compiler replaces the generic type parameters in the
IL with the specific value type, and compiles it to native code. However, the JIT compiler keeps track of
type-specific server code it already generated. If the JIT compiler is asked to compile the generic server
with a value type it has already compiled to machine code, it simply returns a reference to that server
code. Because the JIT compiler uses the same value-type-specific server code in all further encounters,
there is no code bloating.
If the client specifies a reference type, then the JIT compiler replaces the generic parameters in the
server IL with Object, and compiles it into native code. That code will be used in any further request for
a reference type instead of a generic type parameter. Note that this way the JIT compiler only reuses
actual code. Instances are still allocated according to their size off the managed heap, and there is no
casting.
Generics Benefits
Generics in .NET let you reuse code and the effort you put into implementing it. The types and internal
data can change without causing code bloat, regardless of whether you are using value or reference
types. You can develop, test, and deploy your code once, reuse it with any type, including future types,
all with full compiler support and type safety. Because the generic code does not force the boxing and
unboxing of value types, or the down casting of reference types, performance is greatly improved. With
value types there is typically a 200 percent performance gain, and with reference types you can expect
up to a 100 percent performance gain in accessing the type (of course, the application as a whole may or
may not experience any performance improvements). The source code available with this article includes
a micro-benchmark application, which executes a stack in a tight loop. The application lets you
experiment with value and reference types on an Object-based stack and a generic stack, as well as
changing the number of loop iterations to see the effect generics have on performance.
Applying Generics
Because of the native support for generics in the IL and the CLR, most CLR-compliant language can take
advantage of generic types. For example, here is some Visual Basic® .NET code that uses the generic
stack of Code block 2:
Dim stack As Stack(Of Integer)
stack = new Stack(Of Integer)
stack.Push(3)
Dim number As Integer
number = stack.Pop()
You can use generics in classes and in structs. Here is a useful generic point struct:
public struct Point<T>
{
public T X;
public T Y;
}
You can use the generic point for integer coordinates, for example:
Point<int> point;
point.X = 1;
point.Y = 2;
Or for charting coordinates that require floating point precision:
Point<double> point;
point.X = 1.2;
point.Y = 3.4;
Besides the basic generics syntax presented so far, C# 2.0 overloads some of the syntax of C# 1.1 to
specialize it when using generics. For example, consider the Pop() method of Code block 2. Suppose
instead of throwing an exception when the stack is empty, you would like to return the default value of
the type stored in the stack. If you were using an Object-based stack, you would simply return null, but
a generic stack could be used with value types as well. To address this issue, every generic type
parameter supports a property called default, which returns the default value of a type.
Here is how you can use default in the implementation of the Pop() method:
public T Pop()
{
m_StackPointer--;
if(m_StackPointer >= 0)
{
return m_Items[m_StackPointer];
}
else
{
m_StackPointer = 0;
return T.default;
}
}
The default value for reference types is null, and the default value for value types (such as integers,
enum, and structures) is a zero whitewash (filling the structure with zeros). Consequently, if the stack is
constructed with strings, the Pop() methods return null when the stack is empty, and when the stack is
constructed with integers, the Pop() methods return zero when the stack is empty.
Multiple Generic Types
A single type can define multiple generic types parameters. For example, consider the generic linked list
shown in Code block 3.
Code block 3. Generic linked list
class Node<K,T>
{
public Node()
{
Key = K.default;
Item = T.default;
NextNode = null;
}
public Node(K key,T item,Node<K,T> nextNode)
{
Key = key;
Item = item;
NextNode = nextNode;
}
public K Key;
public T Item;
public Node<K,T> NextNode;
}
public class LinkedList<K,T>
{
public LinkedList()
{
m_Head = new Node<K,T>();
}
public void AddHead(K key,T item)
{
Node<K,T> newNode = new Node<K,T>(key,item,m_Head.NextNode);
m_Head.NextNode = newNode;
}
Node<K,T> m_Head;
}
The linked list stores nodes:
class Node<K,T>
{...}
Each node contains a key (of the generic type parameter K) and a value (of the generic type parameter
T). Each node also has a reference to the next node in the list. The linked list itself is defined in terms of
the generic type parameters K and T:
public class LinkedList<K,T>
{...}
This allows the list to expose generic methods like AddHead():
public void AddHead(K key,T item);
Whenever you declare a variable of a type that uses generics, you must specify the types to use. However,
the specified types can themselves be generic types. For example, the linked list has a member variable
called m_Head of type Node, used for referencing the first item in the list. m_Head is declared using the
list's own generic type parameters K and T
Node<K,T> m_Head;
You need to provide specific types when instantiating a node, and again, you can use the linked list's own
generic type parameters:
public void AddHead(K key,T item)
{
Node<K,T> newNode = new Node<K,T>(key,item,m_Head.NextNode);
m_Head.NextNode = newNode;
}
Note that the fact the list uses the same names as the node for the generic type parameters is purely for
readability purposes, and it could have used other names, such as:
public class LinkedList<U,V>
{...}
Or:
public class LinkedList<KeyType,DataType>
{...}
In which case, m_Head would have been declared as:
Node<KeyType,DataType> m_Head;
When the client is using the linked list, the client has to provide specific types. The client can choose
integers as keys and strings as data items:
LinkedList<int,string> list = new LinkedList<int,string>();
list.AddHead(123,"AAA");
But the client can choose any other combination, such as a time stamp for keys:
LinkedList<DateTime,string> list = new LinkedList<DateTime,string>();
list.AddHead(DateTime.Now,"AAA");
Sometimes is it useful to alias a particular combination of specific types. You can do that through the
using statement, as shown in Code block 4. Note that the scope of aliasing is the scope of the file, so you
have to repeat aliasing across the project files in the same way you would with using namespaces.
Code block 4. Generic type aliasing
using List = LinkedList<int,string>;
class ListClient
{
static void Main(string[] args)
{
List list = new List();
list.AddHead(123,"AAA");
}
}
Generic Constraints
With C# generics, the compiler compiles the generic code into IL independent of any specific types that
the clients will use. As a result, the generic code could try to use methods, properties, or members of the
generic type parameters that are incompatible with the specific types the client uses. This is
unacceptable because it amounts to lack of type safety. In C# you need to instruct the compiler which
constraints the client-specified types must obey in order for them to be used instead of the generic type
parameters. There are two types of constraints. A derivation constraint indicates to the compiler that the
generic type parameter derives from a base type such an interface or a particular base class. A default
constructor constraint indicates to the compiler that the generic type parameter exposes a default public
constructor (a public constructor with no parameters). A generic type can employ multiple constraints,
and you even get IntelliSense reflecting the constraints when using the generic type parameter, such as
suggesting methods or members from the base type.
It is important to note that although constraints are optional, they are often essential when developing
a generic type. Without them, the compiler takes the more conservative, type-safe approach and only
allows access to Object-level functionality in your generic type parameters. Constraints are part of the
generic type metadata so that the client-side compiler can take advantage of them as well. The
client-side compiler only allows the client developer to use types that comply with the constraints, thus
enforcing type safety.
An example will go a long way to explain the need and use of constraints. Suppose you would like to add
indexing ability or searching by key to the linked list of Code block 3:
public class LinkedList<K,T>
{
T Find(K key)
{...}
public T this[K key]
{
get{return Find(key);}
}
}
This allows the client to write the following code:
LinkedList<int,string> list = new LinkedList<int,string>();
list.AddHead(123,"AAA");
list.AddHead(456,"BBB");
string item = list[456];
Debug.Assert(item == "BBB");
To implement the search, you need to scan the list, compare each node's key with the key you're looking
for, and return the item of the node whose key matches. The problem is that the following
implementation of Find() does not compile:
T Find(K key)
{
Node<K,T> current = m_Head;
while(current.NextNode != null)
{
if(current.Key == key) //Will not compile
break;
else
current = current.NextNode;
}
return current.Item;
}
The reason is that the compiler will refuse to compile this line:
if(current.Key == key)
The line above will not compile because the compiler does not know whether K (or the actual type
supplied by the client) supports the == operator. For example, structs by default do not provide such an
implementation. You could try to overcome the == operator limitation by using the IComparable
interface:
public interface IComparable
{
int CompareTo(object obj);
}
CompareTo() returns 0 if the object you compare to is equal to the object implementing the interface,
so the Find() method could use it as follows:
if(current.Key.CompareTo(key) == 0)
Unfortunately, this does not compile either because the compiler has no way of knowing whether K (or
the actual type supplied by the client) is derived from IComparable.
You could explicitly cast to IComparable to force the compiler to compile the comparing line, except
doing so is at the expense of type safety:
if(((IComparable)(current.Key)).CompareTo(key) == 0)
If the type the client uses does not derive from IComparable, it results in a run-time exception. In
addition, when the key type used is a value type instead of the key type parameter, you force a boxing
of the key, and that may have some performance implications.
Derivation Constraints
In C# 2.0 you use the where reserved keyword to define a constraint. Use the where keyword on the
generic type parameter followed by a derivation colon to indicate to the compiler that the generic type
parameter implements a particular interface. For example, here is the derivation constraint required to
implement the Find() method of LinkedList:
public class LinkedList<K,T> where K : IComparable
{
T Find(K key)
{
Node<K,T> current = m_Head;
while(current.NextNode != null)
{
if(current.Key.CompareTo(key) == 0)
break;
else
current = current.NextNode;
}
return current.Item;
}
//Rest of the implementation
}
Note that even though the constraint allows you to use IComparable, it does not eliminate the boxing
penalty when the key used is a value type, such as an integer. To overcome this, the
System.Collections.Generics namespace defines the generic interface IComparable<T>:
public interface IComparable<T>
{
int CompareTo(T other);
}
You can constrain the key type parameter to support IComparable<T> with the key's type as the type
parameter, and by doing so you gain not only type safety but also eliminate the boxing of value types
when used as keys:
public class LinkedList<K,T> where K : IComparable<K>
{...}
In fact, all the types that supported IComparable in .NET 1.1 support IComparable<T> in .NET 2.0.
This enables the use for keys of common types such as int, string, Guid, DateTime, and so on.
In C# 2.0, all constraints must appear after the actual derivation list of the generic class. For example,
if LinkedList derives from the IEnumerable<T> interface (for iterator support), you would put the
where keyword immediately after it:
public class LinkedList<K,T> : IEnumerable<T> where K : IComparable<K>
{...}
Note that the constraint appears where the generic type parameter K is defined, at the declaration line of
the LinkedList class. This permits the class LinkedList to treat the generic type parameter K as
implementing IComparable<T> with K as the type parameter. You will also get IntelliSense support on
the methods of interfaces you constrain.
When the client declares a variable of type LinkedList providing a concrete type for the list's key, the
client-side compiler will insist that the key type is derived from IComparable<T> (with the key's type
as the type parameter) and will refuse to build the client code otherwise.
In general, only define a constraint at the level where you require it. In the linked list example, it is
pointless to define the IComparable<T> derivation constraint at the node level, because the node itself
does not compare keys. If you do so, you will have to place the constraint at the LinkedList level as well,
even if the list does not compare keys. This is because the list contains a node as a member variable,
causing the compiler to insist that the key type defined at the list level complies with the constraint
placed by the node on the generic key type.
In other words, if you define the node as so:
class Node<K,T> where K : IComparable<K>
{...}
Then you would have to repeat the constraint at the list level, even if you do not provide the Find()
method, or any other method for that matter:
public class LinkedList<KeyType,DataType> where KeyType :
IComparable<KeyType>
{
Node<KeyType,DataType> m_Head;
}
You can constrain multiple interfaces on the same generic type parameter, separated by a comma. For
example:
public class LinkedList<K,T> where K : IComparable<K>,IConvertible
{...}
You can provide constraints for every generic type parameter your class uses, for example:
public class LinkedList<K,T> where K : IComparable<K>
where T : ICloneable
{...}
You can have a base class constraint, meaning, stipulating that the generic type parameter is derived
from a particular base class:
public class MyBaseClass
{...}
public class LinkedList<K,T> where K : MyBaseClass
{...}
However, at most one base class can be used in a constraint because C# does not support multiple
inheritance of implementation. Obviously, the base class you constrain cannot be a sealed class, and the
compiler enforces that. In addition, you cannot constrain System.Delegate or System.Array as a base
class .
You can constrain both a base class and one or more interfaces, but the base class must appear first in
the derivation constraint list:
public class LinkedList<K,T> where K : MyBaseClass, IComparable<K>
{...}
C# does not allow you to specify a naked generic type parameter as a constraint:
public class LinkedList<K,T,U> where K : U //Will not compile
{...}
However, C# does let you use another generic type as a constraint:
public interface ISomeInterface<T>
{...}
public class LinkedList<K,T> where K : ISomeInterface<int>
{...}
When constraining anther generic type as a base type, you can keep that type generic by specifying the
generic type parameters of your own type parameter. For example, in the case of a generic interface
constraint:
public class LinkedList<K,T> where K : ISomeInterface<T>
{...}
Or a generic base class constraint:
public class MySubClass<T> where T : MyBaseClass<T>
{...}
Finally, note that when providing a derivation constraint, the base type (interface or base class) you
constrain must have consistent visibility with that of the generic type parameter you define. For instance,
the following constraint is valid, because internal types can use public types:
public class MyBaseClass
{}
internal class MySubClass<T> where T : MyBaseClass
{}
However, if the visibility of the two classes were reversed, such as:
internal class MyBaseClass
{}
public class MySubClass<T> where T : MyBaseClass
{}
Then the compiler would issue an error, because no client from outside the assembly will ever be able to
use the generic type MySubClass, rendering MySubClass in effect as an internal rather than a public
type. The reason outside clients cannot use MySubClass is that to declare a variable of type
MySubClass, they require to make use of a type that derives from the internal type MyBaseClass.
Constructor Constraint
Suppose you want to instantiate a new generic object inside a generic class. The problem is the C#
compiler does not know whether the specific type the client will use has a matching constructor, and it
will refuse to compile the instantiation line.
To address this problem, C# allows you to constrain a generic type parameter such that it must support
a public default constructor. This is done using the new() constraint. For example, here is a different way
of implementing the default constructor of the generic Node <K,T> from Code block 3.
class Node<K,T> where T : new()
{
public Node()
{
Key = K.default;
Item = new T();
NextNode = null;
}
public K Key;
public T Item;
public Node<K,T> NextNode;
}
You can combine the constructor constraint with derivation constraint, provided the constructor
constraint appears last in the constraint list:
public class LinkedList<K,T> where K : IComparable<K>,new()
{...}
Generics and Casting
The C# compiler only lets you implicitly cast generic type parameters to Object, or to constraint-specified
types, as shown in Code block 5. Such implicit casting is type safe because any incompatibility is
discovered at compile-time.
Code block 5. Implicit casting of generic type parameters
interface ISomeInterface
{...}
class BaseClass
{...}
class MyClass<T> where T : BaseClass,ISomeInterface
{
void SomeMethod(T t)
{
ISomeInterface obj1 = t;
BaseClass obj2 = t;
object obj3 = t;
}
}
The compiler lets you explicitly cast generic type parameters to any other interface, but not to a class:
interface ISomeInterface
{...}
class SomeClass
{...}
class MyClass<T>
{
void SomeMethod(T t)
{
ISomeInterface obj1 = (ISomeInterface)t;//Compiles
SomeClass obj2 = (SomeClass)t; //Does not compile
}
}
However, you can force a cast from a generic type parameter to any other type using a temporary Object
variable:
class SomeClass
{...}
class MyClass<T>
{
void SomeMethod(T t)
{
object temp = t;
SomeClass obj = (SomeClass)temp;
}
}
Needless to say, such explicit casting is dangerous because it may throw an exception at run time if the
concrete type used instead of the generic type parameter does not derive from the type to which you
explicitly cast. Instead of risking a casting exception, a better approach is to use the is and as operators,
as shown in Code block 6. The is operator returns true if the generic type parameter is of the queried
type, and as will perform a cast if the types are compatible, and will return null otherwise. You can use
is and as on both naked generic type parameters and on generic classes with specific parameters.
Code block 6. Using 'is' and 'as' operators on generic type parameters
public class MyClass<T>
{
public void SomeMethod(T t)
{
if(t is int)
{...}
if(t is LinkedList<int,string>)
{...}
string str = t as string;
if(str != null)
{...}
LinkedList<int,string> list = t as LinkedList<int,string>;
if(list != null)
{...}
}
}
Inheritance and Generics
When deriving from a generic base class, you must provide a specific type instead of the base-class's
generic type parameter:
public class BaseClass<T>
{...}
public class SubClass : BaseClass<int>
{...}
If the subclass is generic, instead of a concrete type, you can use the subclass generic type parameter as
the specified type for the generic base class:
public class SubClass<T> : BaseClass<T>
{...}
When using the subclass generic type parameters, you must repeat any constraints stipulated at the
base class level at the subclass level. For example, derivation constraint:
public class BaseClass<T> where T : ISomeInterface
{...}
public class SubClass<T> : BaseClass<T> where T : ISomeInterface
{...}
Or constructor constraint:
public class BaseClass<T> where T : new()
{
public T SomeMethod()
{
return new T();
}
}
public class SubClass<T> : BaseClass<T> where T : new()
{...}
A base class can define virtual methods whose signatures use generic type parameters. When overriding
them, the subclass must provide the corresponding types in the method signatures:
public class BaseClass<T>
{
public virtual T SomeMethod()
{...}
}
public class SubClass: BaseClass<int>
{
public override int SomeMethod()
{...}
}
If the subclass is generic it can also use its own generic type parameters for the override:
public class SubClass<T>: BaseClass<T>
{
public override T SomeMethod()
{...}
}
You can define generic interfaces, generic abstract classes, and even generic abstract methods. These
types behave like any other generic base type:
public interface ISomeInterface<T>
{
T SomeMethod(T t);
}
public abstract class BaseClass<T>
{
public abstract T SomeMethod(T t);
}
public class SubClass<T> : BaseClass<T>
{
public override T SomeMethod(T t)
{...)
}
There is an interesting use for generic abstract methods and generic interfaces. In C# 2.0, it is impossible
to use operators such as + or += on generic type parameters. For example, the following code does not
compile because C# 2.0 does not have operator constraints:
public class Calculator<T>
{
public T Add(T arg1,T arg2)
{
return arg1 + arg2;//Does not compile
}
//Rest of the methods
}
Nonetheless, you can compensate using abstract methods (or preferably interfaces) by defining generic
operations. Since an abstract method cannot have any code in it, you can specify the generic operations
at the base class level, and provide a concrete type and implementation at the subclass level:
public abstract class BaseCalculator<T>
{
public abstract T Add(T arg1,T arg2);
public abstract T Subtract(T arg1,T arg2);
public abstract T Divide(T arg1,T arg2);
public abstract T Multiply(T arg1,T arg2);
}
public class MyCalculator : BaseCalculator<int>
{
public override int Add(int arg1, int arg2)
{
return arg1 + arg2;
}
//Rest of the methods
}
A generic interface will yield a somewhat cleaner solution as well:
public interface ICalculator<T>
{
T Add(T arg1,T arg2);
//Rest of the methods
}
public class MyCalculator : ICalculator<int>
{
public int Add(int arg1, int arg2)
{
return arg1 + arg2;
}
//Rest of the methods
}
Generic Methods
In C# 2.0, a method can define generic type parameters, specific to its execution scope:
public class MyClass<T>
{
public void MyMethod<X>(X x)
{...}
}
This is an important capability because it allows you to call the method with a different type every time,
which is very handy for utility classes.
You can define method-specific generic type parameters even if the containing class does not use
generics at all:
public class MyClass
{
public void MyMethod<T>(T t)
{...}
}
This ability is for methods only. Properties or indexers can only use generic type parameters defined at
the scope of the class.
When calling a method that defines generic type parameters, you can provide the type to use at the call
site:
MyClass obj = new MyClass();
obj.MyMethod<int>(3);
That said, when the method is invoked the C# compiler is smart enough to infer the correct type based
on the type of parameter passed in, and it allows omitting the type specification altogether:
MyClass obj = new MyClass();
obj.MyMethod(3);
This ability is called generic type inference. Note that the compiler cannot infer the type based on the
type of the returned value alone:
public class MyClass
{
public T MyMethod<T>()
{}
}
MyClass obj = new MyClass();
int number = obj.MyMethod();//Does not compile
When a method defines its own generic type parameters, it can also define constraints for these types:
public class MyClass
{
public void SomeMethod<T>(T t) where T : IComparable<T>
{...}
}
However, you cannot provide method-level constraints for class-level generic type parameters. All
constraints for class-level generic type parameters must be defined at the class scope.
When overriding a virtual method that defines generic type parameters, the subclass method must
redefine the method-specific generic type parameter:
public class BaseClass
{
public virtual void SomeMethod<T>(T t)
{...}
}
public class SubClass : BaseClass
{
public override void SomeMethod<T>(T t)
{...}
}
The subclass implementation must repeat all constraints that appeared at the base method level:
public class BaseClass
{
public virtual void SomeMethod<T>(T t) where T : new()
{...}
}
public class SubClass : BaseClass
{
public override void SomeMethod<T>(T t) where T : new()
{...}
}
Note that the method override cannot define new constraints that did not appear at the base method.
In addition, if the subclass method calls the base class implementation of the virtual method, it must
specify the type to use instead of the generic base method type parameter. You can either explicitly
specify it yourself or rely on type inference if it is available:
public class BaseClass
{
public virtual void SomeMethod<T>(T t)
{...}
}
public class SubClass : BaseClass
{
public override void SomeMethod<T>(T t)
{
base.SomeMethod<T>(t);
base.SomeMethod(t);
}
}
Generic Static Method
C# allows you to define static methods that use generic type parameters. However, when invoking such
a static method, you need to provide the concrete type for the containing class at the call site, such as in
this example:
public class MyClass<T>
{
public static T SomeMethod(T t)
{...}
}
int number = MyClass<int>.SomeMethod(3);
Static methods can define method-specific generic type parameters and constraints, similar to instance
methods. When calling such methods, you need to provide the method-specific types at the call site,
either explicitly:
public class MyClass<T>
{
public static T SomeMethod<X>(T t,X x)
{..}
}
int number = MyClass<int>.SomeMethod<string>(3,"AAA");
Or rely on type inference when possible:
int number = MyClass<int>.SomeMethod(3,"AAA");
Generic static methods are subjected to all constraints imposed on the generic type parameter they use
at the class level. As with instance method, you can provide constraints for generic type parameters
defined by the static method:
public class MyClass
{
public static T SomeMethod<T>(T t) where T : IComparable<T>
{...}
}
Operators in C# are nothing more than static methods and C# allows you to overload operators for your
generic types. Imagine that the generic LinkedList of Code block 3 provided the + operator for
concatenating linked lists. The + operator enables you to write the following elegant code:
LinkedList<int,string> list1 = new LinkedList<int,string>();
LinkedList<int,string> list2 = new LinkedList<int,string>();
...
LinkedList<int,string> list3 = list1+list2;
Code block 7 shows the implementation of the generic + operator on the LinkedList class. Note that
operators cannot define new generic type parameters.
Code block 7. Implementing a generic operator
public class LinkedList<K,T>
{
public static LinkedList<K,T> operator+(LinkedList<K,T> lhs,
LinkedList<K,T> rhs)
{
return concatenate(lhs,rhs);
}
static LinkedList<K,T> concatenate(LinkedList<K,T> list1,
LinkedList<K,T> list2)
{
LinkedList<K,T> newList = new LinkedList<K,T>();
Node<K,T> current;
current = list1.m_Head;
while(current != null)
{
newList.AddHead(current.Key,current.Item);
current = current.NextNode;
}
current = list2.m_Head;
while(current != null)
{
newList.AddHead(current.Key,current.Item);
current = current.NextNode;
}
return newList;
}
//Rest of LinkedList
}
Generic Delegates
A delegate defined in a class can take advantage of the generic type parameter of that class. For
example:
public class MyClass<T>
{
public delegate void GenericDelegate(T t);
public void SomeMethod(T t)
{...}
}
When specifying a type for the containing class, it affects the delegate as well:
MyClass<int> obj = new MyClass<int>();
MyClass<int>.GenericDelegate del;
del = new MyClass<int>.GenericDelegate(obj.SomeMethod);
del(3);
C# 2.0 allows you to make a direct assignment of a method reference into a delegate variable:
MyClass<int> obj = new MyClass<int>();
MyClass<int>.GenericDelegate del;
del = obj.SomeMethod;
I am calling this feature delegate inference. The compiler is capable of inferring the type of the delegate
you assign into, finding if the target object has a method by the name you specify, and verifying that the
method's signature matches. Then, the compiler creates a new delegate of the inferred argument type
(including the correct type instead of the generic type parameter), and assigns the new delegate into the
inferred delegate.
Like classes, structs, and methods, delegates can define generic type parameters too:
public class MyClass<T>
{
public delegate void GenericDelegate<X>(T t,X x);
}
Delegates defined outside the scope of a class can use generic type parameters. In that case, you have
to provide the specific types for the delegate when declaring and instantiating it:
public delegate void GenericDelegate<T>(T t);
public class MyClass
{
public void SomeMethod(int number)
{...}
}
MyClass obj = new MyClass();
GenericDelegate<int> del;
del = new GenericDelegate<int>(obj.SomeMethod);
del(3);
Alternatively, you can use delegate inference when assigning the delegate:
MyClass obj = new MyClass();
GenericDelegate<int> del;
del = obj.SomeMethod;
And naturally, a delegate can define constraints to accompany its generic type parameters:
public delegate void MyDelegate<T>(T t) where T : IComparable<T>;
The delegate-level constraints are enforced only on the using side, when declaring a delegate variable
and instantiating a delegate object, similar to any other constraint at the scope of types or methods.
Generic delegates are especially useful when it comes to events. You can literally define a limited set of
generic delegates, distinguished only by the number of the generic type parameters they require, and
use these delegates for all of your event handling needs. Code block 8 demonstrates the use of a
generic delegate and a generic event-handling method.
Code block 8. Generic event handling
public delegate void
GenericEventHandler<SenderType,ArgsType>(SenderType sender,
ArgsType args);
public class MyPublisher
{
public event GenericEventHandler<MyPublisher,EventArgs> MyEvent;
public void FireEvent()
{
MyEvent(this,EventArgs.Empty);
}
}
public class MySubscriber<ArgsType> //Optional: can be a specific type
{
public void SomeMethod(MyPublisher sender,ArgsType args)
{...}
}
MyPublisher publisher = new MyPublisher();
MySubscriber<EventArgs> subs = new MySubscriber<EventArgs>();
publisher.MyEvent += subs.SomeMethod;
Code block 8 uses a generic delegate called GenericEventHandler, which accepts a generic sender
type and a generic type parameter. Obviously, if you need more parameters, you can simply add more
generic type parameters, but I wanted to model GenericEventHandler after the non-generic .NET
EventHandler, defined as:
public void delegate EventHandler(object sender,EventArgs args);
Unlike EventHandler, GenericEventHandler is type safe, as shown in Code block 8 because it
accepts only objects of the type MyPublisher as senders, rather than mere Object.
Generics and Reflection
In .NET 2.0, reflection is extended to support generic type parameters. The type Type can now represent
generic types with specific type parameters (called bounded types), or unspecified (unbounded) types.
As in C# 1.1, you can obtain the Type of any type by using the typeof operator or by calling the
GetType() method that every type supports. Regardless of the way you choose, both yield the same
Type. For example, in the following code sample, type1 is identical to type2.
LinkedList<int,string> list = new LinkedList<int,string>();
Type type1 = typeof(LinkedList<int,string>);
Type type2 = list.GetType();
Debug.Assert(type1 == type2);
Both typeof and GetType() can operate on naked generic type parameters:
public class MyClass<T>
{
public void SomeMethod(T t)
{
Type type = typeof(T);
Debug.Assert(type == t.GetType());
}
}
Type has new methods and properties designed to provide reflection information about the generic
aspect of the type. Code block 9 shows the new methods.
Code block 9. Type's generic reflection members
public abstract class Type : //Base types
{
public int GenericParameterPosition{virtual get;}
public bool HasGenericParameters{get;}
public bool HasUnboundGenericParameters{virtual get;}
public bool IsGenericParameter{virtual get;}
public bool IsGenericTypeDefinition{virtual get;}
public virtual Type BindGenericParameters(Type[] typeArgs);
public virtual Type[] GetGenericParameters();
public virtual Type GetGenericTypeDefinition();
//Rest of the members
}
The most useful of these new members are the HasGenericParameters and
HasUnboundGenericParameters properties, and GetGenericParameters() and
GetGenericTypeDefinition() methods. The rest of Type's new members are for advanced and
somewhat esoteric scenarios beyond the scope of this article. As its name indicates,
HasGenericParameters is set to true if the type represented by the Type object uses generic type
parameters. GetGenericParameters() returns an array of Types corresponding to the bounded types
used. GetGenericTypeDefinition() returns a Type representing the generic form of the underlying
type. HasUnboundGenericParameters is true if the Type object has any unspecified generic type
parameters. It follows that HasUnboundGenericParameters is set to true on the Type returned from
GetGenericTypeDefinition() when called on a generic type.
Code block 10 demonstrates using these new Type members to obtain generic reflection information on
the LinkedList from Code block 3.
Code block 10. Using Type for generic reflection
LinkedList<int,string> list = new LinkedList<int,string>();
Type boundedType = list.GetType();
Trace.WriteLine(boundedType.ToString());
//Writes 'LinkedList[System.Int32,System.String]'
Debug.Assert(boundedType.HasGenericParameters);
Type[] parameters = boundedType.GetGenericParameters();
Debug.Assert(parameters.Length == 2);
Debug.Assert(parameters[0] == typeof(int));
Debug.Assert(parameters[1] == typeof(string));
Type unboundedType = boundedType.GetGenericTypeDefinition();
Debug.Assert(unboundedType.HasUnboundGenericParameters);
Trace.WriteLine(unboundedType.ToString());
//Writes 'LinkedList[K,T]'
As shown in Code block 10, a Type can refer to a generic type with bounded parameters
(boundedType in Code block 10), or to a generic type with unbounded parameters (unboundedType
in Code block 10).
Similar to Type, MethodInfo and its base class MethodBase have new members that reflect generic
method information.
As in C# 1.1, you can use MethodInfo (as well as a number of other options) for late binding invocation.
However, the type of the parameters you pass for the late binding must match the bounded types used
instead of the generic type parameters (if any):
LinkedList<int,string> list = new LinkedList<int,string>();
Type type = list.GetType();
MethodInfo methodInfo = type.GetMethod("AddHead");
object[] args = {1,"AAA"};
methodInfo.Invoke(list,args);
Attributes and Generics
When defining an attribute, you can instruct the compiler that the attribute should target generic type
parameters, using the new GenericParameter value of the enum AttributeTargets:
[AttributeUsage(AttributeTargets.GenericParameter)]
public class SomeAttribute : Attribute
{...}
Note that C# 2.0 does not allow you to define generic attributes.
//Does not compile:
public class SomeAttribute<T> : Attribute
{...}
Yet internally, an attribute class can take advantage of generics by using generic types or define helper
generic methods, like any other type:
public class SomeAttribute : Attribute
{
void SomeMethod<T>(T t)
{...}
LinkedList<int,string> m_List = new LinkedList<int,string>();
}
Generics and the .NET Framework
To conclude this article, here are how some other areas in .NET besides C# itself are taking advantage of
or interacting with generics.
Generic Collections
The data structures in System.Collections are all Object-based, and thus inheriting the two problems
described at the beginning of this article, namely, poor performance and lack of type safety. .NET 2.0
introduces a new set of generic collections in the System.Collections.Generics namespace. For example,
there are generic Stack<T> and a generic Queue<T> classes. The Dictionary<K,T> data structure is
equivalent to the non-generic HashTable, and there is also a SortedDictionary<K,T> class somewhat
like SortedList. The class List<T> is analogous to the non-generic ArrayList. Table 1 maps the new
types of System.Collections.Generics to those of System.Collections.
Table 1. Mapping System.Collections.Generics to System.Collections
System.Collections.Generics System.Collections
Comparer<T> Comparer
Dictionary<K,T> HashTable
List<T> ArrayList
Queue<T> Queue
SortedDictionary<K,T> SortedList
Stack<T> Stack
ICollection<T> ICollection
IComparable<T> System.IComparable
IComparer<T> IComparer
IDictionary<K,T> IDictionary
IEnumerable<T> IEnumerable
IEnumerator<T> IEnumerator
IKeyComparer<T> IKeyComparer
IList<T> IList
All the generic collections in System.Collections.Generics also implement the generic IEnumerable<T>
interface, defined as:
public interface IEnumerable<T>
{
IEnumerator<T> GetEnumerator();
}
public interface IEnumerator<T> : IDisposable
{
T Current{get;}
bool MoveNext();
}
Briefly, IEnumerable<T> provides access to the IEnumerator<T> iterator interface, used for
abstracted iterations over the collection. All the collections implement IEnumerable<T> on a nested
struct, where the generic type parameter T is the type the collection stores.
Of particular interest is the way the dictionary collections define their iterators. A dictionary is actually a
collection of not one but two types of generic parameter—the key and the value.
System.Collection.Generics provides a generic struct called KeyValuePair<K,V> defined as:
struct KeyValuePair<K,V>
{
public KeyValuePair(K key,V value);
public K Key(get;set;)
public V Value(get;set;)
}
KeyValuePair<K,V> simply stores a pair of a generic key and a generic value, and in doing so, defines in
effect a new generic type. This new type is what the dictionary manages as a collection, and what it uses
for its implementation of IEnumerable<T>. The Dictionary class specifies the generic
KeyValuePair<K,V> structure as the item type for IEnumerable<T> and ICollection<T>:
public class Dictionary<K,T> : IEnumerable<KeyValuePair<K,T>>,
ICollection<KeyValuePair<K,T>>,
//More interfaces
{...}
The key and value type parameters used in KeyValuePair<K,V> are of course the dictionary's own
generic key and value type parameters. I am calling this technique compound generics.
You can certainly take advantage of compound generics in your own generic data structures that use
pairs of keys and values. For example:
public class LinkedList<K,T> : IEnumerable<KeyValuePair<K,T>> where K :
IComparable<K>
{...}
Serialization and Generics
.NET allows you to have serializable generic types:
[Serializable]
public class MyClass<T>
{...}
When serializing a type, besides the state of the object members, .NET persists metadata about the
object and its type. If the serializable type is generic and it contains bounded types, the metadata about
the generic type contains type information about the bounded types as well. Consequently, each
permutation of a generic type with a specific argument type is considered a unique type. For example,
you cannot serialize an object type MyClass<int> but deserialize it into an object of type
MyClass<string>. Serializing an instance of a generic type is no different from serializing a non-generic
type. However, when deserializing that type, you need to declare a variable with matching specific types,
and specify these types again when down casting the Object returned from Deserialize. Code block 11
shows serialization and deserialization of a generic type.
Code block 11. Client-side serialization of a generic type
[Serializable]
public class MyClass<T>
{...}
MyClass<int> obj1 = new MyClass<int>();
IFormatter formatter = new BinaryFormatter();
Stream stream = new
FileStream("obj.bin",FileMode.Create,FileAccess.ReadWrite);
formatter.Serialize(stream,obj1);
stream.Seek(0,SeekOrigin.Begin);
MyClass<int> obj2;
obj2 = (MyClass<int>)formatter.Deserialize(stream);
stream.Close();
When providing custom serialization, you have to add values to a property bag called SerializationInfo,
which you also get values from during deserialization. You implement the interface ISerializable with
the single method GetObjectData() that provides the SerializationInfo used in serialization. You also
must implement a special constructor that accepts the SerializationInfo from which to get the object
state.
SerializationInfo provides methods for getting or adding field values. Each field is identified by a string.
SerializationInfo has type-safe methods for most of the CLR basic types, such as int and string:
public sealed class SerializationInfo
{
public void AddValue(string name, int value);
public int GetInt32(string name);
//Other methods and properties
}
The problem when providing custom serialization on a generic type is that you do not know which of the
adding or getting methods to use. To address this issue, SerializationInfo provides methods to add or
get an Object and its type:
public void AddValue(string name, object value, Type type);
public object GetValue(string name,Type type);
Note These methods are available in .NET 1.1 as well.
When using AddValue(), obtain the type of the generic parameter type. When calling GetValue(), use
a cast to the generic parameter type because GetValue() returns an Object. Code block 12
demonstrates the use of these AddValue() and GetValue() methods.
Code block 12. Custom serialization of a generic class
[Serializable]
public class MyClass<T> : ISerializable
{
public MyClass()
{}
public void GetObjectData(SerializationInfo info,StreamingContext
ctx)
{
info.AddValue("m_T",m_T,typeof(T));
}
private MyClass(SerializationInfo info,StreamingContext context)
{
m_T = (T)info.GetValue("m_T",typeof(T));
}
T m_T;
}
However, there is a better, safer way for providing custom serialization on a generic type. Code block
13 presents the GenericSerializationInfo utility class that exposes generic AddValue() and
GetValue() methods. By encapsulating a regular SerializationInfo, GenericSerializationInfo
shields the client code from the type retrieval and explicit casting.
Code block 13. The GenericSerializationInfo utility class.
public class GenericSerializationInfo
{
SerializationInfo m_SerializationInfo;
public GenericSerializationInfo(SerializationInfo info)
{
m_SerializationInfo = info;
}
public void AddValue<T>(string name,T value)
{
m_SerializationInfo.AddValue(name,value,value.GetType());
}
public T GetValue<T>(string name)
{
object obj = m_SerializationInfo.GetValue(name,typeof(T));
return (T)obj;
}
}
Code block 14 shows the same custom serialization code as in Code block 12, except it uses
GenericSerializationInfo. Note the use of type inference in the call to AddValue().
Code block 14. Using GenericSerializationInfo
[Serializable]
public class MyClass<T> : ISerializable
{
public MyClass()
{}
public void GetObjectData(SerializationInfo info,StreamingContext
ctx)
{
GenericSerializationInfo genericInfo = new
GenericSerializationInfo(info);
genericInfo.AddValue("m_T",m_T); //using type inference
}
private MyClass(SerializationInfo info,StreamingContext context)
{
GenericSerializationInfo genericInfo = new
GenericSerializationInfo(info);
m_T = genericInfo.GetValue<T>("m_T");
}
T m_T;
}
I use GenericSerializationInfo as a cleaner way of implementing custom serialization, even on
non-generic class members. For example, if MyClass in Code block 14 had a class member of type
string called m_Name, you could write:
genericInfo.AddValue("m_Name",m_Name);
and:
m_Name = genericInfo.GetValue<string>("m_Name");
Generics and Remoting
You can define and deploy remote classes that utilize generics, and you can use programmatic or
administrative configuration. Consider the class MyServer that uses generics and is derived from
MarshalByRefObject:
public class MyServer<T> : MarshalByRefObject
{...}
When using administrative type registration, you need to specify the exact types to use instead of the
generic type parameters. You must name the types in a language-neutral manner, and provide fully
qualified namespaces. For example, suppose the class MyServer is defined in the namespace
RemoteServer in the assembly ServerAssembly, and you want to use it with integer instead of the
generic type parameter T, in client activated mode. In that case, the required client-side type registration
entry in the configuration file would be:
<client url="...some url goes here...">
<activated
type="RemoteServer.MyServer[[System.Int32]],ServerAssembly"/>
</client>
The matching host-side type registration entry in the configuration file is:
<service>
<activated
type="RemoteServer.MyServer[[System.Int32]],ServerAssembly"/>
</service>
The double square brackets are used to specify multiple types. For example:
LinkedList[[System.Int32],[System.String]]
When using programmatic configuration, you configure activation modes and type registration similar to
C# 1.1, except when defining the type of the remote object you have to provide specific types instead of
the generic type parameters. For example, for host-side activation mode and type registration you would
write:
Type serverType = typeof(MyServer<int>);
RemotingConfiguration.RegisterActivatedServiceType(serverType);
For client-side type activation mode and location registration, you have the following:
Type serverType = typeof(MyServer<int>);
string url = ...; //some url initialization
RemotingConfiguration.RegisterWellKnownClientType(serverType,url);
When instantiating the remote server, simply provide the specific types, just as if you where using a local
generic type:
MyServer<int> obj;
obj = new MyServer<int>();
//Use obj
Instead of using new, the client can optionally use the methods of the Activator class to connect to
remote objects. When using Activator.GetObject() you need to provide the specific types to use, and
provide the specific types when explicitly casting the returned Object:
string url = ...; //some url initialization
Type serverType = typeof(MyServer<int>);
MyServer<int> obj;
obj = (MyServer<int>)Activator.GetObject(serverType,url);
//Use obj
The Activator.CreateInstance() method has numerous overloaded versions. Some of the versions can
be used with generic types:
Type serverType = typeof(MyServer<int>);
MyServer<int> obj;
obj = (MyServer<int>)Activator.CreateInstance(serverType);
//Use obj
However, for generic types you cannot use the versions of CreateInstance() and
CreateInstanceFrom() that accept the type in string format and return ObjectHandle, such as:
public static ObjectHandle CreateInstance(string assemblyName,string
typeName);
Activator was designed without generics in mind, and it is therefore not completely type safe because
of the explicit cast from Object. It is possible to compensate by defining the class GenericActivator, as
shown in Code block 15.
Code block 15. The GenericActivator
public class GenericActivator
{
public static T CreateInstance<T>()
{
return (T)Activator.CreateInstance(typeof(T));
}
public static T GetObject<T>(string url)
{
return (T)Activator.GetObject(typeof(T),url);
}
}
GenericActivator uses generic static methods to expose the most useful methods of Activator. You
can use GenericActivator instead of Activator to activate generic and non-generic types. For example,
using GetObject():
public class MyServer : MarshalByRefObject
{...}
public class MyGenericServer<T> : MarshalByRefObject
{...}
string url = ...;//some url initialization
MyServer obj1 = GenericActivator.GetObject<MyServer>(url);
MyGenericServer obj2 = GenericActivator.GetObject<MyServer<int>>(url);
Or using CreateInstance():
MyServer obj = GenericActivator.CreateInstance<MyServer>();
What Generics Cannot Do
Under .NET 2.0, you cannot define generic Web services. That is, Web methods that use generic type
parameters. The reason is that none of the Web service standards support generic services.
You also cannot use generic types on a serviced component. The reason is that generics do not meet COM
visibility requirements, which are required for serviced components (just like you could not use C++
templates in COM or COM+).
Conclusion
C# generics are an invaluable addition to your development arsenal. They improve performance, type
safety and quality, reduce repetitive programming tasks, simplify the overall programming model, and
do so with elegant, readable syntax. While the roots of C# generics are C++ templates, C# takes
generics to a new level by providing compile-time safety and support. C# utilizes two-phase compilation,
metadata, and innovative concepts such as constraints and generic methods. No doubt future versions of
C# will continue to evolve generics, adding new capabilities and extending generics to other areas
of .NET Framework such as data access or localization.
Juval Lowy is a software architect and the principal of IDesign, a consulting and training company. Juval
is a Microsoft Regional Director for the Silicon Valley, working with Microsoft on helping the industry
adopt .NET. His latest book is Programming .NET Components (O'Reilly, 2003). The book is dedicated to
component-oriented programming and design, as well as the related system issues. Juval participates in
the Microsoft internal design reviews for future versions of .NET. Microsoft recognized Juval as a Software
Legend and one of the world's top .NET experts and industry leaders. Contact Juval at
http://www.idesign.net.
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