.net入门常见问题（文章超长）

使用业务对象

在业务组件中封装逻辑是实际应用程序的基本组成部分，不论是基于 Web 的应用程序还是任何其他应用程序。在 ASP.NET 中，业务对象是多层 Web 应用程序（如包含数据访问层或通用应用程序规则层的应用程序）的生成块。本节说明如何编写一些简单的组件并将它们包括在应用程序的 Web 窗体页中。 

应用程序 /Bin 目录 

使用 Web 应用程序组件的 COM 模型时遇到的问题是，这些组件必须注册（通常是使用 regsvr32 工具）后才能在传统的 ASP 应用程序中使用。对这些类型的应用程序进行远程管理通常是不可能的，因为注册工具必须在服务器上以本地方式运行。更为棘手的是，这些组件经应用程序加载后便在磁盘上保持锁定状态，必须使整个 Web 服务器停止运行后才能替换或移除这些组件。 

为尝试解决这些问题，ASP.NET 允许将组件放置在已知目录中并在运行时自动找到它们。该已知目录总是命名为 /bin，并紧位于应用程序根目录（一个由 Internet 信息服务 (IIS) 定义的虚拟目录）下。这样做的好处是组件无需注册便可在 ASP.NET Framework 应用程序中使用——只需将组件复制到 /bin 目录或执行 FTP 文件传输便可部署组件。 

除提供零注册方法部署复杂组件外，ASP.NET 不要求这些组件在运行时在磁盘上保持锁定状态。ASP.NET 在后台复制 /bin 中的程序集并加载这些 shadow 副本。原始组件即使在 Web 服务器仍在运行时也可以替换，而对 /bin 目录的更改则由运行时自动获取。当检测到更改时，ASP.NET 允许完成当前执行的请求，并将所有新传入的请求定向到使用新组件的应用程序。 

导入业务对象 

从最基本的层面看，业务组件只是一个类，可以从导入它的 Web 窗体页为其创建实例。下面的示例定义一个简单的 HelloWorld 类。该类有一个公共构造函数（当初次创建该类的实例时执行此构造函数）、一个称为 FirstName 的 String 属性和一个通过 FirstName 属性值打印问候语的 SayHello 方法。 


using System;
using System.Text;

namespace HelloWorld {
  public class HelloObj {
    private String _name;

    public HelloObj() {
        _name = null;

    }

    public String FirstName {
      get {
        return _name;
      }
      set {
        _name = value;
      }
    }

    public String SayHello() {
      StringBuilder sb = new StringBuilder("Hello ");
      if (_name != null)
         sb.Append(_name);
      else
         sb.Append("World");

      sb.Append("!");
      return sb.ToString();
    }
  }
}


Imports System
Imports System.Text

Namespace HelloWorld
  Public Class HelloObj
    Private _name As String

    Public Sub New
      MyBase.New()
      _name = Nothing
    End Sub

    Public Property FirstName As String
      Get
        Return(_name)
      End get
      Set
        _name = value
       End Set
    End Property

    Public Function SayHello() As String
      Dim sb As New StringBuilder("Hello ")
      If (_name <> Nothing) Then
         sb.Append(_name)
      Else
         sb.Append("World")
      End If
      sb.Append("!")
      Return(sb.ToString())
    End Function
  End Class
End Namespace


import System;
import System.Text;

package HelloWorld {
  public class HelloObj {
    private var _name:String;

    public function HelloObj() {
        _name = null;

    }

    public function get FirstName() : String
    {
        return _name;
    }
    public function set FirstName(value:String) : void
    {
        _name = value;
    }

    public function SayHello() : String {
      sb:StringBuilder = new StringBuilder("Hello ");
      if (_name != null)
         sb.Append(_name);
      else
         sb.Append("World");

      sb.Append("!");
      return sb.ToString();
    }
  }
}

 
C#  VB  JScript    
若要编译该类，应从命令行运行 C# 编译器 (Csc.exe)。/t 选项通知编译器生成库 (DLL)，/out 选项通知编译器将结果程序集放置到的位置。本例中，应用程序的 /bin 目录紧位于本教程的“aspplus”虚拟根目录下，并假设该命令从示例目录，即 ...\QuickStart\AspPlus\Samples\WebForms\Busobjs 运行。 


csc /t:library /out:..\..\..\..\bin\HelloObj.dll HelloObj.cs

对于 Visual Basic，等效编译命令是：


vbc /t:library /out:..\..\..\..\bin\HelloObjVB.dll HelloObj.vb

对于 JScript，等效编译命令是：


jsc /out:..\..\..\..\bin\HelloObjJS.dll HelloObj.js

该组件现在可供应用程序中任何需要使用它的 Web 窗体页使用。下面的 HelloObj.aspx 示例阐释了此功能。 

简单的两层 Web 窗体页
外部组件的典型用途是执行数据访问。它简化了页中的代码，增强了可读性并将用户界面 (UI) 逻辑与系统功能分开。下面的实例阐释了简单的两层 Web 窗体页，它使用数据访问组件检索产品信息。 

<%@ Import Namespace="DataLayer" %>







  


  
A Simple Two-Tier WebForm

  Select a Category:

  

  


  

    
      
<%# DataBinder.Eval(Container.DataItem, "ProductName") %>
<%# DataBinder.Eval(Container.DataItem, "UnitPrice", "{0:C}").ToString() %>
    

  

  

using System.Reflection;

[assembly: AssemblyTitle("")]
[assembly: AssemblyDescription("A QuickStart Tutorial Assembly")]
[assembly: AssemblyConfiguration("")]
[assembly: AssemblyCompany("Microsoft Corporation")]
[assembly: AssemblyProduct("Microsoft QuickStart Tutorials")]
[assembly: AssemblyCopyright(" Microsoft Corporation.  All rights reserved.")]
[assembly: AssemblyTrademark("")]
[assembly: AssemblyCulture("")]
[assembly: AssemblyVersion("1.1.*")]

namespace DataLayer
{
using System;
using System.Data;
using System.Data.SqlClient;

  public class DataObj
  {
    private String _connStr;

    public DataObj()
    {
      _connStr = null;
    }

    public DataObj(String connStr)
    {
      _connStr = connStr;
    }

    public String ConnectionString
    {
      get
      {
        return _connStr;
      }
      set
      {
        _connStr = value;
      }
    }

    public DataView GetCategories()
    {
      SqlConnection myConnection = new SqlConnection(_connStr);
      SqlDataAdapter myCommand = new SqlDataAdapter("select distinct CategoryName from Categories", myConnection);

      DataSet ds = new DataSet();
      try {
          myCommand.Fill(ds, "Categories");

          return ds.Tables["Categories"].DefaultView;
      }
      catch (Exception ex){
        throw (ex);
      }
      finally{
        myConnection.Close();
      }
    }

    public DataView GetProductsForCategory(String category)
    {
      SqlConnection myConnection = new SqlConnection(_connStr);
      SqlDataAdapter myCommand = new SqlDataAdapter("select ProductName, ImagePath, UnitPrice, c.CategoryId  from Products p, Categories c where c.CategoryName='" + category + "' and p.CategoryId = c.CategoryId", myConnection);

      DataSet ds = new DataSet();
      try {
          myCommand.Fill(ds, "Products");

          return ds.Tables["Products"].DefaultView;
      }
      catch (Exception ex){
        throw (ex);
      }
      finally{
        myConnection.Close();
      }
    }
  }

}

简单的三层 Web 窗体页

三层应用程序模型扩展了两层方案，在用户界面和数据访问逻辑之间包含了业务规则。此模型允许用户界面开发人员处理更高抽象化的级别，而不是通过低级数据访问组件 API 直接操作数据。中间业务组件通常强制业务规则并确保遵守数据库的关系和主键约束。下面的示例使用中间组件基于客户端输入的两位供应商 ID 计算折扣。 

<%@ Page ClientTarget="DownLevel"%>
<%@ Import Namespace="BusinessLayer" %>







  


  
A Simple Three-Tier Web Form

  Select a Category:

    

  Preferred Customer ID:

    

  

    

  
   Customer ID must be two numeric digits
  

  


  

    
      
<%# DataBinder.Eval(Container.DataItem, "ProductName") %>
<%# DataBinder.Eval(Container.DataItem, "UnitPrice", "{0:C}").ToString() %>
    

  

  

using System.Reflection;

[assembly: AssemblyTitle("")]
[assembly: AssemblyDescription("A QuickStart Tutorial Assembly")]
[assembly: AssemblyConfiguration("")]
[assembly: AssemblyCompany("Microsoft Corporation")]
[assembly: AssemblyProduct("Microsoft QuickStart Tutorials")]
[assembly: AssemblyCopyright(" Microsoft Corporation.  All rights reserved.")]
[assembly: AssemblyTrademark("")]
[assembly: AssemblyCulture("")]
[assembly: AssemblyVersion("1.1.*")]

namespace BusinessLayer
{
using System;
using System.Data;
using System.Data.SqlClient;
using DataLayer;

  public class BusObj
  {
    private DataObj data;

    public BusObj()
    {
      data = new DataObj("server=(local)\\NetSDK;database=grocertogo;Integrated Security=SSPI");
    }

    public DataView GetCategories()
    {
       return data.GetCategories();
    }

    public DataView GetProductsForCategory(String category, int customerid)
    {
       DataView view = data.GetProductsForCategory(category);

       double discount = 0;
       if ((customerid >= 25)&&(customerid < 50))
       {
         discount = .50;
       }
       else if ((customerid >= 50)&&(customerid < 75))
       {
         discount = 1.00;
       }
       else if ((customerid >= 75)&&(customerid < 100))
       {
         discount = 1.50;
       }

       for (int i=0; i


using System.Reflection;

[assembly: AssemblyTitle("")]
[assembly: AssemblyDescription("A QuickStart Tutorial Assembly")]
[assembly: AssemblyConfiguration("")]
[assembly: AssemblyCompany("Microsoft Corporation")]
[assembly: AssemblyProduct("Microsoft QuickStart Tutorials")]
[assembly: AssemblyCopyright(" Microsoft Corporation.  All rights reserved.")]
[assembly: AssemblyTrademark("")]
[assembly: AssemblyCulture("")]
[assembly: AssemblyVersion("1.1.*")]

namespace DataLayer
{
using System;
using System.Data;
using System.Data.SqlClient;

  public class DataObj
  {
    private String _connStr;

    public DataObj()
    {
      _connStr = null;
    }

    public DataObj(String connStr)
    {
      _connStr = connStr;
    }

    public String ConnectionString
    {
      get
      {
        return _connStr;
      }
      set
      {
        _connStr = value;
      }
    }

    public DataView GetCategories()
    {
      SqlConnection myConnection = new SqlConnection(_connStr);
      SqlDataAdapter myCommand = new SqlDataAdapter("select distinct CategoryName from Categories", myConnection);

      DataSet ds = new DataSet();
      try {
          myCommand.Fill(ds, "Categories");

          return ds.Tables["Categories"].DefaultView;
      }
      catch (Exception ex){
        throw (ex);
      }
      finally{
        myConnection.Close();
      }
    }

    public DataView GetProductsForCategory(String category)
    {
      SqlConnection myConnection = new SqlConnection(_connStr);
      SqlDataAdapter myCommand = new SqlDataAdapter("select ProductName, ImagePath, UnitPrice, c.CategoryId  from Products p, Categories c where c.CategoryName='" + category + "' and p.CategoryId = c.CategoryId", myConnection);

      DataSet ds = new DataSet();
      try {
          myCommand.Fill(ds, "Products");

          return ds.Tables["Products"].DefaultView;
      }
      catch (Exception ex){
        throw (ex);
      }
      finally{
        myConnection.Close();
      }
    }
  }

}
向文件写入文本

using System;
using System.IO;

class Test 
{
    public static void Main() 
    {
        // Create an instance of StreamWriter to write text to a file.
        // The using statement also closes the StreamWriter.
        using (StreamWriter sw = new StreamWriter("TestFile.txt")) 
        {
            // Add some text to the file.
            sw.Write("This is the ");
            sw.WriteLine("header for the file.");
            sw.WriteLine("-------------------");
            // Arbitrary objects can also be written to the file.
            sw.Write("The date is: ");
            sw.WriteLine(DateTime.Now);
        }
    }
}

using System;
using System.IO;
public class TextToFile 
{
    private const string FILE_NAME = "MyFile.txt";
    public static void Main(String[] args) 
    {
        if (File.Exists(FILE_NAME)) 
        {
            Console.WriteLine("{0} already exists.", FILE_NAME);
            return;
        }
        StreamWriter sr = File.CreateText(FILE_NAME);
        sr.WriteLine ("This is my file.");
        sr.WriteLine ("I can write ints {0} or floats {1}, and so on.", 
            1, 4.2);
        sr.Close();
    }
}



从文件读取文本
using System;
using System.IO;

class Test 
{
    public static void Main() 
    {
        try 
        {
            // Create an instance of StreamReader to read from a file.
            // The using statement also closes the StreamReader.
            using (StreamReader sr = new StreamReader("TestFile.txt")) 
            {
                String line;
                // Read and display lines from the file until the end of 
                // the file is reached.
                while ((line = sr.ReadLine()) != null) 
                {
                    Console.WriteLine(line);
                }
            }
        }
        catch (Exception e) 
        {
            // Let the user know what went wrong.
            Console.WriteLine("The file could not be read:");
            Console.WriteLine(e.Message);
        }
    }
}


using System;
using System.IO;
public class TextFromFile 
{
    private const string FILE_NAME = "MyFile.txt";
    public static void Main(String[] args) 
    {
        if (!File.Exists(FILE_NAME)) 
        {
            Console.WriteLine("{0} does not exist.", FILE_NAME);
            return;
        }
        StreamReader sr = File.OpenText(FILE_NAME);
        String input;
        while ((input=sr.ReadLine())!=null) 
        {
            Console.WriteLine(input);
        }
        Console.WriteLine ("The end of the stream has been reached.");
        sr.Close();
    }
}

File.AppendText 方法 创建一个 StreamWriter，它将 UTF-8 编码文本追加到现有文件。

public static StreamWriter AppendText(
string path
);

参数
path 
要向其中追加内容的文件的路径。 
返回值
一个 StreamWriter，它将 UTF-8 编码文本追加到现有文件。

将文本追加到文件

using System;
using System.IO;

class Test 
{
    public static void Main() 
    {
        string path = @"c:\temp\MyTest.txt";
        // This text is added only once to the file.
        if (!File.Exists(path)) 
        {
            // Create a file to write to.
            using (StreamWriter sw = File.CreateText(path)) 
            {
                sw.WriteLine("Hello");
                sw.WriteLine("And");
                sw.WriteLine("Welcome");
            }    
        }

        // This text is always added, making the file longer over time
        // if it is not deleted.
        using (StreamWriter sw = File.AppendText(path)) 
        {
            sw.WriteLine("This");
            sw.WriteLine("is Extra");
            sw.WriteLine("Text");
        }    

        // Open the file to read from.
        using (StreamReader sr = File.OpenText(path)) 
        {
            string s = "";
            while ((s = sr.ReadLine()) != null) 
            {
                Console.WriteLine(s);
            }
        }
    }
}



using System;
using System.IO;

class Test 
{
    
    public static void Main() 
    {
        FileInfo fi = new FileInfo(@"c:\temp\MyTest.txt");

        // This text is added only once to the file.
        if (!fi.Exists) 
        {
            //Create a file to write to.
            using (StreamWriter sw = fi.CreateText()) 
            {
                sw.WriteLine("Hello");
                sw.WriteLine("And");
                sw.WriteLine("Welcome");
            }    
        }

        // This text will always be added, making the file longer over time
        // if it is not deleted.
        using (StreamWriter sw = fi.AppendText()) 
        {
            sw.WriteLine("This");
            sw.WriteLine("is Extra");
            sw.WriteLine("Text");
        }    

        //Open the file to read from.
        using (StreamReader sr = fi.OpenText()) 
        {
            string s = "";
            while ((s = sr.ReadLine()) != null) 
            {
                Console.WriteLine(s);
            }
        }
    }
}
=====================================
using System;
using System.IO;

public class AppendTextTest 
{
    public static void Main() 
    {
        // Create a reference to a file, which might or might not exist.
        // If it does not exist, it is not yet created.
        FileInfo fi = new FileInfo("temp.txt");
        // Create a writer, ready to add entries to the file.
        StreamWriter sw = fi.AppendText();
        sw.WriteLine("Add as many lines as you like...");
        sw.WriteLine("Add another line to the output...");
        sw.Flush();
        sw.Close();
        // Get the information out of the file and display it.
        // Remember that the file might have other lines if it already existed.
        StreamReader sr = new StreamReader(fi.OpenRead());
        while (sr.Peek() != -1)
            Console.WriteLine( sr.ReadLine() );
    }
}
 
移动一个文件

将指定文件移到新位置，并提供指定新文件名的选项。

public static void Move(
string sourceFileName,
string destFileName
);
参数
sourceFileName 
要移动的文件的名称。 
destFileName 
文件的新路径。 

using System;
using System.IO;

class Test 
{
    public static void Main() 
    {
        string path = @"c:\temp\MyTest.txt";
        string path2 = path + "temp";
        try 
        {
            if (!File.Exists(path)) 
            {
                // This statement ensures that the file is created,
                // but the handle is not kept.
                using (FileStream fs = File.Create(path)) {}
            }

            // Ensure that the target does not exist.
            if (File.Exists(path2))    
            File.Delete(path2);

            // Move the file.
            File.Move(path, path2);
            Console.WriteLine("{0} was moved to {1}.", path, path2);

            // See if the original exists now.
            if (File.Exists(path)) 
            {
                Console.WriteLine("The original file still exists, which is unexpected.");
            } 
            else 
            {
                Console.WriteLine("The original file no longer exists, which is expected.");
            }            

        } 
        catch (Exception e) 
        {
            Console.WriteLine("The process failed: {0}", e.ToString());
        }
    }
}




将一个文件移动至另一位置并重命名该文件。

using System;
using System.IO;

public class MoveToTest 
{
    public static void Main() 
    {
        // Create a reference to a file, which might or might not exist.
        // If it does not exist, it is not yet created.
        FileInfo fi = new FileInfo("temp.txt");
        // Create a writer, ready to add entries to the file.
        StreamWriter sw = fi.AppendText();
        sw.WriteLine("Add as many lines as you like...");
        sw.WriteLine("Add another line to the output...");
        sw.Flush();
        sw.Close();
        // Ensure that the target file does not exist, since this is disallowed.
        if (File.Exists("newTemp.txt"))
            File.Delete("newTemp.txt");
        // Move this file to another file.
        fi.MoveTo("newTemp.txt");
        // Get the information out of the new file and display it.
        StreamReader sr = new StreamReader( fi.OpenRead() );
        Console.WriteLine("{0}This is the information in the new file '{1}':", Environment.NewLine, fi.Name);
        while (sr.Peek() != -1)
            Console.WriteLine( sr.ReadLine() );
    }
}