反射對象用於在運行時獲得的類型信息。用來接入正在運行的程序的元數據類在System.Reflection命名空間。

System.Reflection命名空間包含的類允許獲取有關應用程序，並動態地添加類型，值和對象的應用程序的信息。

使用反射

反射有以下用途：

• 它允許在運行時查看屬性信息。

• 它允許檢查各類在裝配和實例化這些類型。

• 它允許後期綁定方法和屬性

• 它允許創建新類型的運行時，然後執行使用這些類型的一些任務。

查看元數據

我們曾經提過反射，可以查看屬性信息前一章。

用於發現使用類相關的屬性進行初始化System.Reflection類需要的MemberInfo對象。要做到這一點，定義目標類的對象，如：

System.Reflection.MemberInfo info = typeof(MyClass);

下麵的程序說明了這一點：

using System;

[AttributeUsage(AttributeTargets.All)]
public class HelpAttribute : System.Attribute
{
   public readonly string Url;

   public string Topic  // Topic is a named parameter
   {
      get
      {
         return topic;
      }
      set
      {

         topic = value;
      }
   }

   public HelpAttribute(string url)  // url is a positional parameter
   {
      this.Url = url;
   }

   private string topic;
}
[HelpAttribute("Information on the class MyClass")]
class MyClass
{
}

namespace AttributeAppl
{
   class Program
   {
      static void Main(string[] args)
      {
         System.Reflection.MemberInfo info = typeof(MyClass);
         object[] attributes = info.GetCustomAttributes(true);
         for (int i = 0; i < attributes.Length; i++)
         {
            System.Console.WriteLine(attributes[i]);
         }
         Console.ReadKey();

      }
   }
}

當編譯和運行，它顯示的自定義屬性附加到類名 MyClass:

HelpAttribute

例子

在這個例子中，我們將使用在前麵的章節，並使用反射創建DeBugInfo屬性讀取元數據在Rectangle類中。

using System;
using System.Reflection;
namespace BugFixApplication
{
   //a custom attribute BugFix to be 
	//assigned to a class and its members
   [AttributeUsage(AttributeTargets.Class |
   AttributeTargets.Constructor |
   AttributeTargets.Field |
   AttributeTargets.Method |
   AttributeTargets.Property,
   AllowMultiple = true)]

   public class DeBugInfo : System.Attribute
   {
      private int bugNo;
      private string developer;
      private string lastReview;
      public string message;

      public DeBugInfo(int bg, string dev, string d)
      {
         this.bugNo = bg;
         this.developer = dev;
         this.lastReview = d;
      }

      public int BugNo
      {
         get
         {
            return bugNo;
         }
      }
      public string Developer
      {
         get
         {
            return developer;
         }
      }
      public string LastReview
      {
         get
         {
            return lastReview;
         }
      }
      public string Message
      {
         get
         {
            return message;
         }
         set
         {
            message = value;
         }
      }
   }
   [DeBugInfo(45, "Zara Ali", "12/8/2012",
	Message = "Return type mismatch")]
   [DeBugInfo(49, "Nuha Ali", "10/10/2012",
	Message = "Unused variable")]
   class Rectangle
   {
      //member variables
      protected double length;
      protected double width;
      public Rectangle(double l, double w)
      {
         length = l;
         width = w;
      }
      [DeBugInfo(55, "Zara Ali", "19/10/2012",
	   Message = "Return type mismatch")]
      public double GetArea()
      {
         return length * width;
      }
      [DeBugInfo(56, "Zara Ali", "19/10/2012")]
      public void Display()
      {
         Console.WriteLine("Length: {0}", length);
         Console.WriteLine("Width: {0}", width);
         Console.WriteLine("Area: {0}", GetArea());
      }
   }//end class Rectangle  
   
   class ExecuteRectangle
   {
      static void Main(string[] args)
      {
         Rectangle r = new Rectangle(4.5, 7.5);
         r.Display();
         Type type = typeof(Rectangle);
         //iterating through the attribtues of the Rectangle class
         foreach (Object attributes in type.GetCustomAttributes(false))
         {
            DeBugInfo dbi = (DeBugInfo)attributes;
            if (null != dbi)
            {
               Console.WriteLine("Bug no: {0}", dbi.BugNo);
               Console.WriteLine("Developer: {0}", dbi.Developer);
               Console.WriteLine("Last Reviewed: {0}",
					dbi.LastReview);
               Console.WriteLine("Remarks: {0}", dbi.Message);
            }
         }
         
         //iterating through the method attribtues
         foreach (MethodInfo m in type.GetMethods())
         {
            foreach (Attribute a in m.GetCustomAttributes(true))
            {
               DeBugInfo dbi = (DeBugInfo)a;
               if (null != dbi)
               {
                  Console.WriteLine("Bug no: {0}, for Method: {1}",
						dbi.BugNo, m.Name);
                  Console.WriteLine("Developer: {0}", dbi.Developer);
                  Console.WriteLine("Last Reviewed: {0}",
						dbi.LastReview);
                  Console.WriteLine("Remarks: {0}", dbi.Message);
               }
            }
         }
         Console.ReadLine();
      }
   }
}

當上述代碼被編譯和執行時，它產生了以下結果：

Length: 4.5
Width: 7.5
Area: 33.75
Bug No: 49
Developer: Nuha Ali
Last Reviewed: 10/10/2012
Remarks: Unused variable
Bug No: 45
Developer: Zara Ali
Last Reviewed: 12/8/2012
Remarks: Return type mismatch
Bug No: 55, for Method: GetArea
Developer: Zara Ali
Last Reviewed: 19/10/2012
Remarks: Return type mismatch
Bug No: 56, for Method: Display
Developer: Zara Ali
Last Reviewed: 19/10/2012
Remarks: