Csharp 简明教程

C

Reflection 对象用于在运行时获取类型信息。提供访问正在运行的程序的元数据的类位于 System.Reflection 命名空间中。

Reflection objects are used for obtaining type information at runtime. The classes that give access to the metadata of a running program are in the System.Reflection namespace.

System.Reflection 命名空间中包含的类允许您获得有关应用程序的信息，并动态地向应用程序添加类型、值和对象。

The System.Reflection namespace contains classes that allow you to obtain information about the application and to dynamically add types, values, and objects to the application.

Applications of Reflection

反射有以下应用：

Reflection has the following applications −

It allows view attribute information at runtime.
It allows examining various types in an assembly and instantiate these types.
It allows late binding to methods and properties
It allows creating new types at runtime and then performs some tasks using those types.

Viewing Metadata

我们在前面的章节中提到可以使用反射来查看属性信息。

We have mentioned in the preceding chapter that using reflection you can view the attribute information.

需要初始化 System.Reflection 类的 MemberInfo 对象来发现与类关联的属性。要做到这一点，您定义 target 类的对象，如下所示：

The MemberInfo object of the System.Reflection class needs to be initialized for discovering the attributes associated with a class. To do this, you define an object of the target class, as −

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

以下程序演示了这一点 −

The following program demonstrates this −

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();
      }
   }
}

编译并运行之后，将显示附加到 MyClass 类的自定义属性的名称：

When it is compiled and run, it displays the name of the custom attributes attached to the class MyClass −

HelpAttribute

Example

在这个示例中，我们使用了上一章节创建的 DebugInfo 属性，并使用反射读取 Rectangle 类中的元数据。

In this example, we use the DeBugInfo attribute created in the previous chapter and use reflection to read metadata in the Rectangle class.

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();
      }
   }
}

编译并执行上述代码后，将产生以下结果 −

When the above code is compiled and executed, it produces the following result −

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: