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。

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。

Applications of Reflection

反射有以下应用：

它允许在运行时查看属性信息。
它允许检查程序集中各种类型的详细信息并实例化这些类型。
它允许后期绑定到方法和属性
它允许在运行时创建新的类型，然后使用这些类型执行一些任务。

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。

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：

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

以下程序演示了这一点 −

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：

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。

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 −

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: