C# - 反射

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

System.Reflection 命名空间包含允许您获取有关应用程序的信息并向应用程序动态添加类型、值和对象的类。

反射的应用

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的各种类型并实例化这些类型。

• 它允许后期绑定到方法和属性

• 它允许在运行时创建新类型，然后使用这些类型执行一些任务。

查看元数据

上一章我们已经提到，使用反射可以查看属性信息。

会员信息 System.Reflection 的对象 需要初始化类以发现与类关联的属性。为此，您将目标类的对象定义为 -

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();
      }
   }
}

当它编译并运行时，它会显示附加到类 MyClass 的自定义属性的名称 -

HelpAttribute

示例

在本例中，我们使用 DeBugInfo 上一章创建的属性，并使用反射读取 Rectangle 中的元数据 类。

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();
      }
   }
}

当上面的代码编译并执行时，它会产生以下结果 -

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: