C#反射

Reflection物件用於在執行時獲取型別資訊。可以存取正在執行的程式的後設資料的類在System.Reflection名稱空間中。

System.Reflection名稱空間包含允許獲取有關應用程式的資訊的類，並向應用程式動態新增型別，值和物件。

反射有以下應用：

• 它允許在執行時檢視屬性資訊。
• 它允許檢查裝配中的各種型別並範例化這些型別。
• 它允許後期系結到方法和屬性。
• 它允許在執行時建立新的型別，可使用這些型別執行一些任務。

檢視後設資料

我們在上一章中提到使用反射可以檢視屬性資訊。

需要初始化System.Reflection類的MemberInfo物件，以發現與類相關聯的屬性。可將目標類的物件定義為：

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

以下程式演示如下：

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();
      }
   }
}

當它被編譯並執行時，它顯示附加到類MyClass的自定義屬性的名稱：

HelpAttribute

例子

在這個例子中，我們使用上一章建立的DeBugInfo屬性，並使用反射來讀取Rectangle類中的後設資料。

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, "Sukida", "12/8/2017", Message = "Return type mismatch")]
   [DeBugInfo(49, "Maxsu", "10/10/2017", 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, "Sukida", "19/10/2017", Message = "Return type mismatch")]
      public double GetArea()
      {
         return length * width;
      }
      [DeBugInfo(56, "Sukida", "19/10/2017")]
      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(14.5, 17.5);
         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();
      }
   }
}

當上述程式碼被編譯並執行時，它產生以下結果：

Length: 14.5
Width: 17.5
Area: 253.75
Bug no: 45
Developer: Sukida
Last Reviewed: 12/8/2017
Remarks: Return type mismatch
Bug no: 49
Developer: Maxsu
Last Reviewed: 10/10/2017
Remarks: Unused variable
Bug no: 55, for Method: GetArea
Developer: Sukida
Last Reviewed: 19/10/2017
Remarks: Return type mismatch
Bug no: 56, for Method: Display
Developer: Sukida
Last Reviewed: 19/10/2017
Remarks: