Reflection

Reflection of the C#

The purpose of reflection

Reflection has the following uses:

• It allows you to view attribute information at runtime.
• It allows you to review the various types in the collection and instantiate them.
• It allows methods and properties (property) to be delayed binding.
• It allows you to create new types at runtime and then use those types to perform some tasks.

View metadata

As we mentioned in the previous section, you can view attribute information using Reflection.

The MemberInfo object of the System.Reflection class needs to be initialized to discover the attributes associated with the class. To do this, you can define an object for the target class, as follows:

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

The following program demonstrates this:

using System;
[AttributeUsage(AttributeTargets.All)]
public class HelpAttribute : System.Attribute
{
    public readonly string Url;
    public string Topic  // Topic 是一个命名（named）参数
    {
        get
        {
            return topic;
        }
        set
        {
            topic = value;
        }
    }
    public HelpAttribute(string url)  // url 是一个定位（positional）参数
    {
        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();
        }
    }
}

When the above code is compiled and executed, it displays custom properties attached to the class MyClass:

HelpAttribute

In this example, we'll use the DeBugInfo property created in the previous chapter and use Reflection to read metadata in the Rectangle class.

using System;
using System.Reflection;
namespace BugFixApplication
{
   // 一个自定义属性 BugFix 被赋给类及其成员
   [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
   {
      // 成员变量
      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);
         // 遍历 Rectangle 类的属性
         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);
            }
         }
         
         // 遍历方法属性
         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();
      }
   }
}

When the above code is compiled and executed, it produces the following results:

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: