Csharp/CSharp Tutorial/Attribute/Attributes Reflection

Load class method by Attribute

<source lang="csharp">using System; using System.Reflection; [AttributeUsage(AttributeTargets.Class)] public class ClassTarget : Attribute {

 public ClassTarget()
 {
 }

} [AttributeUsage(AttributeTargets.Method )] public class MethodTarget : Attribute {

 public MethodTarget()
 {
 }

} public class MyClass {

 [MethodTarget]
 public int MyMethod()
 {
   return 5;
 }

}

class MainClass {

 public static void Main(string[] args) 
 {
   ClassTarget rs;
   MethodTarget rm;
   Assembly a = Assembly.LoadFrom("MyClass");
   foreach(Type t in a.GetTypes())
   {
     rs = (ClassTarget) Attribute.GetCustomAttribute(t, typeof(ClassTarget));
     if(rs != null)
     {
       foreach(MethodInfo m in t.GetMethods())
       {
         rm = (MethodTarget) Attribute.GetCustomAttribute(m, typeof(MethodTarget));
         if(rm != null)
         {
           Object o = Activator.CreateInstance(t);
           Object[] aa = new Object[0];
           int i = (int) m.Invoke(o, aa);
         }
       }
     }
   }

 }

}</source>

Reflecting on Attributes

<source lang="csharp">//Code revised from //A Programmer"s Introduction to C# 2.0, Third Edition using System; using System.Reflection; [AttributeUsage(AttributeTargets.Class, AllowMultiple=true)] public class CodeReviewAttribute: System.Attribute {

   public CodeReviewAttribute(string reviewer, string date)
   {
       this.reviewer = reviewer;
       this.date = date;
   }
   public string Comment
   {
       get
       {
           return(comment);
       }
       set
       {
           comment = value;
       }
   }
   public string Date
   {
       get
       {
           return(date);
       }
   }
   public string Reviewer
   {
       get
       {
           return(reviewer);
       }
   }
   string reviewer;
   string date;
   string comment;

} [CodeReview("Name1", "01-12-2000", Comment="comment1")] [CodeReview("Name2", "01-01-2012", Comment="comment2")] class Complex { } class MainClass {

   public static void Main()
   {
       Type type = typeof(Complex);
       foreach (CodeReviewAttribute att in
       type.GetCustomAttributes(typeof(CodeReviewAttribute), false))
       {
           Console.WriteLine("Reviewer: {0}", att.Reviewer);
           Console.WriteLine("Date: {0}", att.Date);
           Console.WriteLine("Comment: {0}", att.rument);
       }
   }

}</source>

Reviewer: Name2
Date: 01-01-2012
Comment: comment2
Reviewer: Name1
Date: 01-12-2000
Comment: comment1

Retrieve Attribute by using reflection

<source lang="csharp">using System; using System.Reflection;

[AttributeUsage(AttributeTargets.All)] public class MyAttribute : Attribute {

 public string remark; 

 public MyAttribute(string comment) { 
   remark = comment; 
 } 

 public string Remark { 
   get { 
     return remark; 
   } 
 } 

}

[MyAttribute("This class uses an attribute.")] class UseAttrib { }

class MainClass {

 public static void Main() {  
   Type t = typeof(UseAttrib); 

   Console.Write("Attributes in " + t.Name + ": "); 

   object[] attribs = t.GetCustomAttributes(false);  
   foreach(object o in attribs) { 
     Console.WriteLine(o); 
   } 

   Console.Write("Remark: "); 

   // Retrieve the MyAttribute. 
   Type tRemAtt = typeof(MyAttribute); 
   MyAttribute ra = (MyAttribute) 
         Attribute.GetCustomAttribute(t, tRemAtt); 

   Console.WriteLine(ra.remark); 
 }  

}</source>

Attributes in UseAttrib: MyAttribute
Remark: This class uses an attribute.

Use Reflection to get the Attribute

<source lang="csharp">using System; [assembly:System.CLSCompliantAttribute(true)] [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct)] public class MyDescriptionAttribute : System.Attribute {

 private string description;
 public string Desc
 {
   get { return description; }
   set { description = value; }
 }
 public MyDescriptionAttribute() {}
 public MyDescriptionAttribute(string desc) 
 { description = desc;}

}

[MyDescriptionAttribute("Info")] public class MyClass {

   public MyClass()
   {
   }

}

public class MainClass {

 public static int Main(string[] args)
 {
   Type t = typeof(MyClass);
 
   // Get all attributes in the assembly.
   object[] customAtts = t.GetCustomAttributes(false);
 
   // List all info.
   Console.WriteLine("Value of MyDescriptionAttribute");
   foreach(MyDescriptionAttribute v in customAtts)
     Console.WriteLine("-> {0}\n", v.Desc);  
   return 0;
 }

}</source>

Value of MyDescriptionAttribute
-> Info

Use the GetCustomAttributes method

<source lang="csharp">using System; public class TrueFalseAttribute : Attribute {

 bool bWritten;
 public bool Written()
 {
   return bWritten;
 }
 public TrueFalseAttribute(bool Written)
 {
   bWritten = Written;
 }

} public class StringAttribute : Attribute {

 string sStage;
 public string Stage()
 {
   return sStage;
 }
 public StringAttribute(string Stage)
 {
   sStage = Stage;
 }

} [TrueFalseAttribute(true)] [StringAttribute("Coding")] public class Class1 { } class MainClass {

 public static void Main() 
 {
   Console.WriteLine("Class1 attributes: ");object[] aAttributes = Attribute.GetCustomAttributes(typeof(Class1));
   foreach (object attr in aAttributes)
   {
     Console.WriteLine(attr);
   }
 }

}</source>

Class1 attributes:
StringAttribute
TrueFalseAttribute