How to write a custom attribute in .Net that do someting on runtime before/after a method?

you can use a conditional attribute.

Marks a conditional method whose execution depends on a specified preprocessing identifier.

sample:

#define TRACE
using System;
using System.Diagnostics;
public class Trace
{
   [Conditional("TRACE")] public static void Msg(string msg)
   {
      Console.WriteLine(msg);
   }
}
class Test
{
   static void A( )
   {
      Trace.Msg("Now in A.");
      B( );
   }
   static void B( )
   {
      Trace.Msg("Now in B.");
   }
   public static void Main( )
   {
      Trace.Msg("Now in Main.");
      A( );
      Console.WriteLine("Done.");
   }
}
-------------------------

Output:
Now in Main.
Now in A.
Now in B.
Done.

If you compile the sample with first line omitted (or changed to #undef DEBUG), the output will be: Done

You might be able to implement what you're looking for using RealProxy.

You still need to inherit your class from a specific class using this technique, but luckily, you need to inherit from MarshalByRefObject, which is a lot more light-weight then ContextBoundObject.

The following article in CodeProject should get you started:

Using Attributes and RealProxy to perform property level processing 

Let us know if you get it working, we all want to [trace]

i think this article can help you

 Decouple Components by Injecting Custom Services into Your Object's Interception Chain

is professional term  call aspect

http://msdn.microsoft.com/msdnmag/issues/03/03/ContextsinNET/default.aspx

Have a look in