.NET Framework Standard Library Annotated Reference, Volume 1: Base Class Library and Extended Numerics Library

Example

#undef A

using System;
using System.Diagnostics;

namespace Samples
{
  public class MyClass
  {
    [ConditionalAttribute("A")]
    public static void Display()
    {
      Console.WriteLine("A was defined");
    }
  }
  public class ConditionalAttributeSample
  {
    public static void Main()
    {
      MyClass.Display();
      Console.WriteLine("Done");
    }
  }
}

The output is

Done