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

Type Summary

						public sealed class ObsoleteAttribute : Attribute
{
  // Constructors
     public ObsoleteAttribute ();
     public ObsoleteAttribute (string message);
     public ObsoleteAttribute (string message,
                               bool error);

  // Properties
     public bool IsError { get; }
     public string Message { get; }
}

BA You'll notice that this attribute is sealed. In general I am not a huge fan of sealing classes by default, but for custom attributes that you expect to be looked up at runtime it is a good idea to seal them, because then reflection can use a slightly faster code path to look them up.