title: Extension method and Explicit casting
body: <p>I'm using class from some assembly(source code is not available), so it is not possible to change their's code
  I need to add extension method for explicit cast operator, is there any way to achieve that? (I have tried to add as regular extension method, but without success)   </p>
  
  <pre><code> public static explicit operator MembershipUser(this MembershipUser membership, User user)
      {
          return new MembershipUser("SimplyMembershipProvider", user.UserName, user.UserId, user.Email, null, null, user.IsApproved, user.IsLocked,
              user.CreateDate, user.LastLoginDate, user.LastActivityDate, user.CreateDate, DateTime.MinValue);
      }
  </code></pre>
  
  <p>how can i solve this?</p>