gistfile1.cs

  // SafeHandle.Close()...

  using System;
  using System.Runtime.InteropServices;

  class MySafeHandle : SafeHandle {

    public MySafeHandle ()
      : base (IntPtr.Zero, ownsHandle:false)
    {
    }

    protected override bool ReleaseHandle ()
    {
      return true;
    }

    public override bool IsInvalid {
      get {return base.handle == IntPtr.Zero;}
    }
  }

  class App {
    public static void Main ()
    {
      var h = new MySafeHandle ();
      h.Close ();
      h.Close ();
    }
  }