[StructLayout(LayoutKind.Sequential)]
struct BP_PAINTPARAMS : IDisposable
[StructLayout(LayoutKind.Explicit)]
struct BP_PAINTPARAMS
{
    private int cbSize;
    public BPPF Flags;
    private IntPtr prcExclude;
    private IntPtr pBlendFunction;
    public int cbSize;
    public BPPF dwFlags;
    public IntPtr prcExclude;
    public IntPtr pBlendFunction;
}

    public BP_PAINTPARAMS(BPPF flags)
    {
      cbSize = Marshal.SizeOf(typeof(BufferedPaintParams));
      Flags = flags;
      prcExclude = pBlendFunction = IntPtr.Zero;
    }

    public Rectangle Exclude
    {
      get { return (Rectangle)Marshal.PtrToStructure(prcExclude, typeof(RECT)); }
      set
      {
    if (prcExclude == IntPtr.Zero)
      prcExclude = Marshal.AllocHGlobal(Marshal.SizeOf(typeof(RECT)));
    Marshal.StructureToPtr(value, prcExclude, false);
      }
    }

    public BlendFunction BlendFunction
    {
      get { return (BlendFunction)Marshal.PtrToStructure(pBlendFunction, typeof(BlendFunction)); }
      set
      {
     if (pBlendFunction == IntPtr.Zero)
        pBlendFunction = Marshal.AllocHGlobal(Marshal.SizeOf(typeof(BlendFunction)));
     Marshal.StructureToPtr(value, pBlendFunction, false);
      }
    }

    public void Dispose()
    {
      if (prcExclude != IntPtr.Zero) Marshal.FreeHGlobal(prcExclude);
      if (pBlendFunction != IntPtr.Zero) Marshal.FreeHGlobal(pBlendFunction);
    }
}