C# Signature:

[DllImport("user32", ExactSpelling = true, SetLastError = true)]
internal static extern int MapWindowPoints(IntPtr hWndFrom, IntPtr hWndTo, [In, Out] ref NativeMethods.RECT rect, [MarshalAs(UnmanagedType.U4)] int cPoints);

[DllImport("user32", ExactSpelling = true, SetLastError = true)]
internal static extern int MapWindowPoints(IntPtr hWndFrom, IntPtr hWndTo, [In, Out] ref System.Drawing.Point pt, [MarshalAs(UnmanagedType.U4)] int cPoints);

VB.Net:

Declare Auto Function MapWindowPoints Lib "user32" (ByVal hwndFrom As IntPtr, ByVal hwndTo As IntPtr, ByRef lppt As drawing.Point, ByVal cPoints As Integer) As Integer

User-Defined Types:

RECT

Notes:

None.

Tips & Tricks:

Please add some!

Sample Code:

internal static class UnsafeNativeMethods
{
   [DllImport("user32", ExactSpelling = true, SetLastError = true)]
   internal static extern int MapWindowPoints(IntPtr hWndFrom, IntPtr hWndTo, [In, Out] ref NativeMethods.RECT rect, [MarshalAs(UnmanagedType.U4)] int cPoints);
   [DllImport("user32", ExactSpelling = true, SetLastError = true)]
   internal static extern int MapWindowPoints(IntPtr hWndFrom, IntPtr hWndTo, [In, Out] ref System.Drawing.Point pt, [MarshalAs(UnmanagedType.U4)] int cPoints);

   public static System.Drawing.Point MapPointToClient(this System.Windows.Forms.IWin32Window ctrl, System.Drawing.Point pt)
   {
     return MapPoint(null, pt, ctrl);
   }

   public static System.Drawing.Point MapPoint(this System.Windows.Forms.IWin32Window ctrl, System.Drawing.Point pt, System.Windows.Forms.IWin32Window newWin = null)
   {
     MapWindowPoints(ctrl == null ? IntPtr.Zero : ctrl.Handle, newWin == null ? IntPtr.Zero : newWin.Handle, ref pt, 1);
     return pt;
   }

   public static System.Drawing.Rectangle MapRectangle(this System.Windows.Forms.IWin32Window ctrl, System.Drawing.Rectangle rectangle, System.Windows.Forms.IWin32Window newWin = null)
   {
     NativeMethods.RECT ir = rectangle;
     MapWindowPoints(ctrl == null ? IntPtr.Zero : ctrl.Handle, newWin == null ? IntPtr.Zero : newWin.Handle, ref ir, 2);
     return ir;
   }

   public static System.Drawing.Point[] MapPoints(this System.Windows.Forms.IWin32Window ctrl, System.Drawing.Point[] points, System.Windows.Forms.IWin32Window newWin = null)
   {
     System.Drawing.Point[] pts = new System.Drawing.Point[points.Length];
     points.CopyTo(pts, 0);
     for (int i = 0; i < pts.Length; i++)
       MapWindowPoints(ctrl == null ? IntPtr.Zero : ctrl.Handle, newWin == null ? IntPtr.Zero : newWin.Handle, ref pts[i], 1);
     return pts;
   }
}

Alternative Managed API:

Do you know one? Please contribute it!