C# Signature:

[DllImport("user32.dll")]
[return: MarshalAs(UnmanagedType.Bool)]
public static extern bool GetWindowRect(HandleRef hwnd, out RECT lpRect);

VB.NET Signature:

<DllImport("user32.dll")> _
Private Shared Function GetWindowRect(ByVal hWnd As HandleRef, ByRef lpRect As RECT) As Boolean
End Function

VB Signature

Public Declare Function GetWindowRect Lib "user32.dll" _
          (ByVal hWnd As Long, _
           prmtypRECT As RECT) As Long

User-Defined Types:

RECT

Notes:

The Win32 RECT is not binary compatible with System.Drawing.Rectangle.

Tips & Tricks:

For .NET CF you may need to replace user32.dll with coredll.dll

Please add some!

Sample Code:

        [DllImport("user32.dll")]
        [return: MarshalAs(UnmanagedType.Bool)]
        static extern bool GetWindowRect(HandleRef hWnd, out RECT lpRect);

        [StructLayout(LayoutKind.Sequential)]
        public struct RECT
        {
            public int Left;        // x position of upper-left corner
            public int Top;         // y position of upper-left corner
            public int Right;       // x position of lower-right corner
            public int Bottom;      // y position of lower-right corner
        }

        Rectangle myRect = new Rectangle();

        private void button1_Click(object sender, System.EventArgs e)
        {
            RECT rct;

            if(!GetWindowRect(new HandleRef(this, this.Handle), out rct ))
            {
                MessageBox.Show("ERROR");
                return;
            }
            MessageBox.Show( rct.ToString() );

            myRect.X = rct.Left;
            myRect.Y = rct.Top;
            myRect.Width = rct.Right - rct.Left + 1;
            myRect.Height = rct.Bottom - rct.Top + 1;
        }

Alternative Managed API:

Omnicoder's Managed Windows API: http://www.fileden.com/files/2008/4/5/1852663/ManWinAPI.zip