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:

Please note that this does NOT work properly with Windows Vista Aero, and reports faulty values.

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