C# Signature:

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

VB Signature:

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

Public Overloads Declare Function GetWindowRect Lib "User32" Alias "GetWindowRect" (ByVal hWnd As IntPtr, ByRef lpRect As RECT) As Int32
Public Overloads Shared Function GetWindowRect(ByVal hWnd As IntPtr) As System.Drawing.Rectangle
     Dim r As New RECT
     GetWindowRect(hWnd, r)
     Return r.ToRectangle
End Function
Public Overloads Shared Function GetWindowRect(ByVal hWnd As Int32) As System.Drawing.Rectangle
     Return GetWindowRect(New IntPtr(hWnd))
End Function

User-Defined Types:

RECT

A simpler approach is:

C# Signature:

[DllImport("user32.dll")]
private static extern bool GetWindowRect(IntPtr hWnd, ref Rectangle 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(IntPtr hWnd, ref RECT lpRect);

        [StructLayout(LayoutKind.Sequential)]
        public struct RECT
        {
            public int Left;    
            public int Top;    
            public int Right;    
            public int Bottom;
        }

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

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

Alternative Managed API:

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