C# Signature:

[DllImport("user32.dll")]
static extern bool GetWindowRect(IntPtr hWnd, out 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

Notes:

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

Tips & Tricks:

Please add some!

Sample Code:

        [DllImport("user32.dll")]
        static extern bool GetWindowRect(IntPtr hWnd, out 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( this.Handle, out rct ))
            {
                MessageBox.Show("ERROR");
            }
            else
            {
                MessageBox.Show( rct.ToString() );
            }
        }

Alternative Managed API:

Do you know one? Please contribute it!