C# Signature:

[DllImport("user32.dll")]
static extern bool GetClientRect(IntPtr hWnd, out RECT lpRect);

VB Signature:

<DllImport("user32.dll", CharSet:=CharSet.Auto)> _
Private Shared Function GetClientRect(ByVal hWnd As System.IntPtr, _
   ByRef lpRECT As RECT) As Integer
    ' Leave function empty    
End Function

User-Defined Types:

RECT

Notes:

The Win32 RECT structure is not compatible with the .NET System.Drawing.Rectangle structure.

Tips & Tricks:

It's easier to use with a tiny wrapper method.

public static RECT GetClientRect(IntPtr hWnd)
{
    RECT result = new RECT();
    GetClientRect(hWnd, out result);
    return result;
}

Sample Code:

Please add some!

Alternative Managed API:

Do you know one? Please contribute it!

ClientRectangle property of the System.Windows.Forms.Control class