using System.Runtime.InteropServices;

[DllImport("user32.dll")]
private static extern int GetWindowRect(IntPtr hwnd,out Rect lpRect);

public struct Rect
{
public int Left;
public int Top;
public int Right;
public int Bottom;
}
演示代碼：
IntPtr hwnd = FindWindow("", "計算器");
Rect rect = new Rect();
GetWindowRect(hwnd, out lpRect);