Specify the namespace for doing P/Invoke stuff i.e. calling Win32 API functions from managed code.

using System.Runtime.InteropServices;

GetWindowThreadProcessId Win32 function retrieves the identifiers of the process and thread that created the specified window.

Here is how we declare GetWindowThreadProcessId for use in managed code (c#).

[DllImport("coredll.dll")]
private static extern uint GetWindowThreadProcessId(IntPtr hWnd, out uint lpdwProcessId);

Description:

• hWnd is the window handle
• lpdwProcessId stores the process identifier after the method returns
• return value of the function is the id of the thread that created the window

Calling GetWindowThreadProcessId via P/Invoke:

// Set the hWnd value below with window handle of your interest
IntPtr hWnd = this.Handle;
uint processid = 0;
uint threadid = GetWindowThreadProcessId((IntPtr)hWnd, out processid);

And there you go….