Proper way of hosting an external window inside WPF using HwndHost

Right before calling 'SetParent' do this:

public const int GWL_STYLE = (-16);public const int WS_CHILD = 0x40000000;SetWindowLong(this.ChildHandle, GWL_STYLE, WS_CHILD);

Looking at the docs:

1. You need a Win32 control compiled for CLI
2. You must create the control inside your WPF app.

It seems not possibile to "attach" an already running Window from another process inside a WPF app.

How do you get the handle you pass to your HwndHostEx constructor?

To add clarification, here's the final code for hosting a process in a WPF app, taken from Jose's answer:

    class HwndHostEx : HwndHost    {        [DllImport("user32.dll")]        static extern IntPtr SetParent(IntPtr hWndChild, IntPtr hWndNewParent);        [DllImport("user32.dll")]        static extern int SetWindowLong(IntPtr hWnd, int nIndex, UInt32 dwNewLong);        private IntPtr ChildHandle = IntPtr.Zero;        public HwndHostEx(IntPtr handle)        {            this.ChildHandle = handle;        }        protected override System.Runtime.InteropServices.HandleRef BuildWindowCore(System.Runtime.InteropServices.HandleRef hwndParent)        {            HandleRef href = new HandleRef();            if (ChildHandle != IntPtr.Zero)            {                const int GWL_STYLE = (-16);                const int WS_CHILD = 0x40000000;                SetWindowLong(this.ChildHandle, GWL_STYLE, WS_CHILD);                SetParent(this.ChildHandle, hwndParent.Handle);                href = new HandleRef(this, this.ChildHandle);            }            return href;        }        protected override void DestroyWindowCore(System.Runtime.InteropServices.HandleRef hwnd)        {        }    }// to create an instance:var processName = "Whatever.exe";var process = System.Diagnostics.Process.GetProcesses() .FirstOrDefault(item => item.ProcessName.ToLowerInvariant() == processName && item.MainWindowHandle != IntPtr.Zero);var handle = process?.MainWindowHandle;if(handle != null){    var host = new HwndHostEx(handle.Value);    YourControl.Grid.Children.Add(host);}