Tuesday, 15 April 2008

Hosting Windows Forms Controls in WPF (Threading)

The threading model in WPF differs somewhat from the conventional Win32 application model.  WPF objects belong to the thread that created them, and cannot be accessed directly by any other thread.  As most objects in WPF inherit from the DispatcherObject class, we can get at an objects Dispatcher property to update any properties on the object from another thread.

So how do we allow access to our object from a Windows form control?  Well, the answer is relatively simple.  Windows forms controls MUST be embedded inside a WindowsFormsHost object (A WPF wrapper for Windows Forms controls).  As the WindowsFormsHost object is  a WPF object, it therefore has a Dispatcher property which can be accessed.

The below example is not a full code sample, what I am trying to highlight here is the use of the Dispatcher to gain access to a Windows Form Control's thread.



Below is a simple Windows Forms User control, which is hosting the Windows Media Player ActiveX control.

Create a new Windows Forms Library project within your WPF solution.  Here we will create our custom Windows Forms Control.


using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Drawing;
using System.Data;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using AxWMPLib;

namespace WmpAxLib
public partial class WmpAxControl : UserControl
public AxWindowsMediaPlayer MediaPlayer
return this.axWindowsMediaPlayer1;
public WmpAxControl()

As you can see, I have exposed the AxWindowsMediaPlayer object as a property Called MediaPlayer.  This is so we can access the Media Player directly in our example from the WPF code.  For now, we will not do anything else with this control but it is worth noting that we could expose events on this object for any Media Player events (for example we could just expose the Ctlcontrols property directly for controlling Play, Stop etc.

Now that we have our simple media player, we can host it inside our WPF application. 

<Window x:Class="MediaPlayer.MainPlayer"
="clr-namespace:WmpAxLib;assembly=WmpAxLib" Loaded="Window_Loaded"

The XAML markup shown above has references to the Windows.Forms.Integration and the Windows.Forms assemblies.  These assemblies are needed in order to host a Windows Form control within your WPF application.  There is also a reference to the Windows Forms Library assembly we created earlier, called AxWMPLib.

Now, personally I prefer to programmatically create a WindowsFormsHost control in the codebehind as a private field.  It is perfectly valid to create a control in your XAML if you wish, either way will work fine.

The next step is to add your Windows Forms control to the newly created WindowsFormsHost control.  To do this, in XAML, you simply embed your control inside the WindowsFormsControl control.  This example will show how to do it in codebehind.

1. Create two private fields ;

WmpAxLib.WmpAxControl _axWmp = new WmpAxLib.WmpAxControl();
WindowsFormsHost _host
= null;

2. In the Window_Loaded event handler, initialise the _host and _axWmp objects and add the _host control to the Windows Children collection;

_host = new System.Windows.Forms.Integration.WindowsFormsHost();
= 0;
= 0;
= _axWmp;

3. Setup a handler for the _axWmp controls "OpenStateChanged" event.

_axWmp.MediaPlayer.OpenStateChange += new AxWMPLib._WMPOCXEvents_OpenStateChangeEventHandler(MediaPlayer_OpenStateChange);

4. In the newly created handler, we want to set the _host width and height to full screen.

void MediaPlayer_OpenStateChange(object sender, AxWMPLib._WMPOCXEvents_OpenStateChangeEvent e)
string name = string.Empty;

new ThreadStart(delegate
= new Thickness(0, -90, 0, 0);
= this.Height;
= this.Width;
= string.Format("Now playing {0}", _axWmp.MediaPlayer.currentMedia.name);

In order to access the ActiveX control which is hosted inside our Windows Forms Control, we need to access its thread.  As the thread is not the same as the current UI Thread, we need to ask the dispatcher object to post a message to the control.  We do this by creating a new ThreadStart() object and rather than pass a delegate , we use an anonymous delegate to do the work.  We are also settings the DispatcherPriority to "Send" which tells the dispatcher to process BEFORE other asynchronous operations. 

The delegate then sets the _host's Margin, Height and Width properties.  It also accesses the MediaPlayer property we set on our Windows control, in order to get the name of the media we are playing and sets it to our locally declared string.

Further Reading