Walkthrough: Hosting a Win32 Control in WPF

 

Windows Presentation Foundation (WPF) provides a rich environment for creating applications. However, when you have a substantial investment in Win32 code, it may be more effective to reuse at least some of that code in your WPF application rather than rewrite it completely. WPF provides a straightforward mechanism for hosting a Win32 window, on a WPF page.

This topic walks you through an application, Hosting a Win32 ListBox Control in WPF Sample, that hosts a Win32 list box control. This general procedure can be extended to hosting any Win32 window.

This topic assumes a basic familiarity with both WPF and Win32 programming. For a basic introduction to WPF programming, see Getting Started (WPF). For an introduction to Win32 programming, you should reference any of the numerous books on the subject, in particular Programming Windows by Charles Petzold.

Because the sample that accompanies this topic is implemented in C#, it makes use of Platform Invocation Services (PInvoke) to access the Win32 API. Some familiarity with PInvoke is helpful but not essential.

System_CAPS_noteNote

This topic includes a number of code examples from the associated sample. However, for readability, it does not include the complete sample code. You can obtain or view complete code from Hosting a Win32 ListBox Control in WPF Sample.

This section outlines the basic procedure for hosting a Win32 window on a WPF page. The remaining sections go through the details of each step.

The basic hosting procedure is:

  1. Implement a WPF page to host the window. One technique is to create a Border element to reserve a section of the page for the hosted window.

  2. Implement a class to host the control that inherits from HwndHost.

  3. In that class, override the HwndHost class member BuildWindowCore.

  4. Create the hosted window as a child of the window that contains the WPF page. Although conventional WPF programming does not need to explicitly make use of it, the hosting page is a window with a handle (HWND). You receive the page HWND through the hwndParent parameter of the BuildWindowCore method. The hosted window should be created as a child of this HWND.

  5. Once you have created the host window, return the HWND of the hosted window. If you want to host one or more Win32 controls, you typically create a host window as a child of the HWND and make the controls children of that host window. Wrapping the controls in a host window provides a simple way for your WPF page to receive notifications from the controls, which deals with some particular Win32 issues with notifications across the HWND boundary.

  6. Handle selected messages sent to the host window, such as notifications from child controls. There are two ways to do this.

    • If you prefer to handle messages in your hosting class, override the WndProc method of the HwndHost class.

    • If you prefer to have the WPF handle the messages, handle the HwndHost class MessageHook event in your code-behind. This event occurs for every message that is received by the hosted window. If you choose this option, you must still override WndProc, but you only need a minimal implementation.

  7. Override the DestroyWindowCore and WndProc methods of HwndHost. You must override these methods to satisfy the HwndHost contract, but you may only need to provide a minimal implementation.

  8. In your code-behind file, create an instance of the control hosting class and make it a child of the Border element that is intended to host the window.

  9. Communicate with the hosted window by sending it Microsoft Windows messages and handling messages from its child windows, such as notifications sent by controls.

The layout for the WPF page that hosts the ListBox Control consists of two regions. The left side of the page hosts several WPF controls that provide a user interface (UI) that allows you to manipulate the Win32 control. The upper right corner of the page has a square region for the hosted ListBox Control.

The code to implement this layout is quite simple. The root element is a DockPanel that has two child elements. The first is a Border element that hosts the ListBox Control. It occupies a 200x200 square in the upper right corner of the page. The second is a StackPanel element that contains a set of WPF controls that display information and allow you to manipulate the ListBox Control by setting exposed interoperation properties. For each of the elements that are children of the StackPanel, see the reference material for the various elements used for details on what these elements are or what they do, these are listed in the example code below but will not be explained here (the basic interoperation model does not require any of them, they are provided to add some interactivity to the sample).

<Window
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  x:Class="WPF_Hosting_Win32_Control.HostWindow"
  Name="mainWindow"
  Loaded="On_UIReady">

  <DockPanel Background="LightGreen">
    <Border Name="ControlHostElement"
    Width="200"
    Height="200"
    HorizontalAlignment="Right"
    VerticalAlignment="Top"
    BorderBrush="LightGray"
    BorderThickness="3"
    DockPanel.Dock="Right"/>
    <StackPanel>
      <Label HorizontalAlignment="Center"
        Margin="0,10,0,0"
        FontSize="14"
        FontWeight="Bold">Control the Control</Label>
      <TextBlock Margin="10,10,10,10" >Selected Text: <TextBlock  Name="selectedText"/></TextBlock>
      <TextBlock Margin="10,10,10,10" >Number of Items: <TextBlock  Name="numItems"/></TextBlock>

      <Line X1="0" X2="200"
        Stroke="LightYellow"
        StrokeThickness="2"
        HorizontalAlignment="Center"
        Margin="0,20,0,0"/>

      <Label HorizontalAlignment="Center"
        Margin="10,10,10,10">Append an Item to the List</Label>
      <StackPanel Orientation="Horizontal">
        <Label HorizontalAlignment="Left"
          Margin="10,10,10,10">Item Text</Label>
        <TextBox HorizontalAlignment="Left"
          Name="txtAppend"
          Width="200"
          Margin="10,10,10,10"></TextBox>
      </StackPanel>

      <Button HorizontalAlignment="Left"
        Click="AppendText"
        Width="75"
        Margin="10,10,10,10">Append</Button>

      <Line X1="0" X2="200"
        Stroke="LightYellow"
        StrokeThickness="2"
        HorizontalAlignment="Center"
        Margin="0,20,0,0"/>

      <Label HorizontalAlignment="Center"
        Margin="10,10,10,10">Delete the Selected Item</Label>

      <Button Click="DeleteText"
        Width="125"
        Margin="10,10,10,10"
        HorizontalAlignment="Left">Delete</Button>
    </StackPanel>
  </DockPanel>
</Window>

The core of this sample is the class that actually hosts the control, ControlHost.cs. It inherits from HwndHost. The constructor takes two parameters, height and width, which correspond to the height and width of the Border element that hosts the ListBox control. These values are used later to ensure that the size of the control matches the Border element.

Public Class ControlHost
	  Inherits HwndHost
	Private hwndControl As IntPtr
	Private hwndHost As IntPtr
	Private hostHeight, hostWidth As Integer

	Public Sub New(ByVal height As Double, ByVal width As Double)
            hostHeight = CInt(height)
            hostWidth = CInt(width)
	End Sub

There is also a set of constants. These constants are largely taken from Winuser.h, and allow you to use conventional names when calling Win32 functions.

Friend Const WS_CHILD As Integer = &H40000000, WS_VISIBLE As Integer = &H10000000, LBS_NOTIFY As Integer = &H00000001, HOST_ID As Integer = &H00000002, LISTBOX_ID As Integer = &H00000001, WS_VSCROLL As Integer = &H00200000, WS_BORDER As Integer = &H00800000

You override this method to create the Win32 window that will be hosted by the page, and make the connection between the window and the page. Because this sample involves hosting a ListBox Control, two windows are created. The first is the window that is actually hosted by the WPF page. The ListBox Control is created as a child of that window.

The reason for this approach is to simplify the process of receiving notifications from the control. The HwndHost class allows you to process messages sent to the window that it is hosting. If you host a Win32 control directly, you receive the messages sent to the internal message loop of the control. You can display the control and send it messages, but you do not receive the notifications that the control sends to its parent window. This means, among other things, that you have no way of detecting when the user interacts with the control. Instead, create a host window and make the control a child of that window. This allows you to process the messages for the host window including the notifications sent to it by the control. For convenience, since the host window is little more than a simple wrapper for the control, the package will be referred to as a ListBox control.

You can use PInvoke to create a host window for the control by creating and registering a window class, and so on. However, a much simpler approach is to create a window with the predefined "static" window class. This provides you with the window procedure you need in order to receive notifications from the control, and requires minimal coding.

The HWND of the control is exposed through a read-only property, such that the host page can use it to send messages to the control.

Public ReadOnly Property hwndListBox() As IntPtr
	  Get
		  Return hwndControl
	  End Get
	End Property

The ListBox control is created as a child of the host window. The height and width of both windows are set to the values passed to the constructor, discussed above. This ensures that the size of the host window and control is identical to the reserved area on the page. After the windows are created, the sample returns a HandleRef object that contains the HWND of the host window.

Protected Overrides Function BuildWindowCore(ByVal hwndParent As HandleRef) As HandleRef
	  hwndControl = IntPtr.Zero
	  hwndHost = IntPtr.Zero

	  hwndHost = CreateWindowEx(0, "static", "", WS_CHILD Or WS_VISIBLE, 0, 0, hostWidth, hostHeight, hwndParent.Handle, New IntPtr(HOST_ID), IntPtr.Zero, 0)

	  hwndControl = CreateWindowEx(0, "listbox", "", WS_CHILD Or WS_VISIBLE Or LBS_NOTIFY Or WS_VSCROLL Or WS_BORDER, 0, 0, hostWidth, hostHeight, hwndHost, New IntPtr(LISTBOX_ID), IntPtr.Zero, 0)

	  Return New HandleRef(Me, hwndHost)
	End Function
'PInvoke declarations
	<DllImport("user32.dll", EntryPoint := "CreateWindowEx", CharSet := CharSet.Unicode)>
	Friend Shared Function CreateWindowEx(ByVal dwExStyle As Integer, ByVal lpszClassName As String, ByVal lpszWindowName As String, ByVal style As Integer, ByVal x As Integer, ByVal y As Integer, ByVal width As Integer, ByVal height As Integer, ByVal hwndParent As IntPtr, ByVal hMenu As IntPtr, ByVal hInst As IntPtr, <MarshalAs(UnmanagedType.AsAny)> ByVal pvParam As Object) As IntPtr
	End Function

In addition to BuildWindowCore, you must also override the WndProc and DestroyWindowCore methods of the HwndHost. In this example, the messages for the control are handled by the MessageHook handler, thus the implementation of WndProc and DestroyWindowCore is minimal. In the case of WndProc, set handled to false to indicate that the message was not handled and return 0. For DestroyWindowCore, simply destroy the window.

Protected Overrides Function WndProc(ByVal hwnd As IntPtr, ByVal msg As Integer, ByVal wParam As IntPtr, ByVal lParam As IntPtr, ByRef handled As Boolean) As IntPtr
	  handled = False
	  Return IntPtr.Zero
	End Function

	Protected Overrides Sub DestroyWindowCore(ByVal hwnd As HandleRef)
	  DestroyWindow(hwnd.Handle)
	End Sub
<DllImport("user32.dll", EntryPoint := "DestroyWindow", CharSet := CharSet.Unicode)>
	Friend Shared Function DestroyWindow(ByVal hwnd As IntPtr) As Boolean
	End Function

To host the control on the page, you first create a new instance of the ControlHost class. Pass the height and width of the border element that contains the control (ControlHostElement) to the ControlHost constructor. This ensures that the ListBox is sized correctly. You then host the control on the page by assigning the ControlHost object to the Child property of the host Border.

The sample attaches a handler to the MessageHook event of the ControlHost to receive messages from the control. This event is raised for every message sent to the hosted window. In this case, these are the messages sent to window that wraps the actual ListBox control, including notifications from the control. The sample calls SendMessage to obtain information from the control and modify its contents. The details of how the page communicates with the control are discussed in the next section.

System_CAPS_noteNote

Notice that there are two PInvoke declarations for SendMessage. This is necessary because one uses the wParam parameter to pass a string and the other uses it to pass an integer. You need a separate declaration for each signature to ensure that the data is marshaled correctly.

Partial Public Class HostWindow
    Inherits Window
    Private selectedItem As Integer
    Private hwndListBox As IntPtr
    Private listControl As ControlHost
    Private app As Application
    Private myWindow As Window
    Private itemCount As Integer

    Private Sub On_UIReady(ByVal sender As Object, ByVal e As EventArgs)
        app = System.Windows.Application.Current
        myWindow = app.MainWindow
        myWindow.SizeToContent = SizeToContent.WidthAndHeight
        listControl = New ControlHost(ControlHostElement.ActualHeight, ControlHostElement.ActualWidth)
        ControlHostElement.Child = listControl
        AddHandler listControl.MessageHook, AddressOf ControlMsgFilter
        hwndListBox = listControl.hwndListBox
        For i As Integer = 0 To 14 'populate listbox
            Dim itemText As String = "Item" & i.ToString()
            SendMessage(hwndListBox, LB_ADDSTRING, IntPtr.Zero, itemText)
        Next i
        itemCount = SendMessage(hwndListBox, LB_GETCOUNT, IntPtr.Zero, IntPtr.Zero)
        numItems.Text = "" & itemCount.ToString()
    End Sub
Private Function ControlMsgFilter(ByVal hwnd As IntPtr, ByVal msg As Integer, ByVal wParam As IntPtr, ByVal lParam As IntPtr, ByRef handled As Boolean) As IntPtr
    Dim textLength As Integer

    handled = False
    If msg = WM_COMMAND Then
        Select Case CUInt(wParam.ToInt32()) >> 16 And &HFFFF 'extract the HIWORD
            Case LBN_SELCHANGE 'Get the item text and display it
                selectedItem = SendMessage(listControl.hwndListBox, LB_GETCURSEL, IntPtr.Zero, IntPtr.Zero)
                textLength = SendMessage(listControl.hwndListBox, LB_GETTEXTLEN, IntPtr.Zero, IntPtr.Zero)
                Dim itemText As New StringBuilder()
                SendMessage(hwndListBox, LB_GETTEXT, selectedItem, itemText)
                selectedText.Text = itemText.ToString()
                handled = True
        End Select
    End If
    Return IntPtr.Zero
End Function
Friend Const LBN_SELCHANGE As Integer = &H1, WM_COMMAND As Integer = &H111, LB_GETCURSEL As Integer = &H188, LB_GETTEXTLEN As Integer = &H18A, LB_ADDSTRING As Integer = &H180, LB_GETTEXT As Integer = &H189, LB_DELETESTRING As Integer = &H182, LB_GETCOUNT As Integer = &H18B

<DllImport("user32.dll", EntryPoint:="SendMessage", CharSet:=CharSet.Unicode)>
Friend Shared Function SendMessage(ByVal hwnd As IntPtr, ByVal msg As Integer, ByVal wParam As IntPtr, ByVal lParam As IntPtr) As Integer
End Function

<DllImport("user32.dll", EntryPoint:="SendMessage", CharSet:=CharSet.Unicode)>
Friend Shared Function SendMessage(ByVal hwnd As IntPtr, ByVal msg As Integer, ByVal wParam As Integer, <MarshalAs(UnmanagedType.LPWStr)> ByVal lParam As StringBuilder) As Integer
End Function

<DllImport("user32.dll", EntryPoint:="SendMessage", CharSet:=CharSet.Unicode)>
Friend Shared Function SendMessage(ByVal hwnd As IntPtr, ByVal msg As Integer, ByVal wParam As IntPtr, ByVal lParam As String) As IntPtr
End Function

You manipulate the control by sending it Windows messages. The control notifies you when the user interacts with it by sending notifications to its host window. The Hosting a Win32 ListBox Control in WPF Sample sample includes a UI that provides several examples of how this works:

  • Append an item to the list.

  • Delete the selected item from the list

  • Display the text of the currently selected item.

  • Display the number of items in the list.

The user can also select an item in the list box by clicking on it, just as they would for a conventional Win32 application. The displayed data is updated each time the user changes the state of the list box by selecting, adding, or appending an item.

To append items, send the list box an LB_ADDSTRING message. To delete items, send LB_GETCURSEL to get the index of the current selection and then LB_DELETESTRING to delete the item. The sample also sends LB_GETCOUNT, and uses the returned value to update the display that shows the number of items. Both these instances of SendMessage use one of the PInvoke declarations discussed in the previous section.

Private Sub AppendText(ByVal sender As Object, ByVal args As EventArgs)
    If txtAppend.Text <> String.Empty Then
        SendMessage(hwndListBox, LB_ADDSTRING, IntPtr.Zero, txtAppend.Text)
    End If
    itemCount = SendMessage(hwndListBox, LB_GETCOUNT, IntPtr.Zero, IntPtr.Zero)
    numItems.Text = "" & itemCount.ToString()
End Sub
Private Sub DeleteText(ByVal sender As Object, ByVal args As EventArgs)
    selectedItem = SendMessage(listControl.hwndListBox, LB_GETCURSEL, IntPtr.Zero, IntPtr.Zero)
    If selectedItem <> -1 Then 'check for selected item
        SendMessage(hwndListBox, LB_DELETESTRING, New IntPtr(selectedItem), IntPtr.Zero)
    End If
    itemCount = SendMessage(hwndListBox, LB_GETCOUNT, IntPtr.Zero, IntPtr.Zero)
    numItems.Text = "" & itemCount.ToString()
End Sub

When the user selects an item changes their selection, the control notifies the host window by sending it a WM_COMMAND message, which raises the MessageHook event for the page. The handler receives the same information as the main window procedure of the host window. It also passes a reference to a Boolean value, handled. You set to handled to true to indicate that you have handled the message and no further processing is needed.

WM_COMMAND is sent for a variety of reasons, so you must examine the notification ID to determine whether it is an event that you wish to handle. The ID is contained in the high word of the wParam parameter. Since Microsoft .NET does not have a HIWORD macro, the sample uses bitwise operators to extract the ID. If the user has made or changed their selection, the ID will be LBN_SELCHANGE.

When LBN_SELCHANGE is received, the sample gets the index of the selected item by sending the control a LB_GETCURSEL message. To get the text, you first create a StringBuilder. You then send the control an LB_GETTEXT message. Pass the empty StringBuilder object as the wParam parameter. When SendMessage returns, the StringBuilder will contain the text of the selected item. This use of SendMessage requires yet another PInvoke declaration.

Finally, set handled to true to indicate that the message has been handled.

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