Visual Basic 2010 : Importing and Using COM Objects

Importing COM Components into Visual Studio

Create a new Windows Forms project with Visual Basic and, when ready, open the Visual Studio toolbox. When done, right-click the toolbox and select Choose Items. This launches the same-named dialog that you already know because of adding .NET controls to the toolbox. Select the COM tab and search for the Windows Media Player item, as shown in Figure 1.

Figure 1. Choosing a COM component to add to the toolbox.

When you click OK, Visual Studio generates two files for you:

• Interop.WMPLib.dll, which is a CLR wrapper for using COM objects exposed by the Windows Media Player type library in a .NET fashion

• AxInterop.WMPLib.dll, which is a Windows Forms proxy that provides the infrastructure required for hosting the control in your forms

At this point notice that the Windows Media Player ActiveX control is available inside the toolbox. Now drag the control over the current form and design the media player as you like. At this point Visual Studio generates some code for you to declare the control and enables you to use it. If you expand the Form1.designer.vb file, you find the following initialization code for the ActiveX control:

Friend WithEvents AxWindowsMediaPlayer1 As AxWMPLib.AxWindowsMediaPlayer
...

Me.AxWindowsMediaPlayer1.Enabled = True
Me.AxWindowsMediaPlayer1.Location = New System.Drawing.Point(24, 13)
Me.AxWindowsMediaPlayer1.Name = "AxWindowsMediaPlayer1"
Me.AxWindowsMediaPlayer1.OcxState = _
CType(resources.GetObject("AxWindowsMediaPlayer1.OcxState"),
System.Windows.Forms.AxHost.State)
Me.AxWindowsMediaPlayer1.Size = New System.Drawing.Size(239, 196)
Me.AxWindowsMediaPlayer1.TabIndex = 0

Now you can work with the ActiveX control in a managed way, as illustrated in next subsection.

Using COM Objects in Code

When you have an instance of the ActiveX control, or of a type library, you can access its members like any other .NET object, thus invoking methods, assigning properties, or handling events. For example the following code assigns the URL property of the media player with a media file to start playing:

Private Sub Form1_Load(ByVal sender As Object,
                       ByVal e As System.EventArgs) Handles Me.Load
    AxWindowsMediaPlayer1.URL = "C:\users\alessandro\music\MySong.mp3"

End Sub

You can also handle events if available, as demonstrated by the following code snippet:

Private Sub AxWindowsMediaPlayer1_MediaError(ByVal sender As Object,
                                  ByVal e As AxWMPLib.
                                  _WMPOCXEvents_MediaErrorEvent) _
                                  Handles AxWindowsMediaPlayer1.
                                  MediaError

    MessageBox.Show("An error occurred while opening media")
End Sub

In this particular case the MediaError event is raised when an error occurs in playing the media file. At a more general level, notice how wrapping an ActiveX control allows importing different kinds of members, including events.

Catching Exceptions

When you implement Try..Catch..End Try blocks, you can intercept and handle only CLS-compliant exceptions, that is, exceptions inheriting from System.Exception. Exceptions wrapped by the COM import tools are not CLS-compliant, so a classic Try block would fail. To intercept exceptions coming from wrapped objects, the .NET Framework offers the System.Runtime.CompilerServices.RuntimeWrappedException that can be used for error handling when working with wrappers. The following code shows an example:

Try
    AxWindowsMediaPlayer1.URL = "C:\users\alessandro\music\MySong.mp3"

Catch ex As RuntimeWrappedException

Catch ex As Exception

End Try

Other than usual exception properties, this class exposes a WrappedException property, of type Object, which represents the occurred problem.

Releasing COM Objects

You should always explicitly release objects that wrap COM components so that associated resources are also released. You accomplish this by invoking the System.Runtime.InteropServices.Marshal.ReleaseCOMObject method. Continuing with the previous example, you release the AxWindowsMediaPlayer1 object as follows:

Private Sub Form1_FormClosing(ByVal sender As Object,
                              ByVal e As System.Windows.Forms.
                              FormClosingEventArgs) Handles _
                              Me.FormClosing
    System.Runtime.InteropServices.Marshal.
    ReleaseComObject(AxWindowsMediaPlayer1)
End Sub

This is important because COM objects treat system resources differently from .NET objects; therefore, an explicit release is required.