September 22, 2014
Hot Topics:
RSS RSS feed Download our iPhone app

Sometimes You Gotta Make a Little Noise

  • May 28, 2004
  • By Paul Kimmel
  • Send Email »
  • More Articles »

Importing the Windows MultiMedia Library

The ability to import an external API library is supported by the DllImportAttribute. In Visual Basic, importing an external library also is manifested with the synonymous Declare modifier. That is, Visual Basic .NET supports the Declare statement. To import the winmm.dll PlaySound method, we can reference the library—which does an implicit LoadLibrary and FreeLibrary in the background—using the following Declare statement:

Private Declare Function PlaySound Lib "winmm.dll" ( _
  ByVal fileName As String, ByVal hmod As IntPtr, _
  ByVal flags As PlaySoundFlags) As Integer

You will have to perform modest translations between managed types and API types on a case by case basis, but the preceding Declare statement is correct for PlaySound.

Wrapping the PlaySound API Method

If we want to make it easier to use the PlaySound API method, we can define our own wrapper that eliminates elements that we won't be changing in a given context. For our example, the module handle will always be zero, the sound will always be played asynchronously, and the filename is the only dynamic parameter. Consequently, we can simplify the API call with the following wrapper method:

Public Shared Sub Play(ByVal fileName As String)
  Try
    PlaySound(fileName, IntPtr.Zero, PlaySoundFlags.SND_FILENAME Or _
      PlaySoundFlags.SND_ASYNC)
  Catch
    Debug.WriteLine("Can't play sound file")
  End Try
End Sub

The IntPtr class and the IntPtr.Zero constant come from the InteropServices namespace. Sometimes, we need a pointer that may not be a native type in a particular .NET language, such as VB.NET. PlaySound supports a null module pointer—as opposed to, for example, the actual address of a module pointer—which is expressed using IntPtr.Zero.

Playing Sounds Asynchronously

The winmm.dll PlaySound method supports synchronous and asynchronous playback of media. If we send PlaySoundFlags.SND_SYNC, our code will have to wait for the API call to complete before we re-attain control. Passing PlaySoundFlags.SND_ASYNC means that the call returns immediately and the media is played in a background thread.

Testing the Wrapper Code

For your convenience, the entire example listing is provided here complete with a test method, beep (see Listing 5). Assuming your Windows directory is Windows and not WINNT and the chord.wav file exists, you should hear the chord.wav file if you call the Sounds.Beep static method. To compare the difference between asynchronous and synchronous behavior, pick a longer .wav file and switch between passing SND_ASYNC and SND_SYNC to see a difference in how the code responds.

Listing 5: The Complete Sounds class.

Imports System
Imports System.Diagnostics
Imports System.IO
Imports System.Runtime.InteropServices

Public Class Sounds
  <Flags()> _
  Public Enum PlaySoundFlags
    SND_SYNC  = 0
    SND_ASYNC = 1
    SND_FILENAME = &H20000
    SND_RESOURCE = &H40004
  End Enum

  Private Declare Function PlaySound Lib "winmm.dll" (ByVal _
    fileName As String, _
    ByVal hmod As IntPtr, ByVal flags As PlaySoundFlags) As Integer

  Public Shared Sub Play(ByVal fileName As String)
    Try
      PlaySound(fileName, IntPtr.Zero, PlaySoundFlags.SND_FILENAME Or _
        PlaySoundFlags.SND_ASYNC)
    Catch
      Debug.WriteLine("Can't play sound file")
    End Try
  End Sub

  Public Shared Sub Beep()
    Try
      Const soundFile As String = "c:\winnt\media\chord.wav"
      If (File.Exists(soundFile)) Then Play(soundFile)

    Catch ex As Exception
      Debug.WriteLine(ex.Message)
      Throw
    End Try
  End Sub

End Class

Summary

A large amount of information can be hidden in a small amount of code. In this article, you learned a little bit about the .NET attributes DllImportAttribute and FlagsAttribute that support the Declare modifier and bitwise Or-behavior for enumerated types. We talked some about InteropServices, and you now have a very simple media player that may add some additional depth to your applications. Enjoy.

Biography

Paul Kimmel is the VB Today columnist, has written several books on .NET programming, and is a software architect. You may contact him at pkimmel@softconcepts.com if you need assistance.





Page 2 of 2



Comment and Contribute

 


(Maximum characters: 1200). You have characters left.

 

 


Sitemap | Contact Us

Rocket Fuel