SoundPlayer.Play實現中的什麼不允許播放超過1個聲音異步？

Question

我試圖理解在執行SoundPlayer.Play時它不允許播放超過1個聲音異步（例如，如解釋here或here）。我看着它的source code，但我無法弄清楚。

爲了方便起見，這裏的兩個SoundPlayer.Play和SoundPlayer.LoadAndPlay的代碼，這是由SoundPlayer.Play調用的方法：

public void Play() { 
      LoadAndPlay(NativeMethods.SND_ASYNC); 
     }  

private void LoadAndPlay(int flags) { 
      // 
      if (String.IsNullOrEmpty(soundLocation) && stream == null) { 
       SystemSounds.Beep.Play(); 
       return; 
      } 

      if (uri != null && uri.IsFile) { 
       // VSW 580992: With more than one thread, someone could call SoundPlayer::set_Location 
       // between the time LoadAndPlay demands FileIO and the time it calls PlaySound under elevation. 
       // 
       // Another scenario is someone calling SoundPlayer::set_Location between the time 
       // LoadAndPlay validates the sound file and the time it calls PlaySound. 
       // The SoundPlayer will end up playing an un-validated sound file. 
       // The solution is to store the uri.LocalPath on a local variable 
       string localPath = uri.LocalPath; 

       // request permission to read the file: 
       // pass the full path to the FileIOPermission 
       FileIOPermission perm = new FileIOPermission(FileIOPermissionAccess.Read, localPath); 
       perm.Demand(); 

       // play the path 
       isLoadCompleted = true; 
       System.Media.SoundPlayer.IntSecurity.SafeSubWindows.Demand(); 

       System.ComponentModel.IntSecurity.UnmanagedCode.Assert(); 
       // ValidateSoundFile calls into the MMIO API so we need UnmanagedCode permissions to do that. 
       // And of course we need UnmanagedCode permissions to all Win32::PlaySound method. 
       try { 
        // don't use uri.AbsolutePath because that gives problems when there are whitespaces in file names 
        ValidateSoundFile(localPath); 
        UnsafeNativeMethods.PlaySound(localPath, IntPtr.Zero, NativeMethods.SND_NODEFAULT | flags); 
       } finally { 
        System.Security.CodeAccessPermission.RevertAssert(); 
       } 
      } else { 
       LoadSync(); 
       ValidateSoundData(streamData); 
       System.Media.SoundPlayer.IntSecurity.SafeSubWindows.Demand(); 

       System.ComponentModel.IntSecurity.UnmanagedCode.Assert(); 
       try { 
        UnsafeNativeMethods.PlaySound(streamData, IntPtr.Zero, NativeMethods.SND_MEMORY | NativeMethods.SND_NODEFAULT | flags); 
       } finally { 
        System.Security.CodeAccessPermission.RevertAssert(); 
       } 
      } 
     } 

Answer 1

UnsafeNativeMethods.PlaySound（WINMM.DLL）僅允許單個聲音在特定播放時間，每個過程。