Single instance application crashes

This article helps you resolve the problem where single instance application crashes if the application uses WindowsFormsApplicationBase.

Original product version:   Visual C#, Visual Basic.NET
Original KB number:   2834636

Symptoms

You have a single instance Visual Basic.NET application or a C# application that uses the WindowsFormsApplicationBase class to make the application single instance and you are running the application as in an environment with IP Virtualization turned on. The application may crash. if a debugger is attached to the application, you will notice that the exception is CantStartSingleInstanceException.

Cause

This is because the Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase type is not designed to run in an environment with IP Virtualization turned on.

Resolution

To work around this issue, you can use a named mutex or enumerate all the processes and see if a previous instance of the application is running and activate it.

Sample code that uses a named mutex

• Visual C# code

  [STAThread]
  public static void Main()
  {
      bool prevInstance;
      _ = new System.Threading.Mutex(true, "Application Name", out prevInstance);
      if (prevInstance == false)
      {
          MessageBox.Show("There is another instance running");
          return;
      }
      Application.Run(new Form1());
  }
  

• Visual Basic .NET code

  Public mutex As System.Threading.Mutex
  Public Sub main()
      Dim prevInstance As Boolean
      mutex = New System.Threading.Mutex(True, "Application Name", prevInstance)
      If (prevInstance = False) Then
          MessageBox.Show("There is another instance running")
          Return
      End If
      Application.Run(New Form1())
  End Sub
  

Sample code that enumerates processes to see if a previous instance is running and activates it

• Visual C# code

  [DllImport ("user32.dll")]
  private static extern int ShowWindow (IntPtr hWnd, int nCmdShow);
  [DllImport ("user32.dll")]
  private static extern int SetForegroundWindow (IntPtr hWnd);
  [DllImport ("user32.dll")]
  private static extern int IsIconic (IntPtr hWnd);
  private const int SW_RESTORE = 9;
  
  /// <summary>
  /// The main entry point for the application.
  /// </summary>
  [STAThread]
  static void Main ()
  {
      var processes = System.Diagnostics.Process.GetProcessesByName ("WindowsFormsApplication1");
      foreach (System.Diagnostics.Process p in processes)
      {
          //Is this the current process?
          if (System.Diagnostics.Process.GetCurrentProcess ().Id == p.Id)
              continue;
          //if the other process is running, we need to activate it and...
          if (null != p.MainWindowHandle)
          {
              if (IsIconic (p.MainWindowHandle) != 0)
              {
                  ShowWindow (p.MainWindowHandle, SW_RESTORE);
              }
              SetForegroundWindow (p.MainWindowHandle);
          }
          //...exit this instance.
          return;
      }
      Application.EnableVisualStyles ();
      Application.SetCompatibleTextRenderingDefault (false);
      Application.Run (new Form1 ());
  }
  

• Visual Basic .NET code

  <DllImport("user32.dll")> _
  Private Function ShowWindow(hWnd As IntPtr, nCmdShow As Integer) As Integer
  End Function
  <DllImport("user32.dll")> _
  Private Function SetForegroundWindow(hWnd As IntPtr) As Integer
  End Function
  <DllImport("user32.dll")> _
  Private Function IsIconic(hWnd As IntPtr) As Integer
  End Function
  Private Const SW_RESTORE As Integer = 9
  
  ''' <summary>
  ''' The main entry point for the application.
  ''' </summary>
  <STAThread> _
  Public Sub Main()
      Dim processes = System.Diagnostics.Process.GetProcessesByName("WindowsApplication1")
      For Each p As System.Diagnostics.Process In processes
  
      'Is this the current process?
      If System.Diagnostics.Process.GetCurrentProcess().Id = p.Id Then
          Continue For
      End If
      'if the other process is running, we need to activate it and...
      If p.MainWindowHandle = Nothing Then
      If IsIconic(p.MainWindowHandle) <> 0 Then
          ShowWindow(p.MainWindowHandle, SW_RESTORE)
      End If
  
          SetForegroundWindow(p.MainWindowHandle)
      End If
      '...exit this instance.
      Return
      Next
  
      Application.EnableVisualStyles()
      Application.SetCompatibleTextRenderingDefault(False)
      Application.Run(New Form1())
  End Sub
  

More information

For more information, see Ensuring that only a single instance of a .NET application is running.