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

Note

If you are using a named mutex to limit your application to a single instance, a malicious user can create this mutex before you do and prevent your application from starting. To prevent this situation, create a randomly named mutex and store the name so that it can only be obtained by an authorized user. Alternatively, you can use a file for this purpose. To limit your application to one instance per user, create a locked file in the user's profile directory.

  • Visual C# code

    public System.Threading.Mutex mutex;
    public void main ()
    {
        bool prevInstance = false;
        mutex = new System.Threading.Mutex (true, 'Application Name', 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.