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WindowsFormsApplicationBase.StartupNextInstance Event

Occurs when attempting to start a single-instance application and the application is already active.

Namespace:  Microsoft.VisualBasic.ApplicationServices
Assembly:  Microsoft.VisualBasic (in Microsoft.VisualBasic.dll)

member StartupNextInstance : IEvent<StartupNextInstanceEventHandler,

A single-instance application raises the StartupNextInstance event when you attempt to restart the application when it is already active. When a single-instance application starts for the first time, it raises the Startup event. For more information, see Startup.

This event is part of the Visual Basic Application model. For more information, see Overview of the Visual Basic Application Model.

This event is raised on the application's main thread with the other user-interface events. This allows the event handler to access directly the application's user interface. However, if the application is busy handling another user-interface event when this event is raised, this event cannot be processed until the other event handler finishes or calls the DoEvents method.


The StartupNextInstance event is raised only in single-instance applications. To enable single-instance behavior for your application, you must check the Make single instance application check box in the Project Designer.

To make an application single instance

  1. With a project selected in Solution Explorer, click Properties on the Project menu.

  2. Click the Application tab.

  3. Select the Make single instance application check box.

You must use the CommandLine property of the e parameter to access the arguments for subsequent attempts to start a single-instance application. The CommandLineArgs property provides the arguments used to start the first instance of a single-instance application.

The code for the StartupNextInstance event handler is stored in the ApplicationEvents.vb file, which is hidden by default.

To access the Code Editor window for application events

  1. With a project selected in Solution Explorer, click Properties on the Project menu.

  2. Click the Application tab.

  3. Click the View Application Events button to open the Code Editor.

    For more information, see Application Page, Project Designer (Visual Basic).

The following table lists examples of tasks involving the My.Application.StartupNextInstance event.



Check the command-line arguments of the first application instance


Availability by Project Type

Project type


Windows Forms Application


Class Library


Console Application


Windows Forms Control Library


Web Control Library


Windows Service


Web Site


This example uses the e parameter of the StartupNextInstance event handler to examine the application's command-line arguments. If an argument is found that starts with /input=, the rest of that argument is displayed.

Private Sub MyApplication_StartupNextInstance( 
    ByVal sender As Object, 
    ByVal e As Microsoft.VisualBasic.ApplicationServices.StartupNextInstanceEventArgs 
) Handles Me.StartupNextInstance
    Dim inputArgument As String = "/input=" 
    Dim inputName As String = "" 

    For Each s As String In e.CommandLine
        If s.ToLower.StartsWith(inputArgument) Then
            inputName = s.Remove(0, inputArgument.Length)
        End If 

    If inputName = "" Then
        MsgBox("No input name")
        MsgBox("Input name: " & inputName)
    End If 
End Sub

You must enter the code in the Code Editor window for application events. To access this window, follow the instructions from this topic's Remarks section. For more information, see Application Page, Project Designer (Visual Basic).

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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