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Single Instance of Application

, 12 Oct 2003
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Shows a very generic way to find out if another instance of the application is already running and activate the running instance's main window by bringing it to the foreground.


Brief preamble: Some time ago I was looking for a way to find out if another instance of the application is already running. A trivial task as you may say and I would agree with you instantly as long as you are familiar with Win32 API. I looked around on various sites including this one and found a few interesting articles, including an article talking at length how to set up a well-known object to pass command line parameters to an existing instance (a good DDE replacement), which is nice but not exactly what I was looking for. I needed something quick and dirty and so here it goes.

Public NotInheritable Class Win32Helper 
<System.Runtime.InteropServices.DllImport("user32.dll", _ 
EntryPoint:="SetForegroundWindow", _
CallingConvention:= Runtime.InteropServices.CallingConvention.StdCall, _
CharSet:=Runtime.InteropServices.CharSet.Unicode, SetLastError:=True)> _
Public Shared Function _ 
     SetForegroundWindow(ByVal handle As IntPtr) As Boolean
' Leave function empty
End Function

<System.Runtime.InteropServices.DllImport("user32.dll", _ 
EntryPoint:="ShowWindow", _
CallingConvention:=Runtime.InteropServices.CallingConvention.StdCall, _
CharSet:=Runtime.InteropServices.CharSet.Unicode, SetLastError:=True)> _
Public Shared Function ShowWindow(ByVal handle As IntPtr, _ 
                             ByVal nCmd As Int32) As Boolean
' Leave function empty 
End Function

End Class ' End Win32Helper 

Public Shared Function GetRunningInstance(ByVal _ 
                   processName As String) As Process
    Dim proclist() As Process = _ 
    For Each p As Process In proclist
        If p.Id <> Process.GetCurrentProcess().Id Then
            Return p
        End If
    Return Nothing
End Function

'The main entry point for the application
Public Shared Sub Main()
    Dim pInstance As Process = _ 
    If Not pInstance Is Nothing Then
        Dim handle As IntPtr = pInstance.MainWindowHandle
        If Not IntPtr.Zero.Equals(handle) Then
            Win32Helper.ShowWindow(handle, 1)
        End If
        Dim context As ApplicationContext = _ 
           New ApplicationContext(New MainFrame) 
    End If
End Sub

Few last comments: I had to resort to a no-so-graceful approach (as compared to using remoting) as needed to activate a main window of the running instance if one was found. Mutexes wouldn't help either.

For those on a look out for any trace plagiarism I should perhaps add, this is my adaptation of already well known methods. This is just my humble contribution for others who may find this all somewhat useful.


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Comments and Discussions

GeneralMain() function PinmemberAnu.....S/W Developer11-Nov-08 23:18 
General4+ years later, exactly what I needed. Thx! (n/t) PinmemberGary Noter10-Dec-07 10:50 
GeneralDoes not activate main window PinmemberDeepak Sakpal14-Feb-07 22:58 
GeneralRe: Does not activate main window Pinmemberpetervn2-May-07 17:19 
GeneralExcellent Code PinmemberKestrel25-Oct-06 9:29 
GeneralThanks PinmemberMr. Beasimer20-Jul-06 12:10 
GeneralRe: Thanks PinmemberLBeasimer18-Sep-06 7:23 
Generalrestore from minimized state Pinmemberjkakermott12-Apr-06 11:09 
GeneralVery good code! PinmemberGiakkomino4-Mar-06 5:56 
GeneralNeeds Process Performance Counter Pinmemberjtencate7-Feb-06 8:47 
GeneralSingle Instance Code PinmemberFade (Amit BS)7-May-05 3:24 
QuestionHow can Start .exe program depending the Principal Process PinmemberVitoto4-May-05 10:24 
GeneralValue of article PinmemberRede14-Oct-03 1:46 

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