Single Instance of Application

Introduction

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 = _ 
       Process.GetProcessesByName(processName)
    For Each p As Process In proclist
        If p.Id <> Process.GetCurrentProcess().Id Then
            Return p
        End If
    Next
    Return Nothing
End Function

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