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There are some instances you may want to disallow the user to launch your program if it is already running. There are few ways to check whether the previous instance of your application is already running. Here is one of the way to check whether previous instance of your application is already running or not:

VB.Net Code:

Imports System.Runtime.InteropServices
Module modMain
  Public Const ERROR_ALREADY_EXISTS As Integer = 183
  Public Declare Function CreateMutexA Lib "Kernel32.dll" (ByVal lpSecurityAttributes As Integer, ByVal bInitialOwner As Boolean, ByVal lpName As String) As Integer
  Public Declare Function GetLastError Lib "Kernel32.dll" () As Integer

  Public Function IsProcessAlreadyRunning() As Boolean
    'Attempt to create defualt mutex owned by process, MyApp can e.g. be the name of the executable/Product
    CreateMutexA(0, True, "MyApp")
    Return (GetLastError() = ERROR_ALREADY_EXISTS)
  End Function
End Module

C#.Net Code:

using System.Runtime.InteropServices;
static class modMain
{
    public const int ERROR_ALREADY_EXISTS = 183;
    [DllImport("Kernel32.dll", CharSet = CharSet.Ansi, SetLastError = true, ExactSpelling = true)]
    public static extern int CreateMutexA(int lpSecurityAttributes, bool bInitialOwner, string lpName);
    [DllImport("Kernel32.dll", CharSet = CharSet.Ansi, SetLastError = true, ExactSpelling = true)]
    public static extern int GetLastError();

    public static bool IsProcessAlreadyRunning()
    {
        ///Attempt to create defualt mutex owned by process, MyApp can e.g. be the name of the executable/Product
        CreateMutexA(0, true, "MyApp");
        return (GetLastError() == ERROR_ALREADY_EXISTS);
    }
}

Another Simple way to accomplish this:

bool PrevInstance()
{
return Process.GetProcessesByName(Process.GetCurrentProcess.ProcessName).Length>1;
}

Tested with VS 2008

If you found any issues with the code posted above, please leave a comment. We will fix the code as soon as possible.