Just Programing!

---VB--

Private Sub Form_Initialize()

    If App.PrevInstance Then End

End Sub

---C#---

//方法1. check process
//需using System.Diagnostics
public bool PrevInstance() {
     return (Process.GetProcessesByName(Process.GetCurrentProcess().ProcessName).Length > 1) ;
}

//方法二
//需using System.Threading
private bool IsMyMutex(string apName) {
    bool IsExist;
    Mutex MyMutex = new Mutex(true, apName, out  IsExist); //out=true表示只有一個
    return (!IsExist);
}

if(IsMyMutex("TestAP"))
    MessageBox.Show("The program is already running");