using unmanaged dll in c# application

You need to use Platform Invoke: http://msdn.microsoft.com/en-us/library/26thfadc.aspx[^]

It's easier than it sounds (depending on the parameters you need to hand over) - at a minimum, it's just a case of adding an attribute to a method declaration:

[DllIport("User32")]
private static extern int SetForegroundWindow(IntPtr hwnd);

Uses the method SetForegroundWindow from the system DLL file "User32.dll"
You can then just call it in C#:

public static void SingleInstance(this Process thisProcess)
    {
    foreach (Process proc in Process.GetProcessesByName(thisProcess.ProcessName))
        {
        if (proc.Id != thisProcess.Id)
            {
            ShowWindow(proc.MainWindowHandle, SW_RESTORE);
            SetForegroundWindow(proc.MainWindowHandle);
            thisProcess.Kill();
            }
        }
    }

[edit]Typo: SetForgrondWindow for SetForegroundWindow - OriginalGriff[/edit]