WPF Application single instance

for WPF if you want to have a single instance (for one session) you can use this code

public class ApplicationSingleInstance : Application
{
    protected override void OnStartup(StartupEventArgs e)
    {
        var currentSessionID = Process.GetCurrentProcess().SessionId;
        var currentProcessId = Process.GetCurrentProcess().Id;
        var exists = System.Diagnostics.Process.GetProcessesByName(System.IO.Path.GetFileNameWithoutExtension(System.Reflection.Assembly.GetEntryAssembly().Location));
        var result = exists.Where(p => p.SessionId == currentSessionID && p.Id != currentProcessId)
                        .OrderBy(p => p.Id)
                        .ToList();
        if (result.Count > 0)
        {
#if MESSAGE
            string s = "";
            int i = 0;
            foreach (var item in result)
            {
                s += "[" + i + "] handle window = " + result[i].MainWindowHandle + " Process ID = " + result[i].Id + "\n";
                i++;
            }
            MessageBox.Show(s);
#endif
            UnMinimizeClass.Reactivate(result.First());
            Environment.Exit(0);
            return;
        }
        base.OnStartup(e);
    }
}

public static class UnMinimizeClass
{
    #region Imports

    [DllImport("user32.dll")]
    static extern bool AllowSetForegroundWindow(int dwProcessId);

    [DllImport("user32.dll")]
    private static extern bool SetForegroundWindow(IntPtr hWnd);

    [DllImport("user32.dll")]
    public static extern bool ShowWindow(IntPtr hWnd, int nCmdShow);

    [DllImport("user32.dll", SetLastError = true)]
    static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter, int X, int Y, int cx, int cy, long uFlags);

    [DllImport("user32.dll", SetLastError = true)]
    static extern IntPtr FindWindow(string lpClassName, string lpWindowName);

    [DllImport("user32.dll", SetLastError = true)]
    static extern IntPtr FindWindowEx(IntPtr hwndParent, IntPtr hwndChildAfter, string lpszClass, string lpszWindow);

    [DllImport("user32.dll", SetLastError = true)]
    static extern uint GetWindowThreadProcessId(IntPtr hWnd, out uint lpdwProcessId);

    [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    static extern int GetWindowText(IntPtr hWnd, StringBuilder lpString, int nMaxCount);

    [DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Auto)]
    static extern int GetWindowTextLength(IntPtr hWnd);

    [DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Auto)]
    static extern int GetClassName(IntPtr hWnd, StringBuilder lpClassName, int nMaxCount);

    [DllImport("user32.dll")]
    public static extern int EnumWindows(CallBackPtr callPtr, int lPar);

    [DllImport("user32.dll")]
    [return: MarshalAs(UnmanagedType.Bool)]
    static extern bool IsWindowVisible(IntPtr hWnd);

    [DllImport("user32.dll")]
    [return: MarshalAs(UnmanagedType.Bool)]
    static extern bool IsIconic(IntPtr hWnd);

    [DllImport("user32.dll")]
    static extern bool IsZoomed(IntPtr hWnd);

    public delegate bool CallBackPtr(IntPtr hwnd, int lParam);

    #endregion

    #region const
    private const int SW_RESTORE = 9;
    private const int SW_HIDE = 0;
    private const int SW_MINIMIZE = 6;
    private const int SW_MAXIMIZE = 3;
    private const int SW_SHOW = 5;

    static readonly IntPtr HWND_TOPMOST = new IntPtr(-1);
    static readonly IntPtr HWND_NOTOPMOST = new IntPtr(-2);
    static readonly IntPtr HWND_TOP = new IntPtr(0);
    static readonly IntPtr HWND_BOTTOM = new IntPtr(1);

    private static class SWP
    {
        public static readonly int
        NOSIZE = 0x0001,
        NOMOVE = 0x0002,
        NOZORDER = 0x0004,
        NOREDRAW = 0x0008,
        NOACTIVATE = 0x0010,
        DRAWFRAME = 0x0020,
        FRAMECHANGED = 0x0020,
        SHOWWINDOW = 0x0040,
        HIDEWINDOW = 0x0080,
        NOCOPYBITS = 0x0100,
        NOOWNERZORDER = 0x0200,
        NOREPOSITION = 0x0200,
        NOSENDCHANGING = 0x0400,
        DEFERERASE = 0x2000,
        ASYNCWINDOWPOS = 0x4000;
    }
    #endregion

    #region  private functions
    private static IntPtr GetWindowHandle(uint processId)
    {
        IntPtr result = IntPtr.Zero;
        UnMinimizeClass.EnumWindows((hCurWnd, param) =>
        {
            uint dwProcessID = 0;
            GetWindowThreadProcessId(hCurWnd, out dwProcessID);
            if (dwProcessID == processId && IsWindowVisible(hCurWnd))
            {
                result = hCurWnd;
                return false;
            }
            return true;
        }, 0);
        return result;
    }

    private static string GetWindowClassName(IntPtr hWnd)
    {
        StringBuilder ClassName = new StringBuilder(256);
        int nRet = GetClassName(hWnd, ClassName, ClassName.Capacity);
        return nRet != 0 ? ClassName.ToString() : null;
    }

    private static string GetWindowTitle(IntPtr hWnd)
    {
        // Allocate correct string length first
        int length = GetWindowTextLength(hWnd);
        StringBuilder sb = new StringBuilder(length + 1);
        GetWindowText(hWnd, sb, sb.Capacity);
        return sb.ToString();
    }

    private static int GetWindowStat(IntPtr hWnd)
    {
        if (!IsWindowVisible(hWnd))
            return SW_HIDE;
        else if (IsIconic(hWnd))
            return SW_MINIMIZE;
        else if (IsZoomed(hWnd))
            return SW_MAXIMIZE;
        else
            return SW_SHOW;
    }
    #endregion

    #region  public Reactivate
    public static void Reactivate(Process process)
    {
        IntPtr handle = process.MainWindowHandle;
        if (process.MainWindowHandle.ToInt32() == 0)
        {
            handle = GetWindowHandle((uint)process.Id);
#if MESSAGE
            MessageBox.Show("pointeur a zero = " + handle + " window handle = " + FindWindow(null, "XXX"));
#endif
        }
        Reactivate(handle, process.Id);
    }
    public static void Reactivate(IntPtr handle, int processId)
    {
        int stat = GetWindowStat(handle);

        if (stat == SW_MAXIMIZE)
        {
            SetForegroundWindow(handle);
            SetWindowPos(handle, (IntPtr)HWND_TOP, 0, 0, 0, 0, SWP.NOMOVE | SWP.NOSIZE | SWP.SHOWWINDOW);
        }
        else if (stat == SW_HIDE)
        {
            ShowWindow(handle, SW_SHOW);
            SetForegroundWindow(handle);
            SetWindowPos(handle, (IntPtr)HWND_TOP, 0, 0, 0, 0, SWP.NOMOVE | SWP.NOSIZE | SWP.SHOWWINDOW);
        }
        else
        {
            ShowWindow(handle, (int)SW_RESTORE);
            SetForegroundWindow(handle);
            SetWindowPos(handle, (IntPtr)HWND_TOP, 0, 0, 0, 0, SWP.NOMOVE | SWP.NOSIZE | SWP.SHOWWINDOW);
        }
    }
#endregion     
}

and hinerite your App class from ApplicationSingleInstance

don’t forget to change in App Xaml like this

<local:ApplicationSingleInstance x:Class="XX.App"
                           xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                           xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                           xmlns:local="clr-namespace:XX"
...

if your namespace is XX