fix shutdown didn't work issue

WinAlfred/App.xaml.cs

@@ -1,11 +1,28 @@
-using System.Windows;
-
-namespace WinAlfred
-{
-    /// <summary>
-    /// App.xaml 的交互逻辑
-    /// </summary>
-    public partial class App : Application
-    {
-    }
-}
+using System;
+using System.Threading;
+using System.Windows;
+
+namespace WinAlfred
+{
+    /// <summary>
+    /// App.xaml 的交互逻辑
+    /// </summary>
+    public partial class App : Application
+    {
+        protected override void OnStartup(StartupEventArgs e)
+        {
+            base.OnStartup(e);
+            bool startupFlag;
+            Mutex mutex = new Mutex(true, "WinAlfred", out startupFlag);
+            if (!startupFlag)
+            {
+                Environment.Exit(0);
+            }
+            else
+            {
+                MainWindow mainWindow = new MainWindow();
+                mainWindow.Show();
+            }
+        }
+    }
+}