SEBWIN-220: Implemented mechanism to prevent (accidental) closing of taskbar by ALT+F4.

SafeExamBrowser.Client/Behaviour/ClientController.cs

@@ -7,6 +7,7 @@
  */
 
 using System;
+using System.ComponentModel;
 using System.IO;
 using SafeExamBrowser.Contracts.Behaviour;
 using SafeExamBrowser.Contracts.Behaviour.OperationModel;
@@ -250,7 +251,7 @@ namespace SafeExamBrowser.Client.Behaviour
 			shutdown.Invoke();
 		}
 
-		private void Taskbar_QuitButtonClicked()
+		private void Taskbar_QuitButtonClicked(CancelEventArgs args)
 		{
 			var result = messageBox.Show(TextKey.MessageBox_Quit, TextKey.MessageBox_QuitTitle, MessageBoxAction.YesNo, MessageBoxIcon.Question);
 
@@ -266,6 +267,10 @@ namespace SafeExamBrowser.Client.Behaviour
 					messageBox.Show(TextKey.MessageBox_QuitError, TextKey.MessageBox_QuitErrorTitle, icon: MessageBoxIcon.Error);
 				}
 			}
+			else
+			{
+				args.Cancel = true;
+			}
 		}
 
 		private void WindowMonitor_WindowChanged(IntPtr window)

SafeExamBrowser.Contracts/Browser/DownloadRequestedEventHandler.cs

@@ -8,5 +8,8 @@
 
 namespace SafeExamBrowser.Contracts.Browser
 {
+	/// <summary>
+	/// Event handler used to control (e.g. allow or prohibit) download requests.
+	/// </summary>
 	public delegate void DownloadRequestedEventHandler(string fileName, DownloadEventArgs args);
 }

SafeExamBrowser.Contracts/SafeExamBrowser.Contracts.csproj

@@ -138,6 +138,7 @@
     <Compile Include="UserInterface\Browser\IBrowserWindow.cs" />
     <Compile Include="UserInterface\MessageBox\IMessageBox.cs" />
     <Compile Include="UserInterface\IProgressIndicator.cs" />
+    <Compile Include="UserInterface\Taskbar\QuitButtonClickedEventHandler.cs" />
     <Compile Include="UserInterface\Windows\IRuntimeWindow.cs" />
     <Compile Include="UserInterface\MessageBox\MessageBoxResult.cs" />
     <Compile Include="UserInterface\Taskbar\INotificationButton.cs" />

SafeExamBrowser.Contracts/UserInterface/Taskbar/ITaskbar.cs

@@ -8,8 +8,6 @@
 
 namespace SafeExamBrowser.Contracts.UserInterface.Taskbar
 {
-	public delegate void QuitButtonClickedEventHandler();
-
 	/// <summary>
 	/// Defines the functionality of the application taskbar. The taskbar is the main user interface element via which the user can access
 	/// the browser, third-party applications, system controls and so on.

SafeExamBrowser.Contracts/UserInterface/Taskbar/QuitButtonClickedEventHandler.cs

@@ -0,0 +1,17 @@
+/*
+ * Copyright (c) 2018 ETH Zürich, Educational Development and Technology (LET)
+ * 
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+using System.ComponentModel;
+
+namespace SafeExamBrowser.Contracts.UserInterface.Taskbar
+{
+	/// <summary>
+	/// Event handler used to define the control flow when the <see cref="ITaskbar"/>'s quit button is clicked.
+	/// </summary>
+	public delegate void QuitButtonClickedEventHandler(CancelEventArgs args);
+}

SafeExamBrowser.UserInterface.Classic/Controls/QuitButton.xaml.cs

@@ -7,6 +7,7 @@
  */
 
 using System;
+using System.ComponentModel;
 using System.Windows;
 using System.Windows.Controls;
 using SafeExamBrowser.Contracts.UserInterface.Taskbar;
@@ -16,7 +17,7 @@ namespace SafeExamBrowser.UserInterface.Classic.Controls
 {
 	public partial class QuitButton : UserControl
 	{
-		public QuitButtonClickedEventHandler Clicked;
+		public event QuitButtonClickedEventHandler Clicked;
 
 		public QuitButton()
 		{
@@ -26,7 +27,7 @@ namespace SafeExamBrowser.UserInterface.Classic.Controls
 
 		private void Button_Click(object sender, RoutedEventArgs e)
 		{
-			Clicked?.Invoke();
+			Clicked?.Invoke(new CancelEventArgs());
 		}
 
 		private void LoadIcon()

SafeExamBrowser.UserInterface.Classic/Taskbar.xaml.cs

@@ -16,6 +16,7 @@ namespace SafeExamBrowser.UserInterface.Classic
 {
 	public partial class Taskbar : Window, ITaskbar
 	{
+		private bool allowClose;
 		private ILogger logger;
 
 		public event QuitButtonClickedEventHandler QuitButtonClicked;
@@ -28,7 +29,7 @@ namespace SafeExamBrowser.UserInterface.Classic
 
 			Closing += Taskbar_Closing;
 			Loaded += (o, args) => InitializeBounds();
-			QuitButton.Clicked += () => QuitButtonClicked?.Invoke();
+			QuitButton.Clicked += QuitButton_Clicked;
 		}
 
 		public void AddApplication(IApplicationButton button)
@@ -87,8 +88,21 @@ namespace SafeExamBrowser.UserInterface.Classic
 			});
 		}
 
+		private void QuitButton_Clicked(CancelEventArgs args)
+		{
+			QuitButtonClicked?.Invoke(args);
+			allowClose = !args.Cancel;
+		}
+
 		private void Taskbar_Closing(object sender, CancelEventArgs e)
 		{
+			if (!allowClose)
+			{
+				e.Cancel = true;
+
+				return;
+			}
+
 			foreach (var child in SystemControlStackPanel.Children)
 			{
 				if (child is ISystemControl systemControl)

SafeExamBrowser.UserInterface.Windows10/Controls/QuitButton.xaml.cs

@@ -6,6 +6,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/.
  */
 
+using System.ComponentModel;
 using System.Windows;
 using System.Windows.Controls;
 using SafeExamBrowser.Contracts.UserInterface.Taskbar;
@@ -23,7 +24,7 @@ namespace SafeExamBrowser.UserInterface.Windows10.Controls
 
 		private void Button_Click(object sender, RoutedEventArgs e)
 		{
-			Clicked?.Invoke();
+			Clicked?.Invoke(new CancelEventArgs());
 		}
 	}
 }

SafeExamBrowser.UserInterface.Windows10/Taskbar.xaml.cs

@@ -16,6 +16,7 @@ namespace SafeExamBrowser.UserInterface.Windows10
 {
 	public partial class Taskbar : Window, ITaskbar
 	{
+		private bool allowClose;
 		private ILogger logger;
 
 		public event QuitButtonClickedEventHandler QuitButtonClicked;
@@ -26,9 +27,9 @@ namespace SafeExamBrowser.UserInterface.Windows10
 
 			InitializeComponent();
 
-			Loaded += (o, args) => InitializeBounds();
 			Closing += Taskbar_Closing;
-			QuitButtonClicked += Taskbar_QuitButtonClicked;
+			Loaded += (o, args) => InitializeBounds();
+			QuitButtonClicked += QuitButton_Clicked;
 		}
 
 		public void AddApplication(IApplicationButton button)
@@ -87,13 +88,21 @@ namespace SafeExamBrowser.UserInterface.Windows10
 			});
 		}
 
-		private void Taskbar_QuitButtonClicked()
+		private void QuitButton_Clicked(CancelEventArgs args)
 		{
-			QuitButtonClicked?.Invoke();
+			QuitButtonClicked?.Invoke(args);
+			allowClose = !args.Cancel;
 		}
 
 		private void Taskbar_Closing(object sender, CancelEventArgs e)
 		{
+			if (!allowClose)
+			{
+				e.Cancel = true;
+
+				return;
+			}
+
 			foreach (var child in SystemControlStackPanel.Children)
 			{
 				if (child is ISystemControl systemControl)