Log crashes and tell the user instead of vanishing

DesktopClock/App.xaml.cs

@@ -18,6 +18,7 @@ public partial class App : Application
 
     protected override void OnStartup(StartupEventArgs e)
     {
+        CrashHandler.Register(this);
         base.OnStartup(e);
         ThemeManager.Initialize();
 

DesktopClock/Properties/Settings.cs

@@ -85,6 +85,23 @@ private Settings()
     /// </remarks>
     public static bool CanBeSaved { get; private set; }
 
+    /// <summary>
+    /// Saves one last time if the settings were loaded and can be saved, without throwing, for when the app is about to close after a crash.
+    /// </summary>
+    public static void TrySaveIfLoaded()
+    {
+        if (!_default.IsValueCreated || !CanBeSaved)
+            return;
+
+        try
+        {
+            Default.Save();
+        }
+        catch
+        {
+        }
+    }
+
     /// <summary>
     /// Checks if the settings file exists on the disk.
     /// </summary>

DesktopClock/Utilities/CrashHandler.cs

@@ -0,0 +1,93 @@
+using System;
+using System.IO;
+using System.Runtime.InteropServices;
+using System.Threading;
+using System.Windows;
+using DesktopClock.Properties;
+
+namespace DesktopClock.Utilities;
+
+/// <summary>
+/// Handles crashes so the clock doesn't just vanish: it saves settings one last time, writes the error to a log file, tells the user where it is, and then exits.
+/// </summary>
+public static class CrashHandler
+{
+    /// <summary>
+    /// The log starts over once it grows past this, so repeated crashes can't fill the disk.
+    /// </summary>
+    private const long MaxLogSize = 1024 * 1024;
+
+    private static int _handling;
+
+    /// <summary>
+    /// Handles unhandled exceptions on the UI thread and on background threads, such as the clock's timer.
+    /// </summary>
+    public static void Register(Application app)
+    {
+        app.DispatcherUnhandledException += (_, e) => HandleAndExit(e.Exception);
+        AppDomain.CurrentDomain.UnhandledException += (_, e) => HandleAndExit(e.ExceptionObject as Exception);
+    }
+
+    private static void HandleAndExit(Exception exception)
+    {
+        // Only the first crash is handled; another one while it's being handled exits right away instead of stacking up dialogs.
+        if (Interlocked.Exchange(ref _handling, 1) == 0)
+        {
+            Settings.TrySaveIfLoaded();
+            var logPath = TryWriteLog(exception);
+            ShowMessage(logPath);
+        }
+
+        // Staying open in an unknown state could leave a clock that looks fine but has stopped, so always close.
+        Environment.Exit(1);
+    }
+
+    /// <summary>
+    /// Appends the error to a log next to the app, or in the temp folder if that one can't be written to.
+    /// </summary>
+    /// <returns>The path of the log that was written, or <c>null</c> if neither could be.</returns>
+    private static string TryWriteLog(Exception exception)
+    {
+        var fileName = Path.GetFileNameWithoutExtension(App.MainFileInfo.Name) + ".log";
+        var entry =
+            $"{DateTimeOffset.Now:yyyy-MM-dd HH:mm:ss zzz}\r\n" +
+            $"{App.MainFileDisplayText} on {RuntimeInformation.OSDescription.Trim()}, {RuntimeInformation.FrameworkDescription}\r\n" +
+            $"{exception}\r\n\r\n";
+
+        foreach (var folder in new[] { App.MainFileInfo.DirectoryName, Path.GetTempPath() })
+        {
+            try
+            {
+                var path = Path.Combine(folder, fileName);
+
+                if (File.Exists(path) && new FileInfo(path).Length > MaxLogSize)
+                    File.Delete(path);
+
+                File.AppendAllText(path, entry);
+                return path;
+            }
+            catch
+            {
+            }
+        }
+
+        return null;
+    }
+
+    private static void ShowMessage(string logPath)
+    {
+        try
+        {
+            var details = logPath == null ? "" : $"Details were saved to:\n{logPath}\n\n";
+
+            MessageBox.Show(
+                "DesktopClock ran into a problem and needs to close.\n\n" +
+                details +
+                "Please report it at https://github.com/danielchalmers/DesktopClock/issues and include that file.",
+                "DesktopClock", MessageBoxButton.OK, MessageBoxImage.Error);
+        }
+        catch
+        {
+        }
+    }
+}