Add experimental debug-only file association for Multiplayer saves

Source/Client/Settings/MpSettingsUI.cs

@@ -93,6 +93,17 @@ public static void DoGeneralSettings(MpSettings settings, Rect inRect, Rect page
             listing.TextFieldNumericLabeled("Desync radius:  ", ref settings.desyncTracesRadius, ref desyncRadiusBuffer, 1f,
                 200f);
 
+            if (MpVersion.IsDebug && FileAssoc.IsSupported())
+            {
+                if (FileAssoc.IsRegistered())
+                {
+                    if (listing.ButtonText("Remove file associations")) FileAssoc.Remove();
+                }
+                else
+                {
+                    if (listing.ButtonText("Register file associations")) FileAssoc.Register();
+                }
+            }
 #if DEBUG
             using (MpStyle.Set(TextAnchor.MiddleCenter))
                 if (listing.ButtonTextLabeled("Desync tracing mode", settings.desyncTracingMode.ToString()))

Source/Client/Util/FileAssoc.cs

@@ -0,0 +1,77 @@
+#nullable enable
+using System;
+using System.Diagnostics;
+using System.IO;
+using Microsoft.Win32;
+using UnityEngine;
+
+namespace Multiplayer.Client.Util;
+
+/// Only Windows support for now. Calling methods of this class from another OS is a no-op.
+public static class FileAssoc
+{
+    public const string ReplayProgId = "RimworldMultiplayer.Replay.1";
+    public const string ReplayExtension = ".rwmts"; // rimworld multiplayer save
+
+    // Create the associations only for the current user.
+    // RimWorld is not installed system-wide, so this works fine and avoids requiring admin rights.
+    private static RegistryKey GetAssociationsRootKey() =>
+        CreateSubKeyOrThrow(Registry.CurrentUser, @"Software\Classes");
+
+    public static void Register()
+    {
+        if (!IsSupported()) return;
+        var appPath = Process.GetCurrentProcess().MainModule?.FileName ?? Environment.GetCommandLineArgs()[0];
+        appPath = Path.GetFullPath(appPath);
+        RegisterApp(appPath);
+        RegisterExtension(ReplayExtension);
+    }
+
+    public static void Remove()
+    {
+        if (!IsSupported()) return;
+        var rootKey = GetAssociationsRootKey();
+        rootKey.DeleteSubKeyTree(ReplayProgId, throwOnMissingSubKey: false);
+        rootKey.DeleteSubKey(ReplayExtension, throwOnMissingSubKey: false);
+    }
+
+    public static bool IsRegistered()
+    {
+        if (!IsSupported()) return false;
+        var rootKey = GetAssociationsRootKey();
+        return rootKey.OpenSubKey(ReplayProgId) != null || rootKey.OpenSubKey(ReplayExtension) != null;
+    }
+
+    public static bool IsSupported() => Application.platform == RuntimePlatform.WindowsPlayer;
+
+    private static void RegisterApp(string appPath)
+    {
+        // https://learn.microsoft.com/en-us/windows/win32/shell/fa-progids
+        var rootKey = GetAssociationsRootKey();
+        using var progIdKey = CreateSubKeyOrThrow(rootKey, ReplayProgId);
+        progIdKey.SetValue("", "RimWorld Multiplayer Save");
+        progIdKey.SetValue("FriendlyTypeName", "RimWorld Multiplayer Save");
+
+        using (var defaultIcon = CreateSubKeyOrThrow(progIdKey, "DefaultIcon"))
+        {
+            defaultIcon.SetValue("", $"\"{appPath}\",0");
+        }
+
+        using (var command = CreateSubKeyOrThrow(progIdKey, @"shell\open\command"))
+        {
+            command.SetValue("", $"\"{appPath}\" -{MultiplayerStatic.MpHostReplayCmdLineArg}=\"%1\"",
+                RegistryValueKind.ExpandString);
+        }
+    }
+
+    private static void RegisterExtension(string ext)
+    {
+        if (!ext.StartsWith(".")) ext = "." + ext;
+        using var extKey = CreateSubKeyOrThrow(GetAssociationsRootKey(), ext);
+        extKey.SetValue("", ReplayProgId);
+    }
+
+    private static RegistryKey CreateSubKeyOrThrow(RegistryKey parent, string subkey) =>
+        parent.CreateSubKey(subkey) ??
+        throw new InvalidOperationException($"Cannot access registry key {parent.Name}\\{subkey}");
+}