add anti-spam handler that removes the same message in multiple channels

CompatBot/EventHandlers/AntiSpamMessageHandler.cs

@@ -0,0 +1,61 @@
+using CompatBot.Utils.Extensions;
+using Microsoft.Extensions.Caching.Memory;
+
+namespace CompatBot.EventHandlers;
+
+public static class AntiSpamMessageHandler
+{
+    private static readonly MemoryCache MessageCache = new(new MemoryCacheOptions{ ExpirationScanFrequency = TimeSpan.FromMinutes(1) });
+    private static readonly TimeSpan DefaultExpiration = TimeSpan.FromSeconds(10);
+
+    public static async Task<bool> OnMessageCreated(DiscordClient client, MessageCreatedEventArgs args)
+    {
+        var author = args.Author;
+        if (author.IsBotSafeCheck())
+            return true;
+
+#if !DEBUG
+        if (await author.IsSmartlistedAsync(client, args.Guild).ConfigureAwait(false)
+            || await author.IsWhitelistedAsync(client, args.Guild).ConfigureAwait(false))
+            return true;
+#endif
+
+        var msg = args.Message;
+        if (msg.Content is not { Length: > 0 })
+            return true;
+
+        if (MessageCache.TryGetValue(author.Id, out var item)
+            && item is (DiscordMessage { Content.Length: >0 } lastMessage, bool isWarned)
+            && lastMessage.Content == msg.Content
+            && lastMessage.ChannelId != msg.ChannelId)
+        {
+            var removedSpam = false;
+            try
+            {
+                await msg.DeleteAsync("spam").ConfigureAwait(false);
+                removedSpam = true;
+            }
+            catch (Exception e)
+            {
+                Config.Log.Warn(e, $"Faled to delete spam message from user {author.Username} ({author.Id}) in #{msg.Channel?.Name} {msg.JumpLink}");
+            }
+            try
+            {
+                if (!isWarned)
+                {
+                    await author.SendMessageAsync("Please do not spam the same message in multiple channels. Thank you.").ConfigureAwait(false);
+                    isWarned = true;
+                }
+            }
+            catch (Exception e)
+            {
+                Config.Log.Warn(e, $"Faled to send DM to user {author.Username} ({author.Id})");
+            }
+            MessageCache.Set(author.Id, (lastMessage, isWarned), DefaultExpiration);
+            return !removedSpam; // couldn't remove, need to check with filters etc
+        }
+
+        MessageCache.Set(author.Id, (msg, false), DefaultExpiration);
+        return true;
+    }
+}

CompatBot/Program.cs

@@ -236,6 +236,7 @@ internal static class Program
                     ]));
                     config.HandleMessageCreated(new MultiEventHandlerWrapper<MessageCreatedEventArgs>(
                         [
+                            AntiSpamMessageHandler.OnMessageCreated,
                             ContentFilterMonitor.OnMessageCreated, // should be first
                             DiscordInviteFilter.OnMessageCreated,
                         ],