Add Message comparer.

src/Discord.Net.Core/Utils/Comparers.cs

@@ -10,11 +10,13 @@ namespace Discord
         public static IEqualityComparer<IGuild> GuildComparer => _guildComparer ?? (_guildComparer = new EntityEqualityComparer<IGuild, ulong>());
         public static IEqualityComparer<IChannel> ChannelComparer => _channelComparer ?? (_channelComparer = new EntityEqualityComparer<IChannel, ulong>());
         public static IEqualityComparer<IRole> RoleComparer => _roleComparer ?? (_roleComparer = new EntityEqualityComparer<IRole, ulong>());
+        public static IEqualityComparer<IMessage> MessageComparer => _messageComparer ?? (_messageComparer = new MessageEqualityComparer());
 
         private static IEqualityComparer<IUser> _userComparer;
         private static IEqualityComparer<IGuild> _guildComparer;
         private static IEqualityComparer<IChannel> _channelComparer;
         private static IEqualityComparer<IRole> _roleComparer;
+        private static IEqualityComparer<IMessage> _messageComparer;
 
         private sealed class EntityEqualityComparer<TEntity, TId> : EqualityComparer<TEntity>
             where TEntity : IEntity<TId>
@@ -39,5 +41,27 @@ namespace Discord
                 return obj?.Id.GetHashCode() ?? 0;
             }
         }
+
+        private sealed class MessageEqualityComparer : EqualityComparer<IMessage>
+        {
+            public override bool Equals(IMessage x, IMessage y)
+            {
+                bool xNull = x == null;
+                bool yNull = y == null;
+
+                if (xNull && yNull)
+                    return true;
+
+                if (xNull ^ yNull)
+                    return false;
+
+                return x.Channel.Id.Equals(y.Channel.Id) && x.Id.Equals(y.Id);
+            }
+
+            public override int GetHashCode(IMessage obj)
+            {
+                return obj?.Id.GetHashCode() ?? 0;
+            }
+        }
     }
 }