Use IMessageCache which can create itself

src/Discord.Net.WebSocket/DiscordSocketClient.cs

@@ -66,6 +66,7 @@ namespace Discord.WebSocket
         //From DiscordSocketConfig
         internal int TotalShards { get; private set; }
         internal int MessageCacheSize { get; private set; }
+        internal IMessageCache MessageCache { get; private set; }
         internal int LargeThreshold { get; private set; }
         internal ClientState State { get; private set; }
         internal UdpSocketProvider UdpSocketProvider { get; private set; }
@@ -131,6 +132,7 @@ namespace Discord.WebSocket
             ShardId = config.ShardId ?? 0;
             TotalShards = config.TotalShards ?? 1;
             MessageCacheSize = config.MessageCacheSize;
+            MessageCache = config.MessageCache;
             LargeThreshold = config.LargeThreshold;
             UdpSocketProvider = config.UdpSocketProvider;
             WebSocketProvider = config.WebSocketProvider;

src/Discord.Net.WebSocket/DiscordSocketConfig.cs

@@ -57,6 +57,8 @@ namespace Discord.WebSocket
         /// </summary>
         public int MessageCacheSize { get; set; } = 0;
 
+        public IMessageCache MessageCache { get; set; } = null;
+
         /// <summary>
         ///     Gets or sets the max number of users a guild may have for offline users to be included in the READY
         ///     packet. The maximum value allowed is 250.

src/Discord.Net.WebSocket/Entities/Channels/SocketDMChannel.cs

@@ -36,8 +36,7 @@ namespace Discord.WebSocket
         {
             Recipient = recipient;
             recipient.GlobalUser.AddRef();
-            if (Discord.MessageCacheSize > 0)
-                _messages = new MessageCache(Discord);
+            _messages = discord.MessageCache.CreateMessageCache(discord);
         }
         internal static SocketDMChannel Create(DiscordSocketClient discord, ClientState state, Model model)
         {

src/Discord.Net.WebSocket/Entities/Channels/SocketTextChannel.cs

@@ -54,7 +54,7 @@ namespace Discord.WebSocket
             : base(discord, id, guild)
         {
             if (Discord.MessageCacheSize > 0)
-                _messages = new MessageCache(Discord);
+                _messages = discord.MessageCache.CreateMessageCache(discord);
         }
         internal new static SocketTextChannel Create(SocketGuild guild, ClientState state, Model model)
         {

src/Discord.Net.WebSocket/Entities/Messages/IMessageCache.cs

@@ -3,8 +3,10 @@ using Discord.WebSocket;
 
 namespace Discord
 {
-    internal interface IMessageCache
+    public interface IMessageCache
     {
+        public IMessageCache CreateMessageCache(DiscordSocketClient discord);
+
         public IReadOnlyCollection<SocketMessage> Messages { get; }
 
         public void Add(SocketMessage message);

src/Discord.Net.WebSocket/Entities/Messages/MessageCache.cs

@@ -6,7 +6,7 @@ using System.Linq;
 
 namespace Discord.WebSocket
 {
-    internal class MessageCache : IMessageCache
+    public class MessageCache : IMessageCache
     {
         private readonly ConcurrentDictionary<ulong, SocketMessage> _messages;
         private readonly ConcurrentQueue<ulong> _orderedMessages;
@@ -14,6 +14,11 @@ namespace Discord.WebSocket
 
         public IReadOnlyCollection<SocketMessage> Messages => _messages.ToReadOnlyCollection();
 
+        public IMessageCache CreateMessageCache(DiscordSocketClient discord)
+        {
+            return new MessageCache(discord);
+        }
+
         public MessageCache(DiscordSocketClient discord)
         {
             _size = discord.MessageCacheSize;