move `Thread` property to `IUserMessage` & add one to `SocketUserMessage`

src/Discord.Net.Core/Entities/Messages/IUserMessage.cs

@@ -17,6 +17,14 @@ namespace Discord
         IUserMessage ReferencedMessage { get; }
 
         /// <summary>
+        ///     Gets the thread that was started from this message.
+        /// </summary>
+        /// <returns>
+        ///    An <see cref="IThreadChannel"/> object if this message has thread attached; otherwise <see langword="null"/>.
+        /// </returns>
+        IThreadChannel Thread { get; }
+
+        /// <summary>
         ///     Modifies this message.
         /// </summary>
         /// <remarks>

src/Discord.Net.Rest/Entities/Messages/RestMessage.cs

@@ -80,15 +80,6 @@ namespace Discord.Rest
         /// <inheritdoc/>
         public MessageType Type { get; private set; }
 
-
-        /// <summary>
-        ///     Gets the thread that was started from this message.
-        /// </summary>
-        /// <returns>
-        ///    A <see cref="RestThreadChannel"/> object if this message has thread attached; otherwise <see langword="null"/>.
-        /// </returns>
-        public RestThreadChannel Thread { get; private set; }
-
         /// <inheritdoc />
         public MessageRoleSubscriptionData RoleSubscriptionData { get; private set; }
 
@@ -264,11 +255,6 @@ namespace Discord.Rest
                     model.RoleSubscriptionData.Value.MonthsSubscribed,
                     model.RoleSubscriptionData.Value.IsRenewal);
             }
-
-            if (model.Thread.IsSpecified)
-            {
-                Thread = RestThreadChannel.Create(Discord, new RestGuild(Discord, model.Thread.Value.GuildId.Value), model.Thread.Value);
-            }
         }
         /// <inheritdoc />
         public async Task UpdateAsync(RequestOptions options = null)

src/Discord.Net.Rest/Entities/Messages/RestUserMessage.cs

@@ -47,6 +47,12 @@ namespace Discord.Rest
         /// <inheritdoc />
         public IUserMessage ReferencedMessage => _referencedMessage;
 
+        /// <inheritdoc cref="IUserMessage.Thread"/>
+        public RestThreadChannel Thread { get; private set; }
+
+        /// <inheritdoc />
+        IThreadChannel IUserMessage.Thread => Thread;
+
         internal RestUserMessage(BaseDiscordClient discord, ulong id, IMessageChannel channel, IUser author, MessageSource source)
             : base(discord, id, channel, author, source)
         {
@@ -130,6 +136,9 @@ namespace Discord.Rest
                 else
                     _stickers = ImmutableArray.Create<StickerItem>();
             }
+
+            if(model.Thread.IsSpecified)
+                Thread = RestThreadChannel.Create(Discord, guild, model.Thread.Value);
         }
 
         /// <inheritdoc />

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

@@ -49,6 +49,12 @@ namespace Discord.WebSocket
         /// <inheritdoc />
         public IUserMessage ReferencedMessage => _referencedMessage;
 
+        /// <inheritdoc cref="IUserMessage.Thread"/>
+        public SocketThreadChannel Thread { get; private set; }
+
+        /// <inheritdoc />
+        IThreadChannel IUserMessage.Thread => Thread;
+
         internal SocketUserMessage(DiscordSocketClient discord, ulong id, ISocketMessageChannel channel, SocketUser author, MessageSource source)
             : base(discord, id, channel, author, source)
         {
@@ -172,6 +178,11 @@ namespace Discord.WebSocket
                 else
                     _stickers = ImmutableArray.Create<SocketSticker>();
             }
+
+            if (model.Thread.IsSpecified)
+            {
+                Thread = SocketThreadChannel.Create(guild, state, model.Thread.Value);
+            }
         }
 
         /// <inheritdoc />