Implemented passing the gateway URL down from DiscordShardedClient.

src/Discord.Net.WebSocket/DiscordShardedClient.cs

@@ -139,9 +139,9 @@ namespace Discord.WebSocket
 
         internal override async Task OnLoginAsync(TokenType tokenType, string token)
         {
+            var botGateway = await GetBotGatewayAsync().ConfigureAwait(false);
             if (_automaticShards)
             {
-                var botGateway = await GetBotGatewayAsync().ConfigureAwait(false);
                 _shardIds = Enumerable.Range(0, botGateway.Shards).ToArray();
                 _totalShards = _shardIds.Length;
                 _shards = new DiscordSocketClient[_shardIds.Length];
@@ -163,7 +163,12 @@ namespace Discord.WebSocket
 
             //Assume thread safe: already in a connection lock
             for (int i = 0; i < _shards.Length; i++)
+            {
+                // Set the gateway URL to the one returned by Discord, if a custom one isn't set.
+                _shards[i].ApiClient.GatewayUrl = botGateway.Url;
+
                 await _shards[i].LoginAsync(tokenType, token);
+            }
 
             if(_defaultStickers.Length == 0 && _baseConfig.AlwaysDownloadDefaultStickers)
                 await DownloadDefaultStickersAsync().ConfigureAwait(false);
@@ -175,7 +180,12 @@ namespace Discord.WebSocket
             if (_shards != null)
             {
                 for (int i = 0; i < _shards.Length; i++)
+                {
+                    // Reset the gateway URL set for the shard.
+                    _shards[i].ApiClient.GatewayUrl = null;
+
                     await _shards[i].LogoutAsync();
+                }
             }
 
             if (_automaticShards)

src/Discord.Net.WebSocket/DiscordSocketApiClient.cs

@@ -38,6 +38,24 @@ namespace Discord.API
 
         public ConnectionState ConnectionState { get; private set; }
 
+        /// <summary>
+        ///     Sets the gateway URL used for identifies.
+        /// </summary>
+        /// <remarks>
+        ///     If a custom URL is set, setting this property does nothing.
+        /// </remarks>
+        public string GatewayUrl
+        {
+            set
+            {
+                // Makes the sharded client not override the custom value.
+                if (_isExplicitUrl)
+                    return;
+
+                _gatewayUrl = FormatGatewayUrl(value);
+            }
+        }
+
         /// <summary>
         ///     Sets the gateway URL used for resumes.
         /// </summary>
@@ -214,16 +232,13 @@ namespace Discord.API
                 string gatewayUrl;
                 if (_resumeGatewayUrl == null)
                 {
-                    if (!_isExplicitUrl)
+                    if (!_isExplicitUrl && _gatewayUrl == null)
                     {
-                        // TODO: pass this down from DiscordShardedClient, so that it's not requested separately for every single shard
                         var gatewayResponse = await GetBotGatewayAsync().ConfigureAwait(false);
-                        gatewayUrl = _gatewayUrl = FormatGatewayUrl(gatewayResponse.Url);
-                    }
-                    else
-                    {
-                        gatewayUrl = _gatewayUrl;
+                        _gatewayUrl = FormatGatewayUrl(gatewayResponse.Url);
                     }
+
+                    gatewayUrl = _gatewayUrl;
                 }
                 else
                 {