update TextChannel properties to include a Type optional parameter with validation

as of writing, this feature is still only available to verified guilds, which makes it impossible for testing.

src/Discord.Net.Core/Entities/Channels/TextChannelProperties.cs

@@ -38,5 +38,15 @@ namespace Discord
         /// </remarks>
         /// <exception cref="ArgumentOutOfRangeException">Thrown if the value does not fall within [0, 120].</exception>
         public Optional<int> SlowModeInterval { get; set; }
+
+        /// <summary>
+        ///     Gets or sets the type of channel.
+        ///     Only setting to a <see cref="ChannelType.News"/> is supported.
+        /// </summary>
+        /// <remarks>
+        ///     Setting this value to a different type will change the type of channel that is associated with this Id.
+        /// </remarks>
+        /// <exception cref="ArgumentException">Thrown if the type of channel that is being set is not valid.</exception>
+        public Optional<ChannelType> Type { get; set; }
     }
 }

src/Discord.Net.Rest/API/Rest/ModifyTextChannelParams.cs

@@ -12,5 +12,7 @@ namespace Discord.API.Rest
         public Optional<bool> IsNsfw { get; set; }
         [JsonProperty("rate_limit_per_user")]
         public Optional<int> SlowModeInterval { get; set; }
+        [JsonProperty("type")]
+        public Optional<ChannelType> Type { get; set; }
     }
 }

src/Discord.Net.Rest/DiscordRestApiClient.cs

@@ -377,6 +377,15 @@ namespace Discord.API
             Preconditions.NotNullOrEmpty(args.Name, nameof(args.Name));
             Preconditions.AtLeast(args.SlowModeInterval, 0, nameof(args.SlowModeInterval));
             Preconditions.AtMost(args.SlowModeInterval, 120, nameof(args.SlowModeInterval));
+            if (args.Type.IsSpecified)
+            {
+                // can only change Text/NewsChannel into a Text or News
+                var warn = "You may not change a Text or News channel into a Voice, Group, DM, or Category channel.";
+                Preconditions.NotEqual((uint)args.Type.Value, (uint)ChannelType.Voice, nameof(args.Type), warn);
+                Preconditions.NotEqual((uint)args.Type.Value, (uint)ChannelType.Group, nameof(args.Type), warn);
+                Preconditions.NotEqual((uint)args.Type.Value, (uint)ChannelType.DM, nameof(args.Type), warn);
+                Preconditions.NotEqual((uint)args.Type.Value, (uint)ChannelType.Category, nameof(args.Type), warn);
+            }
             options = RequestOptions.CreateOrClone(options);
 
             var ids = new BucketIds(channelId: channelId);

src/Discord.Net.Rest/Entities/Channels/ChannelHelper.cs

@@ -46,6 +46,7 @@ namespace Discord.Rest
                 Topic = args.Topic,
                 IsNsfw = args.IsNsfw,
                 SlowModeInterval = args.SlowModeInterval,
+                Type = args.Type
             };
             return await client.ApiClient.ModifyGuildChannelAsync(channel.Id, apiArgs, options).ConfigureAwait(false);
         }

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

@@ -19,5 +19,6 @@ namespace Discord.WebSocket
             entity.Update(state, model);
             return entity;
         }
+        //TODO: Need to set custom channel properties for this type, as apparently it does not support slow mode or overwrites.
     }
 }