a lot of changes

added:
- channel flags
- `ForumTagBuilder`
- imroved channel modification

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

@@ -0,0 +1,22 @@
+namespace Discord;
+
+/// <summary>
+///     Represents public flags for a channel.
+/// </summary>
+public enum ChannelFlags
+{
+    /// <summary> 
+    ///     Default value for flags, when none are given to a channel.
+    /// </summary>
+    None = 0,
+
+    /// <summary>
+    ///      Flag given to a thread channel pinned on top of parent forum channel.
+    /// </summary>
+    Pinned = 1 << 1,
+
+    /// <summary>
+    ///     Flag given to a forum channel that requires people to select tags when posting.
+    /// </summary>
+    RequireTag = 1 << 4
+}

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

@@ -0,0 +1,50 @@
+using System;
+using System.Collections.Generic;
+
+namespace Discord;
+
+public class ForumChannelProperties : TextChannelProperties
+{
+
+    /// <summary>
+    ///     Gets or sets the topic of the channel.
+    /// </summary>
+    /// <remarks>
+    ///     Not available in forum channels.
+    /// </remarks>
+    public new Optional<int> SlowModeInterval { get; }
+
+    /// <summary>
+    /// Gets or sets rate limit on creating posts in this forum channel.
+    /// </summary>
+    /// <remarks>
+    ///     Setting this value to anything above zero will require each user to wait X seconds before
+    ///     creating another thread; setting this value to <c>0</c> will disable rate limits for this channel.
+    ///     <note>
+    ///         Users with <see cref="Discord.ChannelPermission.ManageMessages"/> or 
+    ///         <see cref="ChannelPermission.ManageChannels"/> will be exempt from rate limits.
+    ///     </note>
+    /// </remarks>
+    /// <exception cref="ArgumentOutOfRangeException">Thrown if the value does not fall within [0, 21600].</exception>
+    public Optional<int> ThreadCreationInterval { get; set; }
+
+
+    /// <summary>
+    /// Gets or sets the default slow-mode for threads in this channel.
+    /// </summary>
+    /// <remarks>
+    ///     Setting this value to anything above zero will require each user to wait X seconds before
+    ///     sending another message; setting this value to <c>0</c> will disable slow-mode for child threads.
+    ///     <note>
+    ///         Users with <see cref="Discord.ChannelPermission.ManageMessages"/> or 
+    ///         <see cref="ChannelPermission.ManageChannels"/> will be exempt from slow-mode.
+    ///     </note>
+    /// </remarks>
+    /// <exception cref="ArgumentOutOfRangeException">Thrown if the value does not fall within [0, 21600].</exception>
+    public Optional<int> DefaultSlowModeInterval { get; set; }
+
+    /// <summary>
+    /// Gets or sets a collection of tags inside of this forum channel.
+    /// </summary>
+    public Optional<IEnumerable<ForumTagProperties>> Tags { get; set; }
+}

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

@@ -36,5 +36,10 @@ namespace Discord
         ///     Gets or sets the permission overwrites for this channel.
         /// </summary>
         public Optional<IEnumerable<Overwrite>> PermissionOverwrites { get; set; }
+
+        /// <summary>
+        ///     Gets or sets the flags of the channel.
+        /// </summary>
+        public Optional<ChannelFlags> Flags { get; set; }
     }
 }

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

@@ -53,6 +53,20 @@ namespace Discord
         /// </returns>
         int DefaultSlowModeInterval { get; }
 
+        /// <summary>
+        ///     Modifies this forum channel.
+        /// </summary>
+        /// <remarks>
+        ///     This method modifies the current forum channel with the specified properties. To see an example of this
+        ///     method and what properties are available, please refer to <see cref="ForumChannelProperties"/>.
+        /// </remarks>
+        /// <param name="func">The delegate containing the properties to modify the channel with.</param>
+        /// <param name="options">The options to be used when sending the request.</param>
+        /// <returns>
+        ///     A task that represents the asynchronous modification operation.
+        /// </returns>
+        Task ModifyAsync(Action<ForumChannelProperties> func, RequestOptions options = null);
+
         /// <summary>
         ///     Creates a new post (thread) within the forum.
         /// </summary>

src/Discord.Net.Core/Entities/ForumTag.cs → src/Discord.Net.Core/Entities/ForumTags/ForumTag.cs

@@ -4,6 +4,8 @@ using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
 
+#nullable enable
+
 namespace Discord
 {
     /// <summary>
@@ -24,23 +26,27 @@ namespace Discord
         /// <summary>
         ///     Gets the emoji of the tag or <see langword="null"/> if none is set.
         /// </summary>
-        public IEmote Emoji { get; }
+        /// <remarks>
+        ///     If the emoji is <see cref="Emote"/> only the <see cref="Emote.Id"/> will be populated.
+        ///     Use <see cref="IGuild.GetEmoteAsync"/> to get the emoji.
+        /// </remarks>
+        public IEmote? Emoji { get; }
 
         /// <summary>
         /// Gets whether this tag can only be added to or removed from threads by a member
         /// with the <see cref="GuildPermissions.ManageThreads"/> permission
         /// </summary>
-        public bool Moderated { get; }
+        public bool IsModerated { get; }
 
         /// <summary>
         /// Gets when the tag was created.
         /// </summary>
         public DateTimeOffset CreatedAt => SnowflakeUtils.FromSnowflake(Id);
 
-        internal ForumTag(ulong id, string name, ulong? emojiId, string emojiName, bool moderated)
+        internal ForumTag(ulong id, string name, ulong? emojiId, string? emojiName, bool moderated)
         {
             if (emojiId.HasValue && emojiId.Value != 0)
-                Emoji = new Emote(emojiId.Value, emojiName, false);
+                Emoji = new Emote(emojiId.Value, null, false);
             else if (emojiName != null)
                 Emoji = new Emoji(name);
             else
@@ -48,7 +54,7 @@ namespace Discord
 
             Id = id;
             Name = name;
-            Moderated = moderated;
+            IsModerated = moderated;
         }
     }
 }

src/Discord.Net.Core/Entities/ForumTags/ForumTagBuilder.cs

@@ -0,0 +1,128 @@
+#nullable enable
+using System;
+
+namespace Discord;
+
+public class ForumTagBuilder
+{
+    private string? _name;
+    private IEmote? _emoji;
+    private bool _moderated;
+
+    /// <summary>
+    ///     Returns the maximum length of name allowed by Discord.
+    /// </summary>
+    public const int MaxNameLength = 20;
+
+    /// <summary>
+    ///     Gets or sets the name of the tag.
+    /// </summary>
+    /// <exception cref="ArgumentException">Name length must be less than or equal to <see cref="MaxNameLength"/>.</exception>
+    public string? Name
+    {
+        get { return _name; }
+        set
+        {
+            if (value?.Length > MaxNameLength)
+                throw new ArgumentException(message: $"Name length must be less than or equal to {MaxNameLength}.", paramName: nameof(Name));
+            _name = value;
+        }
+    }
+
+    /// <summary>
+    ///     Gets or sets the emoji of the tag.
+    /// </summary>
+    public IEmote? Emoji
+    {
+        get { return _emoji; }
+        set { _emoji = value; }
+    }
+
+    /// <summary>
+    /// Gets or sets whether this tag can only be added to or removed from threads by a member
+    /// with the <see cref="GuildPermissions.ManageThreads"/> permission
+    /// </summary>
+    public bool IsModerated
+    {
+        get { return _moderated; }
+        set { _moderated = value; }
+    }
+
+    /// <summary>
+    /// Initializes a new <see cref="ForumTagBuilder"/> class.
+    /// </summary>
+    public ForumTagBuilder()
+    {
+
+    }
+
+    /// <summary>
+    /// Initializes a new <see cref="ForumTagBuilder"/> class with values
+    /// </summary>
+    public ForumTagBuilder(string name)
+    {
+        Name = name;
+        IsModerated = false;
+    }
+
+    /// <summary>
+    /// Initializes a new <see cref="ForumTagBuilder"/> class with values
+    /// </summary>
+    public ForumTagBuilder(string name, IEmote? emoji = null, bool moderated = false)
+    {
+        Name = name;
+        Emoji = emoji;
+        IsModerated = moderated;
+    }
+
+    /// <summary>
+    /// Initializes a new <see cref="ForumTagBuilder"/> class with values
+    /// </summary>
+    public ForumTagBuilder(string name, ulong? emoteId = null, bool moderated = false)
+    {
+        Name = name;
+        if(emoteId is not null)
+            Emoji = new Emote(emoteId.Value, string.Empty,  false);
+        IsModerated = moderated;
+    }
+
+    /// <summary>
+    /// Builds the Tag.
+    /// </summary>
+    /// <returns>An instance of <see cref="ForumTagProperties"/></returns>
+    /// <exception cref="ArgumentNullException">"Name must be set to build the tag"</exception>
+    public ForumTagProperties Build()
+    {
+        if (_name is null)
+            throw new ArgumentNullException(nameof(Name), "Name must be set to build the tag");
+        return new ForumTagProperties(_name!, _emoji, _moderated);
+    }
+
+    /// <summary>
+    /// Sets the name of the tag.
+    /// </summary>
+    /// <exception cref="ArgumentException">Name length must be less than or equal to <see cref="MaxNameLength"/>.</exception>
+    public ForumTagBuilder WithName(string name)
+    {
+        Name = name;
+        return this;
+    }
+
+    /// <summary>
+    /// Sets the emoji of the tag.
+    /// </summary>
+    public ForumTagBuilder WithEmoji(IEmote? emoji)
+    {
+        Emoji = emoji;
+        return this;
+    }
+
+    /// <summary>
+    /// Sets the IsModerated of the tag.
+    /// </summary>
+    public ForumTagBuilder WithModerated(bool moderated)
+    {
+        IsModerated = moderated;
+        return this;
+    }
+}

src/Discord.Net.Core/Entities/ForumTags/ForumTagBuilderExtensions.cs

@@ -0,0 +1,11 @@
+namespace Discord;
+
+public static class ForumTagBuilderExtensions
+{
+    public static ForumTagBuilder ToForumTagBuilder(this ForumTag tag)
+        => new ForumTagBuilder(tag.Name, tag.Emoji, tag.IsModerated);
+
+    public static ForumTagBuilder ToForumTagBuilder(this ForumTagProperties tag)
+        => new ForumTagBuilder(tag.Name, tag.Emoji, tag.IsModerated);
+
+}

src/Discord.Net.Core/Entities/ForumTags/ForumTagProperties.cs

@@ -0,0 +1,29 @@
+namespace Discord;
+
+#nullable enable
+
+public class ForumTagProperties
+{
+    /// <summary>
+    ///     Gets the name of the tag.
+    /// </summary>
+    public string Name { get; }
+
+    /// <summary>
+    ///     Gets the emoji of the tag or <see langword="null"/> if none is set.
+    /// </summary>
+    public IEmote? Emoji { get; }
+
+    /// <summary>
+    /// Gets whether this tag can only be added to or removed from threads by a member
+    /// with the <see cref="GuildPermissions.ManageThreads"/> permission
+    /// </summary>
+    public bool IsModerated { get; }
+
+    internal ForumTagProperties(string name, IEmote? emoji = null, bool isMmoderated = false)
+    {
+        Name = name;
+        Emoji = emoji;
+        IsModerated = isMmoderated;
+    }
+}

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

@@ -0,0 +1,21 @@
+using Newtonsoft.Json;
+
+namespace Discord.API
+{
+    [JsonObject(MemberSerialization = MemberSerialization.OptIn)]
+    internal class ForumTagParams
+    {
+
+        [JsonProperty("name")]
+        public string Name { get; set; }
+
+        [JsonProperty("emoji_id")]
+        public Optional<ulong?> EmojiId { get; set; }
+
+        [JsonProperty("emoji_name")]
+        public Optional<string> EmojiName { get; set; }
+
+        [JsonProperty("moderated")]
+        public bool Moderated { get; set; }
+    }
+}

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

@@ -0,0 +1,17 @@
+using Newtonsoft.Json;
+
+namespace Discord.API.Rest;
+
+
+[JsonObject(MemberSerialization = MemberSerialization.OptIn)]
+internal class ModifyForumChannelParams : ModifyTextChannelParams
+{
+    [JsonProperty("available_tags")]
+    public Optional<ForumTagParams[]> Tags { get; set; }
+
+    [JsonProperty("default_thread_rate_limit_per_user")]
+    public Optional<int> DefaultSlowModeInterval { get; set; }
+
+    [JsonProperty("rate_limit_per_user")]
+    public Optional<int> ThreadCreationInterval { get; set; }
+}

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

@@ -13,5 +13,7 @@ namespace Discord.API.Rest
         public Optional<ulong?> CategoryId { get; set; }
         [JsonProperty("permission_overwrites")]
         public Optional<Overwrite[]> Overwrites { get; set; }
+        [JsonProperty("flags")]
+        public Optional<ChannelFlags?> Flags { get; set; }
     }
 }

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

@@ -22,5 +22,8 @@ namespace Discord.API.Rest
 
         [JsonProperty("applied_tags")]
         public Optional<IEnumerable<ulong>> AppliedTags { get; set; }
+
+        [JsonProperty("flags")]
+        public Optional<ChannelFlags> Flags { get; set; }
     }
 }

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

@@ -38,6 +38,7 @@ namespace Discord.Rest
                         Deny = overwrite.Permissions.DenyValue.ToString()
                     }).ToArray()
                     : Optional.Create<API.Overwrite[]>(),
+                Flags = args.Flags.GetValueOrDefault(),
             };
             return await client.ApiClient.ModifyGuildChannelAsync(channel.Id, apiArgs, options).ConfigureAwait(false);
         }

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

@@ -0,0 +1,49 @@
+using System;
+using System.Linq;
+using System.Threading.Tasks;
+using Model = Discord.API.Channel;
+
+namespace Discord.Rest;
+
+internal static class ForumHelper
+{
+    public static async Task<Model> ModifyAsync(IForumChannel channel, BaseDiscordClient client,
+        Action<ForumChannelProperties> func,
+        RequestOptions options)
+    {
+        var args = new ForumChannelProperties();
+        func(args);
+        var apiArgs = new API.Rest.ModifyForumChannelParams()
+        {
+            Name = args.Name,
+            Position = args.Position,
+            CategoryId = args.CategoryId,
+            Overwrites = args.PermissionOverwrites.IsSpecified
+                ? args.PermissionOverwrites.Value.Select(overwrite => new API.Overwrite
+                {
+                    TargetId = overwrite.TargetId,
+                    TargetType = overwrite.TargetType,
+                    Allow = overwrite.Permissions.AllowValue.ToString(),
+                    Deny = overwrite.Permissions.DenyValue.ToString()
+                }).ToArray()
+                : Optional.Create<API.Overwrite[]>(),
+            DefaultSlowModeInterval = args.DefaultSlowModeInterval,
+            ThreadCreationInterval = args.ThreadCreationInterval,
+            Tags = args.Tags.IsSpecified
+                ? args.Tags.Value.Select(tag => new API.ForumTagParams
+                {
+                    Name = tag.Name,
+                    EmojiId = tag.Emoji is Emote emote
+                        ? emote.Id
+                        : Optional<ulong?>.Unspecified,
+                    EmojiName = tag.Emoji is Emoji emoji
+                        ? emoji.Name
+                        : Optional<string>.Unspecified
+                }).ToArray()
+                : Optional.Create<API.ForumTagParams[]>(),
+            Flags = args.Flags.GetValueOrDefault(),
+            Topic = args.Topic,
+        };
+        return await client.ApiClient.ModifyGuildChannelAsync(channel.Id, apiArgs, options).ConfigureAwait(false);
+    }
+}

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

@@ -58,7 +58,8 @@ namespace Discord.Rest
                 AutoArchiveDuration = args.AutoArchiveDuration,
                 Locked = args.Locked,
                 Slowmode = args.SlowModeInterval,
-                AppliedTags = args.AppliedTags
+                AppliedTags = args.AppliedTags,
+                Flags = args.Flags,
             };
             return await client.ApiClient.ModifyThreadAsync(channel.Id, apiArgs, options).ConfigureAwait(false);
         }

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

@@ -63,6 +63,10 @@ namespace Discord.WebSocket
             ).ToImmutableArray();
         }
 
+        /// <inheritdoc />
+        public virtual Task ModifyAsync(Action<ForumChannelProperties> func, RequestOptions options = null)
+            => ForumHelper.ModifyAsync(this, Discord, func, options);
+
         /// <inheritdoc cref="IForumChannel.CreatePostAsync(string, ThreadArchiveDuration, int?, string, Embed, RequestOptions, AllowedMentions, MessageComponent, ISticker[], Embed[], MessageFlags)"/>
         public Task<RestThreadChannel> CreatePostAsync(string title, ThreadArchiveDuration archiveDuration = ThreadArchiveDuration.OneDay, int? slowmode = null, string text = null, Embed embed = null, RequestOptions options = null, AllowedMentions allowedMentions = null, MessageComponent components = null, ISticker[] stickers = null, Embed[] embeds = null, MessageFlags flags = MessageFlags.None)
             => ThreadHelper.CreatePostAsync(this, Discord, title, archiveDuration, slowmode, text, embed, options, allowedMentions, components, stickers, embeds, flags);