feature: Add inline replies (discord-net#1659)

* Add inline replies

* Missed a few things

* Change xml docs, IUserMessage, and other changes

* Missed one when changing

* Fix referencedMessage author

src/Discord.Net.Commands/ModuleBase.cs

@@ -35,9 +35,10 @@ public abstract class ModuleBase<T> : IModuleBase
         ///     Specifies if notifications are sent for mentioned users and roles in the <paramref name="message"/>.
         ///     If <c>null</c>, all mentioned roles and users will be notified.
         /// </param>
-        protected virtual async Task<IUserMessage> ReplyAsync(string message = null, bool isTTS = false, Embed embed = null, RequestOptions options = null, AllowedMentions allowedMentions = null)
+        /// <param name="messageReference">The message references to be included. Used to reply to specific messages.</param>
+        protected virtual async Task<IUserMessage> ReplyAsync(string message = null, bool isTTS = false, Embed embed = null, RequestOptions options = null, AllowedMentions allowedMentions = null, MessageReference messageReference = null)
         {
-            return await Context.Channel.SendMessageAsync(message, isTTS, embed, options, allowedMentions).ConfigureAwait(false);
+            return await Context.Channel.SendMessageAsync(message, isTTS, embed, options, allowedMentions, messageReference).ConfigureAwait(false);
         }
         /// <summary>
         ///     The method to execute before executing the command.

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

@@ -27,11 +27,12 @@ public interface IMessageChannel : IChannel
         ///     Specifies if notifications are sent for mentioned users and roles in the message <paramref name="text"/>.
         ///     If <c>null</c>, all mentioned roles and users will be notified.
         /// </param>
+        /// <param name="messageReference">The message references to be included. Used to reply to specific messages.</param>
         /// <returns>
         ///     A task that represents an asynchronous send operation for delivering the message. The task result
         ///     contains the sent message.
         /// </returns>
-        Task<IUserMessage> SendMessageAsync(string text = null, bool isTTS = false, Embed embed = null, RequestOptions options = null, AllowedMentions allowedMentions = null);
+        Task<IUserMessage> SendMessageAsync(string text = null, bool isTTS = false, Embed embed = null, RequestOptions options = null, AllowedMentions allowedMentions = null, MessageReference messageReference = null);
         /// <summary>
         ///     Sends a file to this message channel with an optional caption.
         /// </summary>
@@ -63,11 +64,12 @@ public interface IMessageChannel : IChannel
         ///     Specifies if notifications are sent for mentioned users and roles in the message <paramref name="text"/>.
         ///     If <c>null</c>, all mentioned roles and users will be notified.
         /// </param>
+        /// <param name="messageReference">The message references to be included. Used to reply to specific messages.</param>
         /// <returns>
         ///     A task that represents an asynchronous send operation for delivering the message. The task result
         ///     contains the sent message.
         /// </returns>
-        Task<IUserMessage> SendFileAsync(string filePath, string text = null, bool isTTS = false, Embed embed = null, RequestOptions options = null, bool isSpoiler = false, AllowedMentions allowedMentions = null);
+        Task<IUserMessage> SendFileAsync(string filePath, string text = null, bool isTTS = false, Embed embed = null, RequestOptions options = null, bool isSpoiler = false, AllowedMentions allowedMentions = null, MessageReference messageReference = null);
         /// <summary>
         ///     Sends a file to this message channel with an optional caption.
         /// </summary>
@@ -96,11 +98,12 @@ public interface IMessageChannel : IChannel
         ///     Specifies if notifications are sent for mentioned users and roles in the message <paramref name="text"/>.
         ///     If <c>null</c>, all mentioned roles and users will be notified.
         /// </param>
+        /// <param name="messageReference">The message references to be included. Used to reply to specific messages.</param>
         /// <returns>
         ///     A task that represents an asynchronous send operation for delivering the message. The task result
         ///     contains the sent message.
         /// </returns>
-        Task<IUserMessage> SendFileAsync(Stream stream, string filename, string text = null, bool isTTS = false, Embed embed = null, RequestOptions options = null, bool isSpoiler = false, AllowedMentions allowedMentions = null);
+        Task<IUserMessage> SendFileAsync(Stream stream, string filename, string text = null, bool isTTS = false, Embed embed = null, RequestOptions options = null, bool isSpoiler = false, AllowedMentions allowedMentions = null, MessageReference messageReference = null);
 
         /// <summary>
         ///     Gets a message from this message channel.

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

@@ -49,6 +49,14 @@ public class AllowedMentions
         /// </summary>
         public List<ulong> UserIds { get; set; } = new List<ulong>();
 
+        /// <summary>
+        ///     Gets or sets whether to mention the author of the message you are replying to or not.
+        /// </summary>
+        /// <remarks>
+        ///     Specifically for inline replies.
+        /// </remarks>
+        public bool? MentionRepliedUser { get; set; } = null;
+
         /// <summary>
         ///     Initializes a new instance of the <see cref="AllowedMentions"/> class.
         /// </summary>

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

@@ -148,11 +148,11 @@ public interface IMessage : ISnowflakeEntity, IDeletable
         MessageApplication Application { get; }
 
         /// <summary>
-        ///     Gets the reference to the original message if it was crossposted.
+        ///     Gets the reference to the original message if it is a crosspost, channel follow add, pin, or reply message.
         /// </summary>
         /// <remarks>
-        ///     Sent with Cross-posted messages, meaning they were published from news channels
-        ///     and received by subscriber channels.
+        ///     Sent with cross-posted messages, meaning they were published from news channels
+        ///     and received by subscriber channels, channel follow adds, pins, and message replies.
         /// </remarks>
         /// <returns>
         ///     A message's reference, if any is associated.

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

@@ -1,5 +1,4 @@
 using System;
-using System.Collections.Generic;
 using System.Threading.Tasks;
 
 namespace Discord
@@ -9,6 +8,14 @@ namespace Discord
     /// </summary>
     public interface IUserMessage : IMessage
     {
+        /// <summary>
+        ///     Gets the referenced message if it is a crosspost, channel follow add, pin, or reply message.
+        /// </summary>
+        /// <returns>
+        ///     The referenced message, if any is associated and still exists.
+        /// </returns>
+        IUserMessage ReferencedMessage { get; }
+
         /// <summary>
         ///     Modifies this message.
         /// </summary>

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

@@ -3,7 +3,7 @@
 namespace Discord
 {
     /// <summary>
-    ///     Contains the IDs sent from a crossposted message.
+    ///     Contains the IDs sent from a crossposted message or inline reply.
     /// </summary>
     [DebuggerDisplay(@"{DebuggerDisplay,nq}")]
     public class MessageReference
@@ -16,13 +16,36 @@ public class MessageReference
         /// <summary>
         ///     Gets the Channel ID of the original message.
         /// </summary>
-        public ulong ChannelId { get; internal set; }
+        /// <remarks>
+        ///     It only will be the default value (zero) if it was instantiated with a <see langword="null"/> in the constructor.
+        /// </remarks>
+        public ulong ChannelId { get => InternalChannelId.GetValueOrDefault(); }
+        internal Optional<ulong> InternalChannelId;
 
         /// <summary>
         ///     Gets the Guild ID of the original message.
         /// </summary>
         public Optional<ulong> GuildId { get; internal set; }
 
+        /// <summary>
+        ///     Initializes a new instance of the <see cref="MessageReference"/> class.
+        /// </summary>
+        /// <param name="messageId">
+        ///     The ID of the message that will be referenced. Used to reply to specific messages and the only parameter required for it.
+        /// </param>
+        /// <param name="channelId">
+        ///     The ID of the channel that will be referenced. It will be validated if sent.
+        /// </param>
+        /// <param name="guildId">
+        ///     The ID of the guild that will be referenced. It will be validated if sent.
+        /// </param>
+        public MessageReference(ulong? messageId = null, ulong? channelId = null, ulong? guildId = null)
+        {
+            MessageId = messageId ?? Optional.Create<ulong>();
+            InternalChannelId = channelId ?? Optional.Create<ulong>();
+            GuildId = guildId ?? Optional.Create<ulong>();
+        }
+
         private string DebuggerDisplay
             => $"Channel ID: ({ChannelId}){(GuildId.IsSpecified ? $", Guild ID: ({GuildId.Value})" : "")}" +
             $"{(MessageId.IsSpecified ? $", Message ID: ({MessageId.Value})" : "")}";

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

@@ -57,5 +57,12 @@ public enum MessageType
         ///     The message for when a news channel subscription is added to a text channel.
         /// </summary>
         ChannelFollowAdd = 12,
+        /// <summary>
+        ///     The message is an inline reply.
+        /// </summary>
+        /// <remarks>
+        ///     Only available in API v8.
+        /// </remarks>
+        Reply = 19,
     }
 }

src/Discord.Net.Core/Entities/Users/IGuildUser.cs

@@ -73,7 +73,7 @@ public interface IGuildUser : IUser, IVoiceState
         /// </summary>
         /// <example>
         ///     <para>The following example checks if the current user has the ability to send a message with attachment in
-        ///     this channel; if so, uploads a file via <see cref="IMessageChannel.SendFileAsync(string, string, bool, Embed, RequestOptions, bool, AllowedMentions)"/>.</para>
+        ///     this channel; if so, uploads a file via <see cref="IMessageChannel.SendFileAsync(string, string, bool, Embed, RequestOptions, bool, AllowedMentions, MessageReference)"/>.</para>
         ///     <code language="cs">
         ///     if (currentUser?.GetPermissions(targetChannel)?.AttachFiles)
         ///         await targetChannel.SendFileAsync("fortnite.png");

src/Discord.Net.Core/Extensions/MessageExtensions.cs

@@ -71,5 +71,25 @@ public static async Task RemoveReactionsAsync(this IUserMessage msg, IUser user,
             foreach (var rxn in reactions)
                 await msg.RemoveReactionAsync(rxn, user, options).ConfigureAwait(false);
         }
+
+        /// <summary>
+        ///     Sends an inline reply that references a message.
+        /// </summary>
+        /// <param name="text">The message to be sent.</param>
+        /// <param name="isTTS">Determines whether the message should be read aloud by Discord or not.</param>
+        /// <param name="embed">The <see cref="Discord.EmbedType.Rich"/> <see cref="Embed"/> to be sent.</param>
+        /// <param name="allowedMentions">
+        ///     Specifies if notifications are sent for mentioned users and roles in the message <paramref name="text"/>.
+        ///     If <c>null</c>, all mentioned roles and users will be notified.
+        /// </param>
+        /// <param name="options">The options to be used when sending the request.</param>
+        /// <returns>
+        ///     A task that represents an asynchronous send operation for delivering the message. The task result
+        ///     contains the sent message.
+        /// </returns>
+        public static async Task<IUserMessage> ReplyAsync(this IUserMessage msg, string text = null, bool isTTS = false, Embed embed = null, AllowedMentions allowedMentions = null, RequestOptions options = null)
+        {
+            return await msg.Channel.SendMessageAsync(text, isTTS, embed, options, allowedMentions, new MessageReference(messageId: msg.Id)).ConfigureAwait(false);
+        }
     }
 }

src/Discord.Net.Rest/API/Common/Message.cs

@@ -56,5 +56,7 @@ internal class Message
         public Optional<MessageFlags> Flags { get; set; }
         [JsonProperty("allowed_mentions")]
         public Optional<AllowedMentions> AllowedMentions { get; set; }
+        [JsonProperty("referenced_message")]
+        public Optional<Message> ReferencedMessage { get; set; }
     }
 }

src/Discord.Net.Rest/API/Common/MessageReference.cs

@@ -8,7 +8,7 @@ internal class MessageReference
         public Optional<ulong> MessageId { get; set; }
 
         [JsonProperty("channel_id")]
-        public ulong ChannelId { get; set; }
+        public Optional<ulong> ChannelId { get; set; } // Optional when sending, always present when receiving
 
         [JsonProperty("guild_id")]
         public Optional<ulong> GuildId { get; set; }

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

@@ -17,6 +17,8 @@ internal class CreateMessageParams
         public Optional<Embed> Embed { get; set; }
         [JsonProperty("allowed_mentions")]
         public Optional<AllowedMentions> AllowedMentions { get; set; }
+        [JsonProperty("message_reference")]
+        public Optional<MessageReference> MessageReference { get; set; }
 
         public CreateMessageParams(string content)
         {

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

@@ -20,6 +20,7 @@ internal class UploadFileParams
         public Optional<bool> IsTTS { get; set; }
         public Optional<Embed> Embed { get; set; }
         public Optional<AllowedMentions> AllowedMentions { get; set; }
+        public Optional<MessageReference> MessageReference { get; set; }
         public bool IsSpoiler { get; set; } = false;
 
         public UploadFileParams(Stream file)