Replace IEnumerable with IReadonlyCollection where possible

src/Discord.Net/WebSocket/Extensions/ChannelExtensions.cs

@@ -8,25 +8,25 @@ namespace Discord.WebSocket.Extensions
         public static IUser GetUser(this IDMChannel channel, ulong id)
             => GetSocketDMChannel(channel).GetUser(id);
 
-        public static IEnumerable<IUser> GetUsers(this IDMChannel channel)
+        public static IReadOnlyCollection<IUser> GetUsers(this IDMChannel channel)
             => GetSocketDMChannel(channel).Users;
 
         public static IUser GetUser(this IGroupChannel channel, ulong id)
             => GetSocketGroupChannel(channel).GetUser(id);
 
-        public static IEnumerable<IUser> GetUsers(this IGroupChannel channel)
+        public static IReadOnlyCollection<IUser> GetUsers(this IGroupChannel channel)
             => GetSocketGroupChannel(channel).Users;
 
         public static IGuildUser GetUser(this ITextChannel channel, ulong id)
             => GetSocketTextChannel(channel).GetUser(id);
 
-        public static IEnumerable<IGuildUser> GetUsers(this ITextChannel channel)
+        public static IReadOnlyCollection<IGuildUser> GetUsers(this ITextChannel channel)
             => GetSocketTextChannel(channel).Members;
 
         public static IGuildUser GetUser(this IVoiceChannel channel, ulong id)
             => GetSocketVoiceChannel(channel).GetUser(id);
 
-        public static IEnumerable<IGuildUser> GetUsers(this IVoiceChannel channel)
+        public static IReadOnlyCollection<IGuildUser> GetUsers(this IVoiceChannel channel)
             => GetSocketVoiceChannel(channel).Members;
 
         internal static SocketDMChannel GetSocketDMChannel(IDMChannel channel)

src/Discord.Net/WebSocket/Extensions/GuildExtensions.cs

@@ -12,7 +12,7 @@ namespace Discord.WebSocket.Extensions
         public static IGuildChannel GetChannel(this IGuild guild, ulong id) =>
             GetSocketGuild(guild).GetChannel(id);
 
-        public static IEnumerable<IGuildChannel> GetChannels(this IGuild guild) =>
+        public static IReadOnlyCollection<IGuildChannel> GetChannels(this IGuild guild) =>
             GetSocketGuild(guild).Channels;
 
         public static ITextChannel GetTextChannel(this IGuild guild, ulong id) =>