Adjust GuildExtensions per Voltana's feedback

Instead of implicitly soft-casting IGuild to SocketGuild, I added a method to soft-cast IGuild to SocketGuild, and throw an InvalidOp if it came up null for some reason.

GetUsers was changed from an IEnumerable to an IReadOnlyCollection to allow quick ".Count"-ing on the collection.

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

@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using System.Linq;
 
 namespace Discord.WebSocket.Extensions
@@ -9,36 +10,44 @@ namespace Discord.WebSocket.Extensions
         // Channels
 
         public static IGuildChannel GetChannel(this IGuild guild, ulong id) =>
-            (guild as SocketGuild).GetChannel(id);
+            GetSocketGuild(guild).GetChannel(id);
 
         public static ITextChannel GetTextChannel(this IGuild guild, ulong id) =>
-            (guild as SocketGuild).GetChannel(id) as ITextChannel;
+            GetSocketGuild(guild).GetChannel(id) as ITextChannel;
 
         public static IEnumerable<ITextChannel> GetTextChannels(this IGuild guild) =>
-            (guild as SocketGuild).Channels.Select(c => c as ITextChannel).Where(c => c != null);
+            GetSocketGuild(guild).Channels.Select(c => c as ITextChannel).Where(c => c != null);
 
 
         public static IVoiceChannel GetVoiceChannel(this IGuild guild, ulong id) =>
-            (guild as SocketGuild).GetChannel(id) as IVoiceChannel;
+            GetSocketGuild(guild).GetChannel(id) as IVoiceChannel;
 
         public static IEnumerable<IVoiceChannel> GetVoiceChannels(this IGuild guild) =>
-            (guild as SocketGuild).Channels.Select(c => c as IVoiceChannel).Where(c => c != null);
+            GetSocketGuild(guild).Channels.Select(c => c as IVoiceChannel).Where(c => c != null);
 
         // Users
 
         public static IGuildUser GetCurrentUser(this IGuild guild) =>
-            (guild as SocketGuild).CurrentUser;
+            GetSocketGuild(guild).CurrentUser;
 
         public static IGuildUser GetUser(this IGuild guild, ulong id) =>
-            (guild as SocketGuild).GetUser(id);
+            GetSocketGuild(guild).GetUser(id);
 
-        public static IEnumerable<IGuildUser> GetUsers(this IGuild guild) =>
-            (guild as SocketGuild).Members;
+        public static IReadOnlyCollection<IGuildUser> GetUsers(this IGuild guild) =>
+            GetSocketGuild(guild).Members;
 
         public static int GetUserCount(this IGuild guild) =>
-            (guild as SocketGuild).MemberCount;
+            GetSocketGuild(guild).MemberCount;
 
         public static int GetCachedUserCount(this IGuild guild) =>
-            (guild as SocketGuild).DownloadedMemberCount;
+            GetSocketGuild(guild).DownloadedMemberCount;
+
+        internal static SocketGuild GetSocketGuild(IGuild guild)
+        {
+            var socketGuild = guild as SocketGuild;
+            if (socketGuild == null)
+                throw new InvalidOperationException("This extension method is only valid on WebSocket Entities");
+            return socketGuild;
+        }
     }
 }