Add various optimizations and cleanups (#1114)

* Change all Select(... as ...) to OfType

* Add changes according to 194a8aa427

src/Discord.Net.Core/Utils/MentionUtils.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Globalization;
 using System.Text;
 
@@ -139,22 +139,22 @@ namespace Discord
                         if (user != null)
                             return $"@{guildUser?.Nickname ?? user?.Username}";
                         else
-                            return $"";
+                            return "";
                     case TagHandling.NameNoPrefix:
                         if (user != null)
                             return $"{guildUser?.Nickname ?? user?.Username}";
                         else
-                            return $"";
+                            return "";
                     case TagHandling.FullName:
                         if (user != null)
                             return $"@{user.Username}#{user.Discriminator}";
                         else
-                            return $"";
+                            return "";
                     case TagHandling.FullNameNoPrefix:
                         if (user != null)
                             return $"{user.Username}#{user.Discriminator}";
                         else
-                            return $"";
+                            return "";
                     case TagHandling.Sanitize:
                         if (guildUser != null && guildUser.Nickname == null)
                             return MentionUser($"{SanitizeChar}{tag.Key}", false);
@@ -176,13 +176,13 @@ namespace Discord
                         if (channel != null)
                             return $"#{channel.Name}";
                         else
-                            return $"";
+                            return "";
                     case TagHandling.NameNoPrefix:
                     case TagHandling.FullNameNoPrefix:
                         if (channel != null)
                             return $"{channel.Name}";
                         else
-                            return $"";
+                            return "";
                     case TagHandling.Sanitize:
                         return MentionChannel($"{SanitizeChar}{tag.Key}");
                 }
@@ -201,13 +201,13 @@ namespace Discord
                         if (role != null)
                             return $"@{role.Name}";
                         else
-                            return $"";
+                            return "";
                     case TagHandling.NameNoPrefix:
                     case TagHandling.FullNameNoPrefix:
                         if (role != null)
                             return $"{role.Name}";
                         else
-                            return $"";
+                            return "";
                     case TagHandling.Sanitize:
                         return MentionRole($"{SanitizeChar}{tag.Key}");
                 }

src/Discord.Net.Core/Utils/Paging/PagedEnumerator.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.Threading;
 using System.Threading.Tasks;
@@ -60,7 +60,7 @@ namespace Discord
                     if (Current.Count == 0)
                         _info.Remaining = 0;
                 }
-                _info.PageSize = _info.Remaining != null ? (int)Math.Min(_info.Remaining.Value, _source.PageSize) : _source.PageSize;
+                _info.PageSize = _info.Remaining != null ? Math.Min(_info.Remaining.Value, _source.PageSize) : _source.PageSize;
 
                 if (_info.Remaining != 0)
                 {
@@ -74,4 +74,4 @@ namespace Discord
             public void Dispose() { Current = null; }
         }
     }
-}
+}

src/Discord.Net.Core/Utils/Permissions.cs

@@ -1,4 +1,4 @@
-using System.Runtime.CompilerServices;
+using System.Runtime.CompilerServices;
 
 namespace Discord
 {
@@ -119,13 +119,11 @@ namespace Discord
                 resolvedPermissions = mask; //Owners and administrators always have all permissions
             else
             {
-                OverwritePermissions? perms;
-
                 //Start with this user's guild permissions
                 resolvedPermissions = guildPermissions;
 
                 //Give/Take Everyone permissions
-                perms = channel.GetPermissionOverwrite(guild.EveryoneRole);
+                var perms = channel.GetPermissionOverwrite(guild.EveryoneRole);
                 if (perms != null)
                     resolvedPermissions = (resolvedPermissions & ~perms.Value.DenyValue) | perms.Value.AllowValue;
 
@@ -133,7 +131,7 @@ namespace Discord
                 ulong deniedPermissions = 0UL, allowedPermissions = 0UL;
                 foreach (var roleId in user.RoleIds)
                 {
-                    IRole role = null;
+                    IRole role;
                     if (roleId != guild.EveryoneRole.Id && (role = guild.GetRole(roleId)) != null)
                     {
                         perms = channel.GetPermissionOverwrite(role);
@@ -151,7 +149,7 @@ namespace Discord
                 if (perms != null)
                     resolvedPermissions = (resolvedPermissions  & ~perms.Value.DenyValue) | perms.Value.AllowValue;
 
-                if (channel is ITextChannel textChannel)
+                if (channel is ITextChannel)
                 {
                     if (!GetValue(resolvedPermissions, ChannelPermission.ViewChannel))
                     {

src/Discord.Net.Core/Utils/Preconditions.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 
 namespace Discord
 {
@@ -198,7 +198,7 @@ namespace Discord
             for (var i = 0; i < roles.Length; i++)
             {
                 if (roles[i] == guildId)
-                    throw new ArgumentException($"The everyone role cannot be assigned to a user", name);
+                    throw new ArgumentException("The everyone role cannot be assigned to a user", name);
             }
         }
     }