Add module-level preconditions and precondition for individual permissions

src/Discord.Net.Commands/Attributes/PreconditionAttribute.cs

@@ -5,6 +5,7 @@ using System.Threading.Tasks;
 
 namespace Discord.Commands
 {
+    [AttributeUsage(AttributeTargets.Method | AttributeTargets.Class, AllowMultiple = true, Inherited = true)]
     public abstract class PreconditionAttribute : Attribute
     {
         public abstract Task<PreconditionResult> CheckPermissions(IMessage context);

src/Discord.Net.Commands/Attributes/Preconditions/RequireDMAttribute.cs

@@ -5,6 +5,7 @@ using System.Threading.Tasks;
 
 namespace Discord.Commands
 {
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = false, Inherited = true)]
     public class RequireDMAttribute : PreconditionAttribute
     {
         public override Task<PreconditionResult> CheckPermissions(IMessage context)

src/Discord.Net.Commands/Attributes/Preconditions/RequireGuildAttribute.cs

@@ -5,6 +5,7 @@ using System.Threading.Tasks;
 
 namespace Discord.Commands
 {
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = false, Inherited = true)]
     public class RequireGuildAttribute : PreconditionAttribute
     {
         public override Task<PreconditionResult> CheckPermissions(IMessage context)

src/Discord.Net.Commands/Attributes/Preconditions/RequirePermission.cs

@@ -0,0 +1,54 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace Discord.Commands.Attributes.Preconditions
+{
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = true, Inherited = true)]
+    public class RequirePermission : RequireGuildAttribute
+    {
+        public GuildPermission? GuildPermission { get; set; }
+        public ChannelPermission? ChannelPermission { get; set; }
+
+        public RequirePermission(GuildPermission permission)
+        {
+            GuildPermission = permission;
+            ChannelPermission = null;
+        }
+
+        public RequirePermission(ChannelPermission permission)
+        {
+            ChannelPermission = permission;
+            GuildPermission = null;
+        }
+
+        public override async Task<PreconditionResult> CheckPermissions(IMessage context)
+        {
+            var result = await base.CheckPermissions(context).ConfigureAwait(false);
+
+            if (!result.IsSuccess)
+                return result;
+
+            var author = context.Author as IGuildUser;
+
+            if (GuildPermission.HasValue)
+            {
+                var guildPerms = author.GuildPermissions.ToList();
+                if (!guildPerms.Contains(GuildPermission.Value))
+                    return PreconditionResult.FromError($"User is missing guild permission {GuildPermission.Value}");
+            }
+
+            if (ChannelPermission.HasValue)
+            {
+                var channel = context.Channel as IGuildChannel;
+                var channelPerms = author.GetPermissions(channel).ToList();
+
+                if (!channelPerms.Contains(ChannelPermission.Value))
+                    return PreconditionResult.FromError($"User is missing channel permission {ChannelPermission.Value}");
+            }
+
+            return PreconditionResult.FromSuccess();
+        }
+    }
+}

src/Discord.Net.Commands/Attributes/Preconditions/RequireRoleAttribute.cs

@@ -5,6 +5,7 @@ using System.Threading.Tasks;
 
 namespace Discord.Commands
 {
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method)]
     public class RequireRoleAttribute : RequireGuildAttribute
     {
         public string Role { get; set; }