Parameter preconditions and typereader overriding

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

@@ -0,0 +1,22 @@
+using System;
+
+using System.Reflection;
+
+namespace Discord.Commands
+{
+    [AttributeUsage(AttributeTargets.Parameter)]
+    public class OverrideTypeReaderAttribute : Attribute
+    {
+        private readonly TypeInfo _typeReaderTypeInfo = typeof(TypeReader).GetTypeInfo();
+
+        public Type TypeReader { get; }
+
+        public OverrideTypeReaderAttribute(Type overridenType)
+        {
+            if (!_typeReaderTypeInfo.IsAssignableFrom(overridenType.GetTypeInfo()))
+                throw new ArgumentException($"{nameof(overridenType)} must inherit from {nameof(TypeReader)}");
+            
+            TypeReader = overridenType;
+        }
+    } 
+}

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

@@ -0,0 +1,11 @@
+using System;
+using System.Threading.Tasks;
+
+namespace Discord.Commands
+{
+    [AttributeUsage(AttributeTargets.Parameter, AllowMultiple = true, Inherited = true)]
+    public abstract class ParameterPreconditionAttribute : Attribute
+    {
+        public abstract Task<PreconditionResult> CheckPermissions(CommandContext context, ParameterInfo parameter, object value);
+    }
+}

src/Discord.Net.Commands/Builders/ModuleClassBuilder.cs

@@ -182,6 +182,10 @@ namespace Discord.Commands
                 // TODO: C#7 type switch
                 if (attribute is SummaryAttribute)
                     builder.Summary = (attribute as SummaryAttribute).Text;
+                else if (attribute is OverrideTypeReaderAttribute)
+                    builder.TypeReader = service.GetTypeReader((attribute as OverrideTypeReaderAttribute).TypeReader);
+                else if (attribute is ParameterPreconditionAttribute)
+                    builder.AddPrecondition(attribute as ParameterPreconditionAttribute);
                 else if (attribute is ParamArrayAttribute)
                 {
                     builder.IsMultiple = true;

src/Discord.Net.Commands/Builders/ParameterBuilder.cs

@@ -1,10 +1,14 @@
 using System;
 using System.Reflection;
 
+using System.Collections.Generic;
+
 namespace Discord.Commands.Builders
 {
     public class ParameterBuilder
     {
+        private readonly List<ParameterPreconditionAttribute> _preconditions; 
+
         public CommandBuilder Command { get; }
         public string Name { get; internal set; }
         public Type ParameterType { get; internal set; }
@@ -16,16 +20,20 @@ namespace Discord.Commands.Builders
         public object DefaultValue { get; set; }
         public string Summary { get; set; }
 
+        public IReadOnlyList<ParameterPreconditionAttribute> Preconditions => _preconditions;
+
         //Automatic
         internal ParameterBuilder(CommandBuilder command)
         {
+            _preconditions = new List<ParameterPreconditionAttribute>();
+
             Command = command;
         }
         //User-defined
         internal ParameterBuilder(CommandBuilder command, string name, Type type)
             : this(command)
         {
-            Preconditions.NotNull(name, nameof(name));
+            Discord.Preconditions.NotNull(name, nameof(name));
 
             Name = name;
             SetType(type);
@@ -49,7 +57,7 @@ namespace Discord.Commands.Builders
         }
         public ParameterBuilder WithDefault(object defaultValue)
         {
-            DefaultValue = defaultValue;            
+            DefaultValue = defaultValue;
             return this;
         }
         public ParameterBuilder WithIsOptional(bool isOptional)
@@ -68,6 +76,12 @@ namespace Discord.Commands.Builders
             return this;
         }
 
+        public ParameterBuilder AddPrecondition(ParameterPreconditionAttribute precondition)
+        {
+            _preconditions.Add(precondition);
+            return this;
+        }
+
         internal ParameterInfo Build(CommandInfo info)
         {
             if (TypeReader == null)

src/Discord.Net.Commands/Info/CommandInfo.cs

@@ -135,9 +135,26 @@ namespace Discord.Commands
             if (map == null)
                 map = DependencyMap.Empty;
 
+            object[] args = null;
+
+            try
+            {
+                args = GenerateArgs(argList, paramList);
+            }
+            catch (Exception ex)
+            {
+                return ExecuteResult.FromError(ex);
+            }
+
+            foreach (var parameter in Parameters)
+            {
+                var result = await parameter.CheckPreconditionsAsync(context, args, map).ConfigureAwait(false);
+                if (!result.IsSuccess)
+                    return ExecuteResult.FromError(result);
+            }
+
             try
             {
-                var args = GenerateArgs(argList, paramList);
                 switch (RunMode)
                 {
                     case RunMode.Sync: //Always sync

src/Discord.Net.Commands/Info/ParameterInfo.cs

@@ -1,5 +1,7 @@
 using System;
 using System.Linq;
+using System.Collections.Generic;
+using System.Collections.Immutable;
 using System.Threading.Tasks;
 
 using Discord.Commands.Builders;
@@ -10,6 +12,17 @@ namespace Discord.Commands
     {
         private readonly TypeReader _reader;
 
+        public CommandInfo Command { get; }
+        public string Name { get; }
+        public string Summary { get; }
+        public bool IsOptional { get; }
+        public bool IsRemainder { get; }
+        public bool IsMultiple { get; }
+        public Type Type { get; }
+        public object DefaultValue { get; }
+
+        public IReadOnlyList<ParameterPreconditionAttribute> Preconditions { get; }
+
         internal ParameterInfo(ParameterBuilder builder, CommandInfo command, CommandService service)
         {
             Command = command;
@@ -23,17 +36,30 @@ namespace Discord.Commands
             Type = builder.ParameterType;
             DefaultValue = builder.DefaultValue;
 
+            Preconditions = builder.Preconditions.ToImmutableArray();
+
             _reader = builder.TypeReader;
         }
 
-        public CommandInfo Command { get; }
-        public string Name { get; }
-        public string Summary { get; }
-        public bool IsOptional { get; }
-        public bool IsRemainder { get; }
-        public bool IsMultiple { get; }
-        public Type Type { get; }
-        public object DefaultValue { get; }
+        public async Task<PreconditionResult> CheckPreconditionsAsync(CommandContext context, object[] args, IDependencyMap map = null)
+        {
+            if (map == null)
+                map = DependencyMap.Empty;
+
+            int position = 0;
+            for(position = 0; position < Command.Parameters.Count; position++)
+                if (Command.Parameters[position] == this)
+                    break;
+
+            foreach (var precondition in Preconditions)
+            {
+                var result = await precondition.CheckPermissions(context, this, args[position]).ConfigureAwait(false);
+                if (!result.IsSuccess)
+                    return result;
+            }
+
+            return PreconditionResult.FromSuccess();
+        }
 
         public async Task<TypeReaderResult> Parse(CommandContext context, string input)
         {