Allow Preconditions on parameters.

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

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

src/Discord.Net.Commands/CommandInfo.cs

@@ -101,6 +101,13 @@ namespace Discord.Commands
                     return result;
             }
 
+            foreach (CommandParameter parameter in Parameters)
+            {
+                var result = await parameter.CheckPreconditions(context, map).ConfigureAwait(false);
+                if (!result.IsSuccess)
+                    return result;
+            }
+
             return PreconditionResult.FromSuccess();
         }
 

src/Discord.Net.Commands/CommandParameter.cs

@@ -1,7 +1,10 @@
 using System;
+using System.Collections.Generic;
+using System.Collections.Immutable;
 using System.Diagnostics;
 using System.Reflection;
 using System.Threading.Tasks;
+using Discord.Commands.Attributes;
 
 namespace Discord.Commands
 {
@@ -18,6 +21,7 @@ namespace Discord.Commands
         public bool IsMultiple { get; }
         public Type ElementType { get; }
         public object DefaultValue { get; }
+        public IReadOnlyList<ParameterPreconditionAttribute> Preconditions { get; }
 
         public CommandParameter(ParameterInfo source, string name, string summary, Type type, TypeReader reader, bool isOptional, bool isRemainder, bool isMultiple, object defaultValue)
         {
@@ -30,6 +34,7 @@ namespace Discord.Commands
             IsRemainder = isRemainder;
             IsMultiple = isMultiple;
             DefaultValue = defaultValue;
+            Preconditions = BuildPreconditions(source);
         }
 
         public async Task<TypeReaderResult> Parse(CommandContext context, string input)
@@ -37,6 +42,23 @@ namespace Discord.Commands
             return await _reader.Read(context, input).ConfigureAwait(false);
         }
 
+        public async Task<PreconditionResult> CheckPreconditions(CommandContext context, IDependencyMap map = null)
+        {
+            foreach (ParameterPreconditionAttribute precondition in Preconditions)
+            {
+                var result = await precondition.CheckPermissions(context, this, map).ConfigureAwait(false);
+                if (!result.IsSuccess)
+                    return result;
+            }
+
+            return PreconditionResult.FromSuccess();
+        }
+
+        private IReadOnlyList<ParameterPreconditionAttribute> BuildPreconditions(ParameterInfo paramInfo)
+        {
+            return paramInfo.GetCustomAttributes<ParameterPreconditionAttribute>().ToImmutableArray();
+        }
+
         public override string ToString() => Name;
         private string DebuggerDisplay => $"{Name}{(IsOptional ? " (Optional)" : "")}{(IsRemainder ? " (Remainder)" : "")}";
     }