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);
+    }
+}