Add nullable type converter to Interaction service

src/Discord.Net.Interactions/InteractionService.cs

@@ -165,7 +165,8 @@ namespace Discord.Interactions
                 [typeof(IUser)] = typeof(DefaultUserConverter<>),
                 [typeof(IMentionable)] = typeof(DefaultMentionableConverter<>),
                 [typeof(IConvertible)] = typeof(DefaultValueConverter<>),
-                [typeof(Enum)] = typeof(EnumConverter<>)
+                [typeof(Enum)] = typeof(EnumConverter<>),
+                [typeof(Nullable<>)] = typeof(NullableConverter<>),
             };
 
             _typeConverters = new ConcurrentDictionary<Type, TypeConverter>
@@ -692,8 +693,8 @@ namespace Discord.Interactions
         {
             if (_typeConverters.TryGetValue(type, out var specific))
                 return specific;
-
-            else if (_genericTypeConverters.Any(x => x.Key.IsAssignableFrom(type)))
+            else if (_genericTypeConverters.Any(x => x.Key.IsAssignableFrom(type)
+            || (x.Key.IsGenericTypeDefinition && type.IsGenericType && x.Key.GetGenericTypeDefinition() == type.GetGenericTypeDefinition())))
             {
                 services ??= EmptyServiceProvider.Instance;
 
@@ -926,6 +927,9 @@ namespace Discord.Interactions
             if (_genericTypeConverters.TryGetValue(type, out var matching))
                 return matching;
 
+            if (type.IsGenericType && _genericTypeConverters.TryGetValue(type.GetGenericTypeDefinition(), out var genericDefinition))
+                return genericDefinition;
+
             var typeInterfaces = type.GetInterfaces();
             var candidates = _genericTypeConverters.Where(x => x.Key.IsAssignableFrom(type))
                 .OrderByDescending(x => typeInterfaces.Count(y => y.IsAssignableFrom(x.Key)));

src/Discord.Net.Interactions/TypeConverters/NullableConverter.cs

@@ -0,0 +1,32 @@
+using System;
+using System.Threading.Tasks;
+
+namespace Discord.Interactions
+{
+    internal class NullableConverter<T> : TypeConverter
+    {
+        private readonly TypeConverter _typeConverter;
+
+        public NullableConverter(InteractionService interactionService, IServiceProvider services)
+        {
+            var type = Nullable.GetUnderlyingType(typeof(T));
+
+            if (type is null)
+                throw new ArgumentException($"No type {nameof(TypeConverter)} is defined for this {type.FullName}", "type");
+
+            _typeConverter = interactionService.GetTypeConverter(type, services);
+        }
+
+        public override bool CanConvertTo(Type type)
+            => Nullable.GetUnderlyingType(type) != null;
+
+        public override ApplicationCommandOptionType GetDiscordType()
+            => _typeConverter.GetDiscordType();
+
+        public override Task<TypeConverterResult> ReadAsync(IInteractionContext context, IApplicationCommandInteractionDataOption option, IServiceProvider services)
+            => _typeConverter.ReadAsync(context, option, services);
+
+        public override void Write(ApplicationCommandOptionProperties properties, IParameterInfo parameter)
+            => _typeConverter.Write(properties, parameter);
+    }
+}