gistfile1.txt

using System;
using System.Collections.Generic;
using System.Net;
using System.Threading.Tasks;
using Xunit;

public class Fixture
{
    [Fact]
    public void Should_FactMethodName()
    {
        var module = new Class1();

        var result = module.routes["/"].Invoke(new object());

        Assert.Equal("non async", result);
    }


    [Fact]
    public async Task Should_work_async()
    {
        var module = new Class1();

        var result = await module.routes["/stuff"].Invoke(new object());

        Assert.Equal("async", result);
    }
}

public class Class1 : Module
{
    public Class1()
    {
        Get["/"] = x =>
        {
            return "non async";
        };

        Get["/stuff"] = Stuff;
    }

    public async Task<dynamic> Stuff(dynamic _)
    {
        await Task.Delay(100);
        return "async";
    }
}

public class Module
{
    public readonly IDictionary<string, Func<dynamic, dynamic>> routes = new Dictionary<string, Func<dynamic, dynamic>>();

    public Builder Get
    {
        get { return new Builder(this); }
    }

    public class Builder
    {
        private readonly Module module;

        public Builder(Module module)
        {
            this.module = module;
        }

        public Func<dynamic, dynamic> this[string path]
        {
            set { this.module.routes.Add(path, value); }
        }

        //public Func<dynamic, Task<dynamic>> this[string path]
        //{
        //    set { this.module.routes.Add(path, value); }
        //}
    }
}