WriteOrWriteAsync.cs

public class Startup
{
    static Task emptyTask = Task.FromResult<object>(null);

    public void Configuration(IAppBuilder app)
    {
        app.Run(context =>
        {
            var name = context.Request.Query.Get("name");
            var x = int.Parse(context.Request.Query.Get("x"));
            var y = int.Parse(context.Request.Query.Get("y"));
            var e = Enum.Parse(typeof(MyEnum), context.Request.Query.Get("e"));

            var mc = new MyClass { Name = name, Sum = (x + y) * (int)e };

            var json = JsonConvert.SerializeObject(mc);
            var enc = System.Text.Encoding.UTF8.GetBytes(json);
            context.Response.ContentType = "application/json";

            // sync write or async write
            if (context.Request.Query.Get("sync") == "true")
            {
                context.Response.Body.Write(enc, 0, enc.Length);
                return emptyTask;
            }
            else
            {
                return context.Response.Body.WriteAsync(enc, 0, enc.Length);
            }
        });
    }
}