simple.cs

using System;
using System.Threading.Tasks;

namespace ConsoleApp8
{
    class Program
    {
        static async Task<int> DoStuff()
        {
            var x = await System.IO.File.ReadAllTextAsync("Foo");
            var y = await System.IO.File.ReadAllTextAsync("Bar");
            return x.Length + y.Length;
        }

        static void Main(string[] args)
        {
            var lengths = DoStuff().Result;
            Console.WriteLine($"Hello World! {lengths}");
        }
    }
}

simple.fs

let doStuff = async {
    let! x = System.IO.File.ReadAllTextAsync "Foo" |> Async.AwaitTask
    let! y = System.IO.File.ReadAllTextAsync "Bar" |> Async.AwaitTask
    return x.Length + y.Length
}

let main args =
    let lengths = doStuff |> Async.RunSynchronously
    printf "Hello World! %d" lengths

simpletask.fs

open FSharp.Control.Tasks.V2

let doStuff () = task {
    let! x = System.IO.File.ReadAllTextAsync "Foo"
    let! y = System.IO.File.ReadAllTextAsync "Bar"
    return x.Length + y.Length
}

let main args =
    let lengths = doStuff().Result
    printf "Hello World! %d" lengths