FailFastAsync.cs

using System;
using System.Threading.Tasks;

namespace FailFastAsync
{
    public class Program
    {
        public static async Task Main(string[] args)
        {
            var functions = new Functions();

            Console.WriteLine(await functions.FailFastAsyncUsingAnonymousFunction("Anonymous Function"));
            Console.WriteLine(await functions.FailFastAsyncUsingLocalFunction("Local Function"));
            Console.WriteLine(await functions.FailFastAsyncUsingMethod("Method"));
        }
    }

    public class Functions
    {
        public Task<string> FailFastAsyncUsingAnonymousFunction(string name)
        {
            if (name == null)
            {
                throw new ArgumentNullException(nameof(name));
            }

            Func<Task<string>> core = async () =>
            {
                await Task.Delay(10);
                return $"Hello {name}";
            };

            return core();
        }

        public Task<string> FailFastAsyncUsingLocalFunction(string name)
        {
            if (name == null)
            {
                throw new ArgumentNullException(nameof(name));
            }

            async Task<string> core()
            {
                await Task.Delay(10);
                return $"Hello {name}";
            };

            return core();
        }

        public Task<string> FailFastAsyncUsingMethod(string name)
        {
            if (name == null)
            {
                throw new ArgumentNullException(nameof(name));
            }

            return FailFastAsyncUsingMethodCore(name);
        }

        private async Task<string> FailFastAsyncUsingMethodCore(string name)
        {
            await Task.Delay(10);
            return $"Hello {name}";
        }
    }
}