using System;

using System.Diagnostics;

using System.Threading.Tasks;

using System.Collections.Concurrent;

public class Program

{

    static async Task Main()

    {

        var keys = new string[] { "a", "b", "error", "b", "a" };

        foreach (var key in keys)

        {

            var sw = new Stopwatch();

            try

            {

                sw.Start();

                var ret = await GetFromCacheOrDbAsync(key);

                Console.WriteLine($"result: {ret}, in {sw.Elapsed}");

            }

            catch (Exception ex)

            {

                sw.Stop();

                Console.WriteLine($"exception: {ex.Message}, in {sw.Elapsed}");

            }

        }

    }

​

​

    static ConcurrentDictionary<string, string> cache = new ConcurrentDictionary<string, string>();

​

    static Task<string> GetFromCacheOrDbAsync(string key)

    {

        if (cache.TryGetValue(key, out string value))

        {

            return Task.FromResult(value);

        }

        return GetFromDbAsync(key).ContinueWith((task) =>

        {

            task.Exception?.Flatten().Handle(ex => throw ex);

            cache.TryAdd(key, task.Result);

            return task.Result;

        });

    }

​

    static async Task<string> GetFromDbAsync(string key)

    {

        await Task.Delay(500);

        if (key == "error")

            throw new Exception(key);

        return key;

    }

}