CustomCache.cs

using System;
using System.Collections.Concurrent;
using System.Threading.Tasks;
using System.Timers;

public class CustomCache
{
    private readonly ConcurrentDictionary<string, Task<(object result, DateTime expirationDate)>> dic = new();
    private readonly TimeSpan _threshold;
    private readonly Timer _timer;
    
    public CustomCache(TimeSpan threshold)
    {
        _threshold = threshold;
        
        _timer = new(TimeSpan.FromMinutes(15).Milliseconds);
        _timer.Elapsed += OnTimedEvent;
        _timer.AutoReset = true;
        _timer.Enabled = true;
    }

    private async void OnTimedEvent(object source, ElapsedEventArgs e)
    {
        foreach (var (key, task) in dic)
        {
            var resp = await task;
            if (IsExpirationValid(resp.expirationDate) is false)
                dic.TryRemove(key, out var _);
        }
    }
  
    private bool IsExpirationValid(DateTime expirationDate)
    {
        return DateTime.UtcNow < expirationDate - _threshold;   
    }

    public async Task<TResult> GetOrAddAsync<TResult>(string key, Func<string, Task<(TResult result, DateTime expirationDate)>> function)
    {
        var usedCache = true;
        var resp = await dic.GetOrAdd(key, async k =>
        {
            usedCache = false;
            return await function(k);
        });

        if (usedCache is false || IsExpirationValid(resp.expirationDate))
        {
            return (TResult)resp.result;
        }

        dic.TryRemove(key, out var _);
        resp = await dic.GetOrAdd(key, async k => await function(k));

        return (TResult)resp.result;
    }
}