Lazy with expiration time

Question

I want to implement an expiration time on a Lazy object. The expiration cooldown must start with the first retrieve of the value. If we get the value, and the expiration time is

Answer 1

I agree with the other commenters that you probably shouldn't touch Lazy at all. Lazy isn't very complicated if you ignore the multiple thread-safety options, so just implement it from scratch.

I quite like the idea by the way, although I don't know if I'd be comfortable using it as a general purpose caching strategy. It might be sufficient for some of the simpler scenarios.

Here's my stab at it. If you don't need it to be thread-safe you can just remove the locking stuff. I don't think it's possible to use the double-checking lock pattern here because of the chance that the cached value may be be invalidated inside the lock.

public class Temporary
{
    private readonly Func factory;
    private readonly TimeSpan lifetime;
    private readonly object valueLock = new object();

    private T value;
    private bool hasValue;
    private DateTime creationTime;

    public Temporary(Func factory, TimeSpan lifetime)
    {
        this.factory = factory;
        this.lifetime = lifetime;
    }

    public T Value
    {
        get
        {
            DateTime now = DateTime.Now;
            lock (this.valueLock)
            {
                if (this.hasValue)
                {
                    if (this.creationTime.Add(this.lifetime) < now)
                    {
                        this.hasValue = false;
                    }
                }

                if (!this.hasValue)
                {
                    this.value = this.factory();
                    this.hasValue = true;

                    // You can also use the existing "now" variable here.
                    // It depends on when you want the cache time to start
                    // counting from.
                    this.creationTime = Datetime.Now;
                }

                return this.value;
            }
        }
    }
}