LazyCache.fs

module CachingAgent
open System
type AgentCacheMessage<'T> = AsyncReplyChannel<'T>
type Agent<'T> = MailboxProcessor<'T>

let CreateCache<'T> timespan (funcGen:Func<'T>) = Agent.Start(fun inbox -> 
    let rec loop expire (cached:Lazy<'T>) = async {
        let! (message:AgentCacheMessage<'T>) = inbox.Receive()
        let renew = expire <= DateTime.Now
        let cached' = if renew then lazy (funcGen.Invoke()) else cached
        let expire' = if renew then DateTime.Now + timespan else expire
        message.Reply(cached'.Force())
        return! loop expire' cached'
    }
    loop (DateTime.Now + timespan) (lazy (funcGen.Invoke()))
)
let GetValue<'T> (agent:Agent<AgentCacheMessage<'T>>) = agent.PostAndAsyncReply(fun rp -> rp) |> Async.StartAsTask

Program.cs

class Program
{
    static void Main(string[] args)
    {
        var taskFactory = new TaskFactory();
        var x = 0;
        Func<int> generator = () => 
        {
            Thread.Sleep(2000);
            Console.WriteLine("Generating value");
            return x++;
        };
        var cache = CachingAgent.CreateCache(TimeSpan.FromSeconds(5), generator);
        for (int i = 0; i < 20; i++)
        {
            var result = CachingAgent.GetValue(cache).Result;
            Console.WriteLine(result);
            Thread.Sleep(1000);
        }
    }
}