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IMemoryCache and GetOrCreateForTimeSpanAsync

· 2 min read
John Reilly
OSS Engineer - TypeScript, Azure, React, Node.js, .NET

One thing that ASP.Net Core really got right was caching. IMemoryCache is a caching implementation that does just what I want.

TimeSpan, TimeSpan Expiration Y'all

To make usage of the IMemoryCache even more lovely I've written an extension method. I follow pretty much one cache strategy: SetAbsoluteExpiration and I just vary the expiration by an amount of time. This extension method implements that in a simple way; I call it GetOrCreateForTimeSpanAsync - catchy right? It looks like this:

using System;
using System.Threading.Tasks;
using Microsoft.Extensions.Caching.Memory;

namespace My.Helpers {

public static class CacheHelpers {

public static async Task<TItem?> GetOrCreateForTimeSpanAsync<TItem>(
this IMemoryCache cache,
string key,
Func<Task<TItem?>> itemGetterAsync,
TimeSpan timeToCache
) {
if (!cache.TryGetValue(key, out object? result))
result = await itemGetterAsync();
if (result == null)
return default(TItem);

var cacheEntryOptions = new MemoryCacheEntryOptions()

cache.Set(key, result, cacheEntryOptions);

return (TItem)result;

Usage looks like this:

private Task<SuperInterestingThing> GetSuperInterestingThingFromCache(Guid superInterestingThingId) =>
key: $"{nameof(MyClass)}:GetSuperInterestingThing:{superInterestingThingId}",
itemGetterAsync: () => GetSuperInterestingThing(superInterestingThingId),
timeToCache: TimeSpan.FromMinutes(5)

Where _cache is an instance of IMemoryCache that can be dependency injected into your class. This helper allows the consumer to provide three things:

  • The key key for the item to be cached with
  • A itemGetterAsync which is the method that is used to retrieve a new value if an item cannot be found in the cache
  • A timeToCache which is the period of time that an item should be cached

If an item can't be looked up by the itemGetterAsync then nothing will be cached and a the default value of the expected type will be returned. This is important because lookups can fail, and there's nothing worse than a lookup failing and you caching null as a result.

Go on, ask me how I know.

This is a simple, clear and helpful API which makes interacting with IMemoryCache even more lovely than it was.