Issues with Memory Cache. TryGetValue returns false

Question

In this code snippet i simply put null in MemoryCache and then check if this key exists:

MemoryCache _cache = new(new MemoryCacheOptions());
_cache.Set<string>(cacheKey, null);
bool isInCache = _cache.TryGetValue(cacheKey, out string nothing);

isInCache is false in this case. Is this behavior expected?

I use .NET Core 2.2 Console app.

Answer 1

Based on the source code for TryGetValue() it will return false if null is returned when it checks type if (result is TItem item). However, the .Count property would return 1. (thanks to @jgoday comment for these details).

An alternative is to have a 'null value' (e.g. Guid.NewGuid()) that you can use to represent a null value, this way something is entered into the cache so you can verify whether it has ever been added.

public class MyCache
{
  private MemoryCache _cache = new MemoryCache(new MemoryCacheOptions());
  private string nullValue = Guid.NewGuid().ToString();

  public void Set(string cacheKey, string toSet)
    => _cache.Set<string>(cacheKey, toSet == null ? nullValue : toSet);

  public string Get(string cacheKey)
  {
    var isInCache = _cache.TryGetValue(cacheKey, out string cachedVal);
    if (!isInCache) return null;

    return cachedVal == nullValue ? null : cachedVal;
  }
}