Null value lingers in cache after Cache.Remove(key)

I added a caching layer between my ASP.NET MVC application and database using the GetOrStore<T> cache helper below.

This includes caching users' system roles.

When I remove a cached roles object for a signed-in user via HttpRuntime.Cache.Remove(userRoleCacheKey), subsequent requests fail with a NullReferenceException because the cache helper is returning a null value for the role cache, even though the cached key should not exist and the helper should regenerate it.

It seems like the cached key lingers around with a null value. The exception won't budge until I request a role-heavy page a few seconds later.

Why is my cache breaking?

public static class CacheExtensions
{
    public static T GetOrStore<T>(this Cache cache, string key, Func<T> generator)
    {
        var result = cache.Get(key);

        if (result == null)
        {
            result = generator();
            if (result != null) // can't store null values in cache.
            {
                cache[key] = result;
            }
        }
        return (T)result;
    }
}

Here is the code that fetches the user's roles and caches it:

public override string[] GetRolesForUser(string userId)
{
    return HttpRuntime.Cache.GetOrStore<string[]>(
        "RolesForUser[" + userId + "]",
        () => Con.Query<string>("SELECT Role FROM vw_UserRo开发者_运维百科les WHERE UserId = @userId", new { userId = Guid.Parse(userId)  }).ToArray());
}

where Con retrieves an open IDbConnection.

Minor issue in your code that can hide the problem from you:

if (result != null) // can't store null values in cache.
{
    cache[key] = result;
}

What about else clause? The result returned anyway, even if it is null value. Throw InvalidOperationException in that case and debug your queries passed as generator parameter.