public class InternalService
{
    private readonly IMemoryCache m_memoryCache;

    private readonly MemoryCacheEntryOptions m_cacheEntryOptions;

    public Service(IMemoryCache memoryCache)
    {
         m_memoryCache = memoryCache;

         // Set cache options: keep in cache for this time, reset time in 1 hour, no matter what.
        m_cacheEntryOptions = new MemoryCacheEntryOptions().SetAbsoluteExpiration(TimeSpan.FromHours(1d));
    }
}

public class Entity
{
    public long Id {get; set;}
}

private async IList<Entity> GetEntitiesAsync(string tenantId)
{
    if (false == m_cache.TryGetValue(tenantId, out IList<Entity> tenantEntities) || tenantEntities.Count == 0)
    {
         // Do the expensive call.
         IList<Entity> tenantEntities = await ExpensiveServiceCallAsync(tenantId);

         m_cache.Set(tenantId, tenantEntities , m_cacheEntryOptions);
    }

    return tenantEntities;
}