C# MemoryCache add to list

Im using some cache control in my c# web-api server.

Im using the following code:

private static MemoryCache _cache = new MemoryCache("ExampleCache");

public static object GetItems(string key) {
    return AddOrGetExisting(key, () => InitItem(key));
}

private static List<T> AddOrGetExisting<T>(string key, Func<List<T>> valueFactory)
{
    var newValue = new Lazy<List<T>>(valueFactory);
    var oldValue = _cache.AddOrGetExisting(key, newValue, new CacheItemPolicy()) as Lazy<List<T>>;
    try
    {
        return (oldValue ?? newValue).Value;
    }
    catch
    {
        _cache.Remove(key);
        throw;
    }       
}

private static List<string> InitItem(string key) {
    // im actually fetching a list from the db..but for the sake of the example
    return new List<string>()
}

now, everything works nicely. BUT, this time I want to update something in my db and after that, I want to update the cache control so I wont have to query my db.

Assume im using an object which looks like this

Public class Foo{
         public string Id;
         public List<string> values;
}

And assume T is Foo in this example

I need to add an item to the list which is stored in the _cache by Foo's Id field. I would insanely appericate if the process will be thread safe.

TIA.