Is this non-locked TryGetValue() dictionary access thread-safe?

private object lockObj = new object();

private Dictionary<int, string> dict = new Dictionary<int, string>();

public string GetOrAddFromDict(int key)
{
    string value;

    // non-locked access:
    if (dict.TryGetValue(key, out value))
        return value;

    lock (this.lockObj)
    {
        if (dict.TryGetValue(key, out value))
            return value;

        string newValue = "value of " + key; // place long operation here
        dict.Add(key, newValue);

        return newValue;
    }
}

Question a: Is it thread-safe? If yes, why?

Question b: How is this double-TryGetValue() pattern called?