Question

I’ve created this extension method

public static void AddIfNullCreate<TKey, TValue>(this IDictionary<TKey, TValue> dictionary, TKey key, TValue value)
 {
     if (dictionary == null)
     {
         dictionary = new Dictionary<TKey, TValue>();
     }

     dictionary.Add(key, value);
 }

But when I use it

    public void DictionaryTest()
    {
        IDictionary<int, string> d = GetD();

        d.AddIfNullCreate(1,"ss");
    }

    private IDictionary<int, string> GetD()
    {
        return null;
    }

After calling AddIfNullCreate is d null. Why is that so ?

Answer 1

Just like any other method, a change to the parameter doesn’t change the caller’s argument unless it’s a ref parameter (which it can’t be for an extension method first parameter). The argument is passed by value, even if that value is a reference.

One option is to return the dictionary too:

public static IDictionary<TKey, TValue> AddIfNullCreate<TKey, TValue>
    (this IDictionary<TKey, TValue> dictionary, TKey key, TValue value)
{
    if (dictionary == null)
    {
        dictionary = new Dictionary<TKey, TValue>();
    }

    dictionary.Add(key, value);
    return dictionary;
}

Then:

d = d.AddIfNullCreate(1, "ss");

However, I’m not sure I’d really do that. I think I’d just conditionally create the dictionary in the method itself:

public void DictionaryTest()
{
    IDictionary<int, string> d = GetD() ?? new Dictionary<int, string>();

    d[1] = "ss";
}