.NET Dictionary: get or create new

Question

I often find myself creating a Dictionary with a non-trivial value class (e.g. List), and then always writing the same code pattern when filling in data.

For example

Answer 1

Here is a solution in case the constructor requires a parameter.

public static TValue GetOrCreate(this IDictionary dict, TKey key, Func createNew)
    {
        if (!dict.TryGetValue(key, out var val))
        {
            val = createNew();
            dict.Add(key, val);
        }

        return val;
    }

Simple to use:

MyDict.GetOrCreate(si.Id, createNew: () => new ObjectKnowingItsId(si.Id))