.NET Dictionary: get existing value or create and add new value

We have a slightly different take on this, but the effect is similar:

public static TValue GetOrCreate<TKey, TValue>(this IDictionary<TKey, TValue> dict, TKey key) 
    where TValue : new()
{
    if (!dict.TryGetValue(key, out TValue val))
    {
        val = new TValue();
        dict.Add(key, val);
    }

    return val;
}

Called:

var dictionary = new Dictionary<string, List<int>>();

List<int> numbers = dictionary.GetOrCreate("key");

It makes use of the generic constraint for public parameterless constructors: where TValue : new().

To help with discovery, unless the extension method is quite specific to a narrow problem, we tend to place extension methods in the namespace of the type they are extending, in this case:

namespace System.Collections.Generic

Most of the time, the person using the type has the using statement defined at the top, so IntelliSense would also find the extension methods for it defined in your code.