[c#] Dictionary with list of strings as value

I have a dictionary where my value is a List. When I add keys, if the key exists I want to add another string to the value (List)? If the key doesn't exist then I create a new entry with a new list with a value, if the key exists then I jsut add a value to the List value ex.

Dictionary<string, List<string>> myDic = new Dictionary<string, List<string>>();
myDic.Add(newKey, add to existing list<strings> and not create new one)

To do this manually, you'd need something like:

List<string> existing;
if (!myDic.TryGetValue(key, out existing)) {
    existing = new List<string>();
    myDic[key] = existing;
}
// At this point we know that "existing" refers to the relevant list in the 
// dictionary, one way or another.
existing.Add(extraValue);

However, in many cases LINQ can make this trivial using ToLookup. For example, consider a List<Person> which you want to transform into a dictionary of "surname" to "first names for that surname". You could use:

var namesBySurname = people.ToLookup(person => person.Surname,
                                     person => person.FirstName);

I'd wrap the dictionary in another class:

public class MyListDictionary
{

    private Dictionary<string, List<string>> internalDictionary = new Dictionary<string,List<string>>();

    public void Add(string key, string value)
    {
        if (this.internalDictionary.ContainsKey(key))
        {
            List<string> list = this.internalDictionary[key];
            if (list.Contains(value) == false)
            {
                list.Add(value);
            }
        }
        else
        {
            List<string> list = new List<string>();
            list.Add(value);
            this.internalDictionary.Add(key, list);
        }
    }

}

A simpler way of doing it is:

var dictionary = list.GroupBy(it => it.Key).ToDictionary(dict => dict.Key, dict => dict.Select(item => item.value).ToList());

Just create a new array in your dictionary

Dictionary<string, List<string>> myDic = new Dictionary<string, List<string>>();
myDic.Add(newKey, new List<string>(existingList));