C# equivalent of Python's defaultdict (for lists) in C#

Question

What is the C# equivalent of doing:

>>> from collections import defaultdict
>>> dct = defaultdict(list)
>>> dct['key1'].append('value1')
>>> dct['key1'].append('value2')
>>> dct
defaultdict(<type 'list'>, {'key1': ['value1', 'value2']})

For now, I have:

Dictionary<string, List<string>> dct = new Dictionary<string, List<string>>();
dct.Add("key1", "value1");
dct.Add("key1", "value2");

but that gives errors like "The best overloaded method match has invalid arguments".

Answer 1

Here's an extension method you can add to your project to emulate the behavior you want:

public static class Extensions
{
    public static void AddOrUpdate<TKey, TValue>(this Dictionary<TKey, List<TValue>> dictionary, TKey key, TValue value)
    {
        if (dictionary.ContainsKey(key))
        {
            dictionary[key].Add(value);
        }
        else
        {
            dictionary.Add(key, new List<TValue>{value});
        }
    }
}

Usage:

Dictionary<string, List<string>> dct = new Dictionary<string, List<string>>();
dct.AddOrUpdate("key1", "value1");
dct.AddOrUpdate("key1", "value2");