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

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".

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