默认字典，带有C＃中的列表

Question

Is there a way to have a Dictionary in C# that will automatically have an empty list associated with any key, other than creating such a class myself? I would like to avoid the following code if possible:

int x = 0;
int y = 42;
Dictionary<int, List<int>> dict = new Dictionary<int, List<int>>();

List<int> newList;
if (dict.containsKey(x))
{
    dict[x].Add(y);
}
else
{
    dict[x] = new List<int>{y};
}

or possibly:

int x = 0;
int y = 42;
Dictionary<int, List<int>> dict = new Dictionary<int, List<int>>();

List<int> newList;
if (dict.TryGetValue(x, out newList))
{
    newList.Add(y);
}
else
{
    dict[x] = new List<int>{y};
}

Answer 1

This behavior can be abstracted nicely with an extension method.

public static TValue GetOrAdd<TKey, TValue>(this IDictionary<TKey, TValue> dictionary, TKey key, Func<TKey, TValue> valueFactory)
{
    if (dictionary == null) throw new ArgumentNullException(nameof(dictionary));
    if (valueFactory == null) throw new ArgumentNullException(nameof(valueFactory));

    TValue value;
    if (!dictionary.TryGetValue(key, out value))
    {
        value = valueFactory.Invoke(key);
        dictionary.Add(key, value);
    }
    return value;
}

The method signature is identical to ConcurrentDictionary.GetOrAdd() .

var list = dict.GetOrAdd(x, (_) => new List<int>());
list.Add(y);

Answer 2

you already have answer, dictionary does not have such ability, using extension method just fakes it and hides the fact that you are looking up dictionary twice (in case key doesn't exist). but you will write one line of code if that makes you happy.

In c#7 how ever you can pack things more a little bit.

if (dict.TryGetValue(x, out var list)) list.Add(y);
else dict.Add(x, new List<int>{y});

this will have one lookup if key already exist, and two lookups if key doesn't exist.

Answer 3

Usually my code is:

if (!dict.containsKey(x)) 
    dict[x] = new List<int>();

dict[x].Add(y);

I believe it's easier to read