c# - 从字典中组合 "Check Add or Fetch"

Question

我厌倦了这个字典习语:

        Dictionary<Guid,Contact> Contacts;
        //...
        if (!Contacts.ContainsKey(id))
        {
            contact = new Contact();
            Contacts[id] = contact;
        }
        else
        {
            contact = Contacts[id];
        }

如果有一种语法允许从默认构造函数隐式创建新值(如果它不存在)(毕竟字典知道值的类型)，那就太好了。有人见过这样做的助手(例如扩展方法)吗？

Answer 1

实现:

public static TValue GetOrAdd<TKey, TValue>(this IDictionary<TKey, TValue> dictionary,
                                            TKey key, Func<TValue> valueCreator)
{
    TValue value;
    if (!dictionary.TryGetValue(key, out value))
    {
        value = valueCreator();
        dictionary.Add(key, value);
    }
    return value;
}

public static TValue GetOrAdd<TKey, TValue>(this IDictionary<TKey, TValue> dictionary,
                                            TKey key) where TValue : new()
{
   return dictionary.GetOrAdd(key, () => new TValue());
}

用法:

var contacts = new Dictionary<Guid, Contact>();
Guid id = ...

contacts.GetOrAdd(id).Name = "Abc"; // ok since Contact has public parameterless ctor
contacts.GetOrAdd(id, () => new Contact { Name = "John Doe" }).Age = 40;