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从字典中组合“检查添加或获取”

extension-methods dictionary syntax collections c#

Brent Arias · 技术社区 · 14 年前

我厌倦了这本字典的成语:

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

如果有语法允许从默认构造函数隐式创建新值(如果不存在的话,字典知道值的类型),那就太好了。有人看到这样做的助手(如扩展方法)吗?

3 回复 | 直到 14 年前

Ani 14 年前

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());
}

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;

Community CDub 8 年前

/// <param name="valueCreator">The expensive value creator function.</param>
public static T GetOrAdd<S, T>(this IDictionary<S, T> dict, S key, 
                               Func<T> valueCreator)
{
    T value;
    return dict.TryGetValue(key, out value) ? value : dict[key] = valueCreator();
}

doesn't have this feature out of the box to do all this in a single lookup

Demetris Leptos 8 年前

Func<K, T>

IDictionary<K,T>

public class WrappedDictionary<K, T> : IDictionary<K, T>
{
    public IDictionary<K, T> WrappedInstance { get; set; }

    public virtual T this[K key]
    {
        get
        {
            // CUSTOM RESOLUTION CODE GOES HERE
            return this.WrappedInstance[key];
        }
        set
        {
            this.WrappedInstance[key] = value;
        }
    }

    public int Count
    {
        get
        {
            return this.WrappedInstance.Count;
        }
    }

    public bool IsReadOnly
    {
        get
        {
            return this.WrappedInstance.IsReadOnly;
        }
    }

    public ICollection<K> Keys
    {
        get
        {
            return this.WrappedInstance.Keys;
        }
    }

    public ICollection<T> Values
    {
        get
        {
            return this.WrappedInstance.Values;
        }
    }

    public void Add(KeyValuePair<K, T> item)
    {
        this.WrappedInstance.Add(item);
    }

    public void Add(K key, T value)
    {
        this.WrappedInstance.Add(key, value);
    }

    public void Clear()
    {
        this.WrappedInstance.Clear();
    }

    public bool Contains(KeyValuePair<K, T> item)
    {
        return this.WrappedInstance.Contains(item);
    }

    public bool ContainsKey(K key)
    {
        return this.WrappedInstance.ContainsKey(key);
    }

    public void CopyTo(KeyValuePair<K, T>[] array, int arrayIndex)
    {
        this.WrappedInstance.CopyTo(array, arrayIndex);
    }

    public IEnumerator<KeyValuePair<K, T>> GetEnumerator()
    {
        return this.WrappedInstance.GetEnumerator();
    }

    public bool Remove(KeyValuePair<K, T> item)
    {
        return this.WrappedInstance.Remove(item);
    }

    public bool Remove(K key)
    {
        return this.WrappedInstance.Remove(key);
    }

    public bool TryGetValue(K key, out T value)
    {
        // CUSTOM RESOLUTION CODE GOES HERE
        return this.WrappedInstance.TryGetValue(key, out value);
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return this.WrappedInstance.GetEnumerator();
    }
}