字典返回默认值，如果该键不存在 [英] Dictionary returning a default value if the key does not exist

问题描述

我发现自己使用目前的格局常常在我的code时下

I find myself using the current pattern quite often in my code nowadays

var dictionary = new Dictionary<type, IList<othertype>>();
// Add stuff to dictionary

var somethingElse = dictionary.ContainsKey(key) ? dictionary[key] : new List<othertype>();
// Do work with the somethingelse variable

或者有时候

var dictionary = new Dictionary<type, IList<othertype>>();
// Add stuff to dictionary

IList<othertype> somethingElse;
if(!dictionary.TryGetValue(key, out somethingElse) {
    somethingElse = new List<othertype>();
}

这两种方式感到相当迂回。我真的希望是像

Both of these ways feel quite roundabout. What I really would like is something like

dictionary.GetValueOrDefault(key)

现在，我可以写字典的类，它这对我来说一个扩展方法，但我想，我可能会丢失已经存在的东西。那么，有没有一种方式，一种方式是更容易对眼睛，而无需编写扩展方法来字典做到这一点？

Now, I could write an extension method for the dictionary class that does this for me, but I figured that I might be missing something that already exists. SO, is there a way to do this in a way that is more "easy on the eyes" without writing an extension method to dictionary?

推荐答案

TryGetValue 将已为类型字典分配默认值，因此你可以使用：

TryGetValue will already assign the default value for the type to the dictionary, so you can just use:

dictionary.TryGetValue(key, out value);

和忽略返回值。然而，真正的将会的只是返回默认（TValue），而不是一些自定义的默认值（也没有，更有效，执行委托的结果） 。没有什么更强大的内置的框架。我建议两拓方法：

and just ignore the return value. However, that really will just return default(TValue), not some custom default value (nor, more usefully, the result of executing a delegate). There's nothing more powerful built into the framework. I would suggest two extension methods:

public static TValue GetValueOrDefault<TKey, TValue>
    (this IDictionary<TKey, TValue> dictionary, 
     TKey key,
     TValue defaultValue)
{
    TValue value;
    return dictionary.TryGetValue(key, out value) ? value : defaultValue;
}

public static TValue GetValueOrDefault<TKey, TValue>
    (this IDictionary<TKey, TValue> dictionary,
     TKey key,
     Func<TValue> defaultValueProvider)
{
    TValue value;
    return dictionary.TryGetValue(key, out value) ? value
         : defaultValueProvider();
}

（您可能希望把参数检查中，当然是：）

(You may want to put argument checking in, of course :)

这篇关于字典返回默认值，如果该键不存在的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！