C＃字典< int，int>会发生什么如果密钥不存在查找？ [英] What happens to C# Dictionary<int, int> lookup if the key does not exist?

问题描述

我尝试检查null，但编译器警告这种情况永远不会发生。我应该寻找什么？

I tried checking for null but the compiler warns that this condition will never occur. What should I be looking for?

推荐答案

假设你想获得价值，如果密钥 存在，请使用 词典< TKey，TValue> .TryGetValue ：

Assuming you want to get the value if the key does exist, use Dictionary<TKey, TValue>.TryGetValue:

int value;
if (dictionary.TryGetValue(key, out value))
{
    // Key was in dictionary; "value" contains corresponding value
} 
else 
{
    // Key wasn't in dictionary; "value" is now 0
}

（使用 ContainsKey 然后索引器使它看起来两次，这是非常无意义的。）

(Using ContainsKey and then the the indexer makes it look the key up twice, which is pretty pointless.)

注意，即使你 / em>使用引用类型，检查null将不起作用 - Dictionary<，> 的索引器将抛出异常，如果您请求缺少的键，而不是返回空值。 （这是 Dictionary<，> 和 Hashtable 之间的一个很大的区别。）

Note that even if you were using reference types, checking for null wouldn't work - the indexer for Dictionary<,> will throw an exception if you request a missing key, rather than returning null. (This is a big difference between Dictionary<,> and Hashtable.)

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