c# - 字典中的无效键

Question

为什么字典在使用无效键索引集合时不只是返回 null？

Answer 1

因为通用字典可以包含值类型的实例，而 null 对值类型无效。例如:

var dict = new Dictionary<string, DateTime>();
DateTime date = dict["foo"]; // What should happen here?  date cannot be null!

您应该改用字典的 TryGetValue 方法:

var dict = new Dictionary<string, DateTime>();
DateTime date;

if (dict.TryGetValue("foo", out date)) {
    // Key was present; date is set to the value in the dictionary.
} else {
    // Key was not present; date is set to its default value.
}

此外，存储引用类型的字典仍将存储空值。并且您的代码可能认为“value is null”不同于“key does not exist”。