Nullable Modifier--from <Essential c# 2.0>

最新推荐文章于 2025-12-03 17:30:03 发布

翻译最新推荐文章于 2025-12-03 17:30:03 发布 · 732 阅读

文章标签：

#c# #null #variables #reference #string

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本文介绍C#中如何使值类型变量能够被赋值为null，通过在变量类型后添加问号(?)实现这一特性，并展示了如何在代码中声明和使用这种可空的值类型变量。

三句话总结：

数值类型的变量不能使用null赋值；

在变量类型前面加上？表示该值为可空；

使用时显示转换为iny型，进行运算时不执行运算，也就是值为空。

原文：

Mark Michaelis

P53

As I pointed out earlier, value typescannot be assigned null because, by definition, they can’t contain reference,including references to nothing.

However, this presents a problem in thereal world, where values are missing. When specifying a count, for example,what do you enter if the count is unknown? One possible solution is todesignate a “magic” value, such as 0 or int.Max, but these are valid integers. Rather,it is desirable to assign null to thevalue type because this is not a valid integer.,

To declare variables that can store nullyou use the nullable modifier, ? . This c#2.0 feature appears in listing 2.17.

static void Main(string[] args)
    {
      int? count;
      //count = null;
      count = 22;
      switch (count)
      {
        case null:
          Console.WriteLine("the value is null:" + count.ToString() + "!");
          break;
        default:
          Console.WriteLine("the value is not null:" + count.ToString() + "!");
          break;
      }
}