无法将null分配给隐式类型的变量 [英] Cannot assign null to an implicitly-typed variable

问题描述

根据Visual Studio，这不行:

According to Visual Studio this is not ok:

var foo = null;

但这没关系:

var foo = false ? (double?)null : null;

为什么?(double?)null 是否还会影响else分支中的 null ?

Why? Is the (double?)null affecting also the null in the else branch?

推荐答案

隐式键入变量声明/赋值有两个目的:

Implicitly typed variable declaration/assignment serves two purposes:

• 确定变量的值，并且
• 确定变量的类型.

您的第一个声明的值带有 null ，没有办法弄清楚类型(它可以是从 System.Object 派生的任何内容，也可以是Nullable< T> ).这就是为什么这是一个错误.

Your first declaration has null for the value, with no way to figure out the type (it could be anything derived from System.Object, or a Nullable<T>). That is why it is an error.

您的第二个声明由于强制类型转换而将类型精确定位为 Nullable< double> .这就是C#允许的原因.

Your second declaration pinpoints the type as Nullable<double> because of the cast. That is why C# allows it.

不用说 double吗?foo = null 会更容易阅读.

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