为什么插入const字符串会导致编译器错误？ [英] Why does interpolating a const string result in a compiler error?

问题描述

为什么C＃中的字符串插值不适用于const字符串？例如：

Why does string interpolation in c# does not work with const strings? For example:

private const string WEB_API_ROOT = "/private/WebApi/";
private const string WEB_API_PROJECT = $"{WEB_API_ROOT}project.json";

从我的角度来看，所有内容在编译时都是已知的。还是要在以后添加的功能？

From my point of view, everything is known at compile time. Or is that a feature that will be added later?

编译器消息：

分配给'DynamicWebApiBuilder.WEB_API_PROJECT'的表达式必须是常量。

The expression being assigned to 'DynamicWebApiBuilder.WEB_API_PROJECT' must be constant.

非常感谢！

推荐答案

插值的字符串仅转换为对 string.Format 的调用。因此，您上面的行实际上是读取

Interpolated strings are simply converted to calls to string.Format. So your above line actually reads

private const string WEB_API_PROJECT = string.Format("{0}project.json", WEB_API_ROOT);

由于包含方法调用，因此这不是编译时间常数。

And this is not compile time constant as a method call is included.

另一方面，字符串 concatenation （具有简单，恒定的字符串常量）可以由编译器完成，因此会起作用：

On the other hand, string concatenation (of simple, constant string literals) can be done by the compiler, so this will work:

private const string WEB_API_ROOT = "/private/WebApi/";
private const string WEB_API_PROJECT = WEB_API_ROOT + "project.json";

或从 const 切换到静态只读：

private static readonly string WEB_API_PROJECT = $"{WEB_API_ROOT}project.json";

因此将初始化字符串（和 string.Format 被调用）在对声明类型的任何成员的首次访问中。

so the string is initialized (and string.Format called) at the first access to any member of the declaring type.

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