对嵌套类型定义使用指令

Question

我正在尝试使用 using 指令以功能方式定义类型，以使代码更具可读性。假设我的 example.cs 文件如下所示：

using A = System.Tuple<int, int>;
using B = List<A>;

我收到错误：

CS0246 The type or namespace name 'A' could not be found (are you missing a using directive or an assembly reference?)

我可以用 using 定义嵌套类型吗？

Answer 1

docs.microsoft.com 上的 C# 语言规范有一节涵盖 Using directive, specifically the type of using directive you're attempting to use is a "Using alias directive":

A using_alias_directive (Using alias directives) introduces an alias for a namespace or type.

文档突出显示了 using_alias_directive 的允许结构，如：

using_alias_directive
    : 'using' identifier '=' namespace_or_type_name ';'
    ;

请注意，这只允许使用 "namespace_or_type_name"，但不允许使用另一个 "using_alias_directive"。可能的原因之一，这可以通过他们的文档部分找到（强调我的）：

The order in which using_alias_directives are written has no significance, and resolution of the namespace_or_type_name referenced by a using_alias_directive is not affected by the using_alias_directive itself or by other using_directives in the immediately containing compilation unit or namespace body. In other words, the namespace_or_type_name of a using_alias_directive is resolved as if the immediately containing compilation unit or namespace body had no using_directives

简而言之，恐怕您不能在另一个别名中使用别名。

对嵌套类型定义使用指令

Using directive for nested type definitions

c#

using-directives