c# - 如何在没有编译时常量的情况下设置可选参数

Question

有没有办法写下面的C#方法:

public string Download(Encoding contentEncoding = null) {
    defaultEncoding = contentEncoding ?? Encoding.UTF8;
    // codes...
}

添加了默认参数，看起来像这样:

public string Download(Encoding contentEncoding = Encoding.UTF8) {
    // codes...
}

不使用编译时常量？

Answer 1

简而言之。没有。

可选参数必须是编译时常量或值类型。

来自 Named and Optional Arguments (C# Programming Guide)在 MSDN 上:

Each optional parameter has a default value as part of its definition. If no argument is sent for that parameter, the default value is used. A default value must be one of the following types of expressions:

• a constant expression;
• an expression of the form new ValType(), where ValType is a value type, such as an enum or a struct;
• an expression of the form default(ValType), where ValType is a value type.

---

你似乎想要实现的可以通过重载来实现:

public string Download()
{
   return Download(Encoding.UTF8);
}

public string Download(Encoding contentEncoding)
{
   defaultEncoding = contentEncoding ?? Encoding.UTF8;
   // codes...
}

请注意，这与可选参数并不完全相同，因为默认值使用可选参数硬编码到调用者中(这就是存在对它们的限制的原因)。