c# – 如何设置可选参数,无需编译时常数

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,如下所示：

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

不使用编译时常数？

解决方法

简而言之.没有.

需要可选参数来编译时间常数或值类型.

从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.

你似乎想要实现的是可以通过重载来实现的：

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

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

请注意,这与可选参数不完全相同,因为默认值通过可选参数进行硬编码到调用者中(这就是为什么存在限制).