c# - Switch/case 语句中的 String.Empty 生成编译器错误

Question

如果 String.Empty 和 "" 一样好，那么编译器怎么会在 case 语句中抛出 string.Empty ？在我看来，没有什么比 string.Empty 更稳定的了。有人知道吗？谢谢!

switch (filter)
            {
     case string.Empty:  // Compiler error "A constant value is expected"

                break;

                case "":  // It's Okay.
                    break;

            }

Answer 1

您可以这样尝试:

switch(filter ?? String.Empty)

string.Empty 是只读字段，而 "" 是编译时常量。您还可以在此处阅读有关代码项目的文章 String.Empty Internals

//The Empty constant holds the empty string value.
//We need to call the String constructor so that the compiler doesn't
//mark this as a literal.
//Marking this as a literal would mean that it doesn't show up as a field 
//which we can access from native.

public static readonly String Empty = ""; 

旁注:

当您在方法中提供默认参数值时 (C# 4.0)，您也会看到此问题:

void myMethod(string filter = string.Empty){}

以上将导致编译时错误，因为默认值需要是常量。