C# switch 语句的默认标签如何处理可为空的枚举？
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How will a C# switch statement's default label handle a nullable enum?
提问by Trey Mack
How will a C# switch statement's default label handle a nullable enum?
C# switch 语句的默认标签如何处理可为空的枚举？
Will the default label catch nulls and any unhandled cases?
默认标签会捕获空值和任何未处理的情况吗？
采纳答案by Trey Mack
If it's null, it will hit the default label.
如果它为空，它将命中默认标签。
public enum YesNo
{
Yes,
No,
}
public class Program
{
public static void Main(string[] args)
{
YesNo? value = null;
switch (value)
{
case YesNo.Yes:
Console.WriteLine("Yes");
break;
case YesNo.No:
Console.WriteLine("No");
break;
default:
Console.WriteLine("default");
break;
}
}
}
The program will print default.
该程序将打印default.
Unless null is handled.
除非处理 null。
public class Program
{
public static void Main(string[] args)
{
YesNo? value = null;
switch (value)
{
case YesNo.Yes:
Console.WriteLine("Yes");
break;
case YesNo.No:
Console.WriteLine("No");
break;
case null:
Console.WriteLine("NULL");
break;
default:
Console.WriteLine("default");
break;
}
}
}
prints NULL.
打印NULL。
If you have an unhandled enum value that was added later:
如果您有稍后添加的未处理的枚举值：
public enum YesNo
{
Yes,
No,
FileNotFound,
}
public class Program
{
public static void Main(string[] args)
{
YesNo? value = YesNo.FileNotFound;
switch (value)
{
case YesNo.Yes:
Console.WriteLine("Yes");
break;
case YesNo.No:
Console.WriteLine("No");
break;
default:
Console.WriteLine("default");
break;
}
}
}
It still prints default.
它仍然打印default.
回答by Glenn Slayden
You can use the null-coalescing operator ??to route nullswitch values to a specific case label other than default:
您可以使用空合并运算符??将null开关值路由到特定案例标签，而不是default：
public static IEnumerable<String> AsStrings(this IEnumerable<Char[]> src)
{
Char[] rgch;
var e = src.GetEnumerator();
while (e.MoveNext())
{
switch ((rgch = e.Current)?.Length ?? -1)
{
case -1: // <-- value when e.Current is 'null'
yield return null;
break;
case 0:
yield return String.Empty;
break;
case 1:
yield return String.Intern(new String(rgch[0], 1));
break;
default: // 2...n
yield return new String(rgch);
break;
}
}
}
回答by Przemek Struciński
It's worth to mention that C# 8.0 introduced a new Property Pattern for a switch expression. Now you can implement default logic to switch by using floor:
值得一提的是，C# 8.0 为 switch 表达式引入了一个新的属性模式。现在您可以使用 floor 实现默认逻辑来切换：
public double Calculate(int left, int right, Operator op) =>
op switch
{
Operator.PLUS => left + right,
Operator.MINUS => left - right,
Operator.MULTIPLY => left * right,
Operator.DIVIDE => left / right,
_ => 0 // default
}
Ref. https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-8
参考 https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-8
