C#8.0 switch 语句和局部变量

Question

试图了解 switch 语句的新 C#8.0 语法的限制我正在尝试将一些旧代码从 C#7.0 转换为 C#8.0。我想使用本地引入的变量（模式匹配），但显然我做错了。

作为参考，propertySelectorExpression 的类型为 Expression<Func<TObject, TProperty>> propertySelectorExpression

旧代码：

PropertyInfo propertyInfo = null;
switch (propertySelectorExpression.Body)
{
    case MemberExpression me:
        propertyInfo = me.Member as PropertyInfo;
        break;
    case UnaryExpression ue:
        propertyInfo = (ue.Operand as MemberExpression)?.Member as PropertyInfo;
        break;
}

新代码（尝试，未编译）：

PropertyInfo propertyInfo = null;
propertyInfo = switch (propertySelectorExpression.Body)
{
    MemberExpression me => me.Member as PropertyInfo,
    UnaryExpression ue => (ue.Operand as MemberExpression)?.Member as PropertyInfo
}

或

PropertyInfo propertyInfo = null;
switch (propertySelectorExpression.Body)
{
    MemberExpression me => propertyInfo = me.Member as PropertyInfo,
    UnaryExpression ue => propertyInfo = (ue.Operand as MemberExpression)?.Member as PropertyInfo
}

我做错了什么或假设错了什么？

Answer 1

试试这个：

PropertyInfo propertyInfo = null;
propertyInfo = propertySelectorExpression.Body switch
{
    MemberExpression me => me.Member as PropertyInfo,
    UnaryExpression ue => (ue.Operand as MemberExpression)?.Member as PropertyInfo
}

更多信息请参阅documentation关于新的开关表达式格式

Answer 2

下面的代码片段应该可以工作，变量出现在 switch 关键字之前

var propertyInfo = propertySelectorExpression.Body switch
{
    MemberExpression me => (me.Member as PropertyInfo),
    UnaryExpression ue => ((ue.Operand as MemberExpression)?.Member as PropertyInfo),
    _ => null
};

您必须管理默认值，在您的案例中为空。 default 案例被替换为 _ 丢弃。

Answer 3

您混淆了 switch 的两种用法的语法。

你有：

切换语句
开关表达式

开关表达式的语法是<expression> switch { <patterns> }，所以这应该编译：

PropertyInfo propertyInfo = null;
propertyInfo = propertySelectorExpression.Body switch // <-- noticed the switched order
{
    MemberExpression me => me.Member as PropertyInfo,
    UnaryExpression ue => (ue.Operand as MemberExpression)?.Member as PropertyInfo
}

（尽管编译器会警告您 switch 表达式无法处理所有可能的输入）

switch statement 的语法（你问题中的最后一个代码示例）使用 case 而不是 lambda 语法，所以这应该编译：

PropertyInfo propertyInfo = null;
switch (propertySelectorExpression.Body)
{
    case MemberExpression me:
        propertyInfo = me.Member as PropertyInfo;
        break;
    case UnaryExpression ue:
        propertyInfo = (ue.Operand as MemberExpression)?.Member as PropertyInfo;
        break;
}

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