C#8.0 switch 语句和局部变量

Question

为了了解 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)，但显然我做错了什么。

供引用，propertySelectorExpression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，不编译):

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

您混淆了 switch 的两种用法的语法.

你有:

• switch 语句
• 切换表达式

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 语句(您问题中的最后一个代码示例)的语法使用 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;
}