c# - 嵌套开关表达式

Question

我有以下使用 switch 表达式的语句:

var levelType = task switch
{
    var t when t.Status == TaskStatus.EXECUTING && t.PlanIDs.Contains(currentPlan.ID) => WorkplaceAssociatedLevelTypes.Critical,
    var t when t.Status == TaskStatus.WAITING && t.PlanIDs.Contains(currentPlan.ID) => WorkplaceAssociatedLevelTypes.Warning,
    _ => WorkplaceAssociatedLevelTypes.Untyped
};

正如我们所见，t.PlanIDs.Contains(currentPlan.ID) 重复了两次，我想避免它。

我不确定是否可以使用 switch 表达式执行嵌套语句。

类似下面的内容:

var levelType = task switch
{
    case (task.PlanIDs.Contains(currentPlan.ID))
      true: 
        var t when t.Status == TaskStatus.EXECUTING => WorkplaceAssociatedLevelTypes.Critical,
        var t when t.Status == TaskStatus.WAITING => WorkplaceAssociatedLevelTypes.Warning,
        _ => WorkplaceAssociatedLevelTypes.Untyped
      false: WorkplaceAssociatedLevelTypes.Untyped
};

上面的代码无法编译，我发明这个只是为了演示我在寻找什么。

使用 if-else 有效:

var levelType = WorkplaceAssociatedLevelTypes.Untyped;

    if (task.PlanIDs.Contains(currentPlan.ID))
    {
        if (task.Status == TaskStatus.EXECUTING)
        {
            levelType = WorkplaceAssociatedLevelTypes.Critical;
        }
        else if (task.Status == TaskStatus.WAITING)
        {
            levelType = WorkplaceAssociatedLevelTypes.Warning;
        }
    }

Answer 1

switch 表达式的语法不会因为你想嵌套它们而改变，它一直都是一样的。

([expr] switch {
    [case] => [expr],
})

([expr] switch {
    [case] => ([expr] switch {
        [case] => [expr]
    }),
})

在你的情况下:

var levelType = task.PlanIDs.Contains(currentPlan.ID) switch
{
    true => t.Status switch
    {
        TaskStatus.EXECUTING => WorkplaceAssociatedLevelTypes.Critical,
        TaskStatus.WAITING => WorkplaceAssociatedLevelTypes.Warning,
        _ => WorkplaceAssociatedLevelTypes.Untyped,
    },
    _ => WorkplaceAssociatedLevelTypes.Untyped,
};

尽管您可能只使用一个 switch 表达式:

var levelType = task.Status switch
{
    // cases are evaluated in the order they are given
    _ when !task.PlanIDs.Contains(currentPlan.ID) => WorkplaceAssociatedLevelTypes.Untyped,
    TaskStatus.EXECUTING => WorkplaceAssociatedLevelTypes.Critical,
    TaskStatus.WAITING => WorkplaceAssociatedLevelTypes.Warning,
    _ => WorkplaceAssociatedLevelTypes.Untyped,
};

考虑到您使用的是 bool 表达式，我可能会选择使用条件表达式:

var levelType = task.PlanIDs.Contains(currentPlan.ID)
    ? task.Status switch
    {
        TaskStatus.EXECUTING => WorkplaceAssociatedLevelTypes.Critical,
        TaskStatus.WAITING => WorkplaceAssociatedLevelTypes.Warning,
        _ => WorkplaceAssociatedLevelTypes.Untyped,
    }
    : WorkplaceAssociatedLevelTypes.Untyped;