F#编译器警告FS0067用于防护时的异常

Question

在F#中,我们可以方便地使用failwith和failwithf抛出异常实例.因此,您可能偶尔需要使用"守卫时"来区分不同的异常情况.

这是一个说明性的例子:

    try
        let isWednesday = DateTime.Now.DayOfWeek = DayOfWeek.Wednesday
        if isWednesday then failwith "Do not run this on Wednesdays!"
    with
    | :? DivideByZeroException -> printfn "You divided by zero."
    | :? Exception as ex when ex.Message.Contains("Wednesdays") -> printfn "I said, %s" ex.Message
    | ex -> printfn "%s" ex.Message

但是,上面的代码会产生两个警告:

Program.fs(14,7): warning FS0067: This type test or downcast will always hold 
Program.fs(14,7): warning FS0067: This type test or downcast will always hold 

我该如何避免这种警告？

Answer 1

删除类的类型测试模式Exception.这是不必要的.

with
| :? DivideByZeroException -> printfn "You divided by zero."
| ex when ex.Message.Contains("Wednesdays") -> printfn "I said, %s" ex.Message
| ex -> printfn "%s" ex.Message