F＃编译器警告FS0067带有when防护的异常

Question

在F＃中，我们可以方便地使用同时抛出Exception实例的failwith和failwithf。因此，您有时可能需要使用“时刻警惕”来区分不同的异常情况。

这是一个说明性示例：

    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，上面的代码会导致两个警告：

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 

如何避免该警告？

Answer 1

删除Exception类的类型测试模式。没必要。

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