Question

我想将null值转换为Nullable（Of）-Type。我可以使用CType（）进行转换，但不能使用System.Convert.ChangeType（）进行转换。

有没有办法这样做？为什么会抛出异常？

Dim b as Boolean? = CType(Nothing, Boolean?) 'ok
System.Convert.ChangeType(Nothing, GetType(Boolean?)) 'Throws System.InvalidCastException

Answer 1

有办法吗？

let obsList = [ for x in 0..80 ->  (Observable.map (fun _-> x) ((Array.get(gameButtons)(x)).Click)) ]

let rec obsMerger obsList =
        match obsList with
        | x1::[] -> x1
        | x1::xs -> Observable.merge x1 (obsMerger xs)
        | [] -> failwith "No Observables"

为什么会引发异常？

由于Dim valueNothing = ConvertHelper.SafeChangeType(Of Boolean)(Nothing) Dim valueTrue = ConvertHelper.SafeChangeType(Of Boolean)(True) Dim valueFalse = ConvertHelper.SafeChangeType(Of Boolean)(False) ' ... Class ConvertHelper Shared Function SafeChangeType(Of T As Structure)(ByVal value As Object) As T? Return If(value Is Nothing, Nothing, DirectCast(Convert.ChangeType(value, GetType(T)), T?)) End Function End Class方法Secure Cookie：

Convert.ChangeType

将Nothing转换为Nullable（Of）

1 个答案: