将 Nothing 转换为 Nullable(Of )

Question

我想将空值转换为 Nullable(Of ) 类型。我可以用 CType() 转换它，但不能用 System.Convert.ChangeType().

转换它

有办法吗？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

Is there a way to do so?

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

Why it throws an exception?

由于Convert.ChangeType方法implementation:

if( value == null ) {
    if(conversionType.IsValueType) {
        throw new InvalidCastException(Environment.GetResourceString("InvalidCast_CannotCastNullToValueType"));
    }
    return null; 
}

将 Nothing 转换为 Nullable(Of )

Convert Nothing into Nullable(Of )

.net

vb.net

casting

nullable