DateDiff不适用于nullables

Question

我正试图找出可以为空的日期之间的区别。 DateDiff(DateInterval.Day, firstDate, secondDate)适用于Date，但适用于Nullable(Of Date)。我的日期都是可以为空的字段。

这是错误消息：

  

重载解析失败，因为没有缩小转换就无法调用可访问的DateDiff：

     

  

• Public Function DateDiff(Interval As String, Date1 As Object, Date2 As Object, [DayOfWeek As Microsoft.VisualBasic.FirstDayOfWeek = FirstDayOfWeek.Sunday], [WeekOfYear As Microsoft.VisualBasic.FirstWeekOfYear = FirstWeekOfYear.Jan1]) As Long：参数匹配参数Interval从Microsoft.VisualBasic.DateInterval缩小为String。

  

• Public Function DateDiff(Interval As Microsoft.VisualBasic.DateInterval, Date1 As Date, Date2 As Date, [DayOfWeek As Microsoft.VisualBasic.FirstDayOfWeek = FirstDayOfWeek.Sunday], [WeekOfYear As Microsoft.VisualBasic.FirstWeekOfYear = FirstWeekOfYear.Jan1]) As Long：参数匹配参数Date1从Date?缩小为Date。

  

• Public Function DateDiff(Interval As Microsoft.VisualBasic.DateInterval, Date1 As Date, Date2 As Date, [DayOfWeek As Microsoft.VisualBasic.FirstDayOfWeek = FirstDayOfWeek.Sunday], [WeekOfYear As Microsoft.VisualBasic.FirstWeekOfYear = FirstWeekOfYear.Jan1]) As Long：参数匹配参数Date2从Date?缩小为Date。

  

Answer 1

您必须在可空的.Value对象上调用Date。

DateDiff(DateInterval.Day, firstDate.Value, secondDate.Value) 

Answer 2

如果您知道他们不是Nothing，请获取他们的值：

DateDiff(DateInterval.Day, firstDate.Value, secondDate.Value)

但是你可以利用operator overloading，因为这不是VB6：

(secondDate.Value - firstDate.Value).Days

Answer 3

2年后回答问题......：）

上述错误可能是由数据类型未命中匹配引起的。 使用DateValue函数，例如

DateDiff(DateInterval.Day, DateValue(stringwiDate.text), DateValue(string2witDate.text))