将 SQL 日期时间设置为空/无 vb.net

Question

我在SQL中有一个Date列（日期时间数据类型）。对于特定场景，我需要在浏览器中将其显示为空白。我试过了

Dim DOB As Nullable(Of Date) = Date.Now
Dim datestring As String = DOB.Value.ToString("d")
DOB = Nothing

但它给了我错误地说DateTime was not in a proper format.

这是怎么回事？

Answer 1

你的代码会抛出DateTime was not in a proper format的原因是因为你很可能不做Dim DOB As Nullable(Of Date) = Date.Now。您从数据库中获取日期，然后直接输入值，值为空。

假设您有一个带有可为空的日期字段的 Sql 表，并且您使用 DataReader 读取它，您可以这样做

Dim strDate as string
if IsDbNull(reader("myDateColumn")) then 
    strDate = String.empty
else
   DirectCast(reader("myDateColumn"), DateTime).ToString("your format here")
End If

较短的版本将是

Dim strDate as string = If(IsDbNull(reader("myDateColumn")), String.empty,
                           DirectCast(reader("myDateColumn"), DateTime).ToString("your format here"))

Answer 2

您可以尝试以下任一操作：

Dim datestring As String = IF(DOB Is Nothing,"", DOB.Value.ToString("d"))

或

Dim datestring As String = ""
If Not DoB Is Nothing Then
  datestring = DOB.Value.ToString("d")
End If

Answer 3

如果

.HasValue Null
类型Nullable(Of..)属性用于签入并.Value属性来获取使用.ToString的DateTime对象

Dim dbdate As Nullable(Of DateTime) = DateTime.Now
Dim datetxt As String = If(dbdate.HasValue = true, 
                           dbdate.Value.ToString("d"), 
                           "")

那么我认为String.Format会更好使用：

Dim dbdate As Nullable(Of DateTime) = DateTime.Now
Dim datetxt As String = String.Format("{0:d}", dbdate)
dbdate = Nothing
datetxt = String.Format("{0:d}", dbdate)