VB .NET空条件

Question

这是我的代码 -

  Dim provider As CultureInfo = CultureInfo.InvariantCulture 
      Dim a1 As DateTime = Nothing 
      insexp = DateTime.ParseExact(date1.SelectedValue, "MMMM yyyy", provider) 
      If a1.Month = Today.Month AndAlso a1.Year = Today.Year Then 
       a1 = Today.Date 
      End If 

这只会工作，如果date1.selectedvalue不为空，但如果是空会崩溃。 我如何给条件来运行这只有如果不是空？ 感谢

Answer 1

If date1.SelectedValue IsNot Nothing Then 
    ... 
End If 

Answer 2

Dim provider As CultureInfo = CultureInfo.InvariantCulture 
     Dim a1 As DateTime = Nothing 
     If not String.IsNullOrEmpty(date1.SelectedValue) Then   
    insexp = DateTime.ParseExact(date1.SelectedValue, "MMMM yyyy", provider) 
     End If 
    If a1.Month = Today.Month AndAlso a1.Year = Today.Year Then 
     a1 = Today.Date 
    End If