.net dropdownlist if condition

I had posted this question before but didn't get the right answer - here's my code again with the if condition.

Dim provider As CultureInfo = CultureInfo.InvariantCulture 
Dim a1 As DateTime = Nothing 
If date1.selectedvalue isnot noth开发者_高级运维ing 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 

date1 is a dropdownlist in the aspx page and does not have a value if you don't select it. Its not a mandatory dropdownlist. When i run it in debugger mode I get the value of date1 = "". And that goes into the if condition and crashes. Please advice how to make this IF condition work

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If Not String.IsNullOrEmpty(date1.sSelectedValue) Then
    insexp = DateTime.ParseExact(date1.SelectedValue, "MMMM yyyy", provider) 
End If

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Check selectedindex instead of selectedvalue