If you want to check the property value or string which is Null or not using the following code in vb .Net.
Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
Dim cust As New Customer()
cust.firstname = "Rasik"
If Not String.IsNullOrEmpty(cust.firstname.ToString()) Then
' yourcode here
Else
' yourcode here
End If
End Sub
The above example clears how to use IsNullOrEmpty, which will return true or false. It indicates whether the property value or string is empty or null.