Question

Can you please tell me why this line of code gives error:- “Object reference not set to instance of an object”

 searchstr = IIf(
    Not searchstr Is Nothing
    AndAlso searchstr.Length > 0,
    searchstr.Replace("Desig_id", "designation_id"), "")  

Also can you provide alternative to make this type of validation?

Answer 1

The IIf function isn’t behaving how you think it does. It’s not a ternary-style operator a la C++ or C#. It’s just a normal function, which means both your conditional check and the searchstr.Replace are being evaluated before passing it to IIf. If searchstr is null, searchstr.Replace(…) is going to throw a NullReferenceException, even though you expected it wouldn’t be called at all.

The If operator was added in VS2008 to support this, but if you’re in 2005, you have to expand it to an If/Else block:

If Not searchstr Is Nothing AndAlso searchstr.Length > 0 Then
    searchstr = searchstr.Replace("Desig_id", "designation_id")
Else
    searchstr = ""
End If