Question

In my code, I want to check if a referrer url exists and if the referrer url contains a specific sub string. I know how to check for the sub string:

If( InStr( Request.UrlReferrer.ToString(), "some sub string here" ) > 0 ) Then

But I don’t know how to check if a referrer exists or not. No refer exists if the url is entered in manually in the address bar. So I tried this, but this does not work:

If (Not (Request.UrlReferrer.ToString() = "")) And (InStr(Request.UrlReferrer.ToString(), "some sub string here") > 0) Then

Anyone know how do check if a referrer exists?

Answer 1

I think your problem is you’re using And when you should be using AndAlso.

And is NOT short-circuit. So the runtime will evaluate each side of the And anyway, then return true if both sides are true. So you’re probably getting a NullReferenceException I guess because you’re trying to look if the referrer contains something, when the referrer is null, because you’ve not used short-circuit evaluation.

Conversely, AndAlso is short-circuit and will not bother to evaluate the second half of the statement if the first half is false.

I’ve never yet found a good, compelling reason to use And in VB instead of AndAlso.

Anyway this should work for you.

If (Not IsNothing(Request.UrlReferrer)) AndAlso Request.UrlReferrer.ToString.Contains("substring") Then
    Dosomething
End If