Why is String.IsNullOrWhiteSpace(vbNullChar) = False?

Question

I am using 3rd party library to obtain data from a machine's variable. The data returns a string type, so after requesting data I'm validating that it is not an empty string like that:

Not String.IsNullOrWhiteSpace(data)

However, my program was failing, and I realized that when the variable in the machine was empty, the function above was returning True.

By debugging I noticed that the function in the 3rd party library returns vbNullChar when the variable is empty and String.IsNullOrWhiteSpace(vbNullChar) evaluates to False, when I would expect it to evaluate to True as String.IsNullOrWhiteSpace(vbNullString) does.

So is my only option to check as follows?

Not String.IsNullOrWhiteSpace(data) AndAlso data <> vbNullChar

Because it is neither a null reference nor an empty string? It is a string that contains 1 character, the \0 code. — Lasse V. Karlsen
– Lasse V. Karlsen, Commented Feb 28, 2019 at 8:54

Lasse V. Karlsen · Accepted Answer · 2019-02-28 11:15:27Z

Because the two things you want to compare here are different "null" things.

String.IsNullOrWhiteSpace returns true if:

The reference being passed to the method is null
or ... every character of the non-null string is categorized as a Whitespace character ("every character" is true if the string has a length of 0)

In this case you're trying to pass in Constants.vbNullChar which is neither, it is a string which consists of 1 character, the \0 character, which is not categorized as Whitespace.

Constants.vbNullString, on the other hand, is a null-reference, which String.IsNullOrWhiteSpace handles just fine.

As for how you should handle this concrete problem, I would do a replacement from vbNullChar to null (vbNullString) immediately after obtaining the results, so that this oddity will have to checked only one place.

Additionally, if you get back a string, does it end with this \0 character? If so then you probably want to remove that as well.

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