Nothing equals String.Empty, null does not equal String.Empty, what am I missing here?

Question

This must've have been asked before but I couldn't locate it. In a mixed code project (VB and C#) we were debugging some old Visual Basic code where a statement as follows could be found:

If Request.Params("xxx") <> "" Then
   'do something

I considered this a bug as Request.Params could be null, in which case the statement would've become false which wasn't the idea. So I thought. I just found out, -- probably for the tenth time and I will keep forgetting -- that the following two statements are not equal, while Nothing in VB should be equal to null in C# (thought I):

if(String.Empty == null)          // always false
If String.Empty = Nothing Then    ' always true

Should I dismiss this as a typical Microsoft backward compatibility effort, or should I consider this a huge error in the VB.NET compiler? Does anybody know the Microsoftean opinion on this oddity?

Answer 1

You want

If Not String.IsNullOrEmpty(Request.Params("xxx") Then
    ...
End If

Or

if (!String.IsNullOrEmpty(Request.Params("xxx")) {
    ...
}

Answer 2

Nothing has a special meaning in VB for strings. To test whether a string reference is null, you need:

If value Is Nothing

From the VB comparison operators documentation:

Numeric comparisons treat Nothing as 0. String comparisons treat Nothing as "" (an empty string).

I suspect this is just for backward compatibility with VB6 - it's not something I'd be happy with, if I were a VB developer.

A comparison of the form

If value = Nothing

is compiled to a call to Microsoft.VisualBasic.CompilerServices.Operators.CompareString which returns 0 (i.e. equal) if one operand is null and the other is empty.