VB.NET- Peformance when testing an empty string

Question

In VB6, I was told that when testing for an empty string, it would be much faster to check this by verifying the length of the string by using :

If Len("ABC") = 0 then 'fast

or

If LenB("ABC") = 0 then 'even faster

instead of:

If "ABC" = "" then 'slower

Do you know by any chance if this is true also in VB.NET? Thank you.

Answer 1

You should use:

If ("Value").Length > 0 Then

End If

Answer 2

I don't exactly the syntax for VB but in C# you can use the String static method IsNullOrEmpty:

String.IsNullOrEmpty("ABC")

When you're debug this method, it will check on the .Length of the parameter. Checking on Lengte is alwasy faster bcause on String compare, the compiler must create a new object to compare (in this case a "ABC" string).

Answer 3

It's not necessary no, I mean are you kidding me? How many people write programs where comparing a string to see if it is zero length has any impact at all on performance. Even if strings didn't retain a length count and VB did a c-style strcmp() string comparison every time, it doesn't take a string comparison function very long to work out that one of the strings is zero length does it? But .NET strings do include a length field and so when you do a string comparison the first thing it's going to check is if the lengths differ, i.e. a straight int comparison. All you save by doing this yourself if a function call and then only if the JIT inlines Len().

By the way, in VB.NET you don't need to call String.IsNullOrEmpty() because comparisons between strings are transformed into a call to String.strcmp() which transforms nulls (Nothing in VB) into references to String.EmptyString. It then goes on to call a native function that Reflector can't examine, but I'd be pretty sure that the first thing it does is check if the lengths of the strings differ.

Answer 4

I think the the best solution for a VB6 equivalent to IsNullOrEmpty is

Public Function IsNullOrEmpty(ByRef s As Variant) As Boolean

  If IsNull(s) Then ' IsNull returns true if s is a Variant that is Null
    IsNullOrEmpty = True ' IsNull returns false if s is a Variant and s = vbEmpty
    Exit Function
  End If

  If LenB(s) = 0 Then ' never compare a string to "" - it's slow
    IsNullOrEmpty = True
    Exit Function
  End If

  If LenB(Trim(s)) = 0 Then
    IsNullOrEmpty = True
    Exit Function
  End If

  IsNullOrEmpty = False

End Function

Ironically, if you have strings or variant that are set to vbEmpty, then this function will return false, because vbEmpty as string is "0". So if you are not careful using this version you could get some strange errors.