not or <>?

  • Thread starter Thread starter kerberoz
  • Start date Start date
K

kerberoz

which is faster?

If strMessage <> String.Empty Then

End If

or

If Not strMessage Is Nothing Then

End If
 
Kerberoz,

Comparing against string.empty is slower as this invokes a call to StrCmp to
compare the two strings and return the result. Take a look at the following
IL for If strMessage <> String.Empty

IL_0008: call int32
[Microsoft.VisualBasic]Microsoft.VisualBasic.CompilerServices.StringType::St
rCmp(string,

string,

bool)

Cheers,
Lubos
 
I would point out that these two equality test are doing very different
things.

If strMessage <> String.Empty Then ' This tests for a zero length string

if If Not strMessage Is Nothing Then ' This tests to see if a reference is
associated with this identifier.

Apples and Pears !


Regards - OHM
 
Hi Kerberoz

In addition to OHT

In this newsgroup we mostly advice to use
strMessage <> ""
or seldom
strMessage <> nothing
but never
strMessage <> String.Empty

Cor
 
kerberoz,


You're testing two different things (empty string and a null reference).

Here is the technique I use to test for both conditions.

-------------------

' Test for Valid String
If (not szMessage Is Nothing) andalso (szMessage.Length > 0) then

' Do stuff for the valid string.

End If

' Test for invalid string
If (szMessage Is Nothing) orelse (szMessage.Length = 0) then

' Do stuff for the invalid string.

End If
-------------------

AFAIK, "szMessage.Length > 0" is faster than "szMesage <> String.Empty"


HTH,

Trev.
 
kerberoz,
Does it really matter?

In addition to the others comments about the differences between what you
are actually comparing.

I normally write semantically (full OOP) correct programs, then when a
specific routine has proven to have a performance issue, via profiling. I
look at optimizing that specific routine.

Hope this helps
Jay
 
Cor said:
Hi Kerberoz

In addition to OHT

In this newsgroup we mostly advice to use
strMessage <> ""
or seldom
strMessage <> nothing
but never
strMessage <> String.Empty

Cor
Click to expand...

What's wrong with strMessage <> String.Empty?

Just curious.

Erik
 
1.) Nothing wrong with strMessage <> String.Empty,

2.) but lots wrong with strMessage <> nothing if you think you are checking
for an empty string

OHM
 
2.) but lots wrong with strMessage said:
if you think you are checking for an empty string
Click to expand...

According to the Help files:

----------------------
When doing a string comparison, a null reference is equivalent to the string
literal ""
----------------------

As demonstrated by
----------------------------
Trace.WriteLine("" = Nothing)
----------------------------
which returns true.

as opposed to
-----------------------------
Trace.WriteLine("" is Nothing)
------------------------------

which returns false.

Not that I'd recommend doing it this way if you're comparing a lot of
strings in a loop (for readability as much as performance). Better to use a
combination of "is nothing" and "string.length = 0"

HTH,

Trev.
 
Hi Trev,

Can you tell where in the helpfiles, because I think this is completly nuts

In my opinion it is this

If string Is nothing means that there is a declared String withouth a
pointer to a virtual memoryadres

If string = nothing
or
if string = string.empty
or
if string = ""
or
if string.length = 0

Means that there is a declared String with a pointer to a virtual memory
adres with a length of zero.

Not that difficult I thought,

Cor
 
I'm amazed and stand corrected. However, Now I think about it more, it
absolutely makes sense.

Thanks for pointing that out.


Regards - OHM
 
Can you tell where in the helpfiles,
because I think this is completly nuts
Click to expand...

Look under the help for the "<>" or "=" operator. It's under the section
about string comparison. I couldn't find the exact page in MSDN online, but
it is in the help files installed with Visual Studio.
If string Is nothing means that there is a
declared String withouth a
pointer to a virtual memoryadres
Click to expand...

I agree, but the implementation of the "=" and "<>" operators for strings
does an extra check and returns True (or false for <>) for "" = Nothing.

Note I'm talking about "=" and not "is". i.e.

"" = Nothing returns true
"" is Nothing returns false

Dim szTest as String

szTest = Nothing returns true
szTest is Nothing returns true

' Assign szTest to an empty string
szTest = ""

szTest = Nothing returns true
szTest is Nothing returns false

I admit, this is slightly confusing, that's why I don't use this method -
it's harder to read.

HTH,

Trev.
 
Not really. If you think about it as strings are immutable in .NET, if you
assing a zero length tsring to a string type identifier, then that means
that it points to nowhere on the heap. This equals nothing as a value,
however the string object istself is still on the stack.

Whereas to

str = "" '//does not mean that str Is Nothing. Just its value is nothing.
'See the difference ?

OHM
 
Hi Trev,
Note I'm talking about "=" and not "is". i.e.
Click to expand...

I was thinking you was talking about IS therefore my message, forget it
than, now I understand what you did want to say.

Cor
 
Hi OHM,

I did understand Trev wrong,

I thought that he wanted to say that "Is nothing" is equal to "= nothing"
with a string.

Forget my message.

Cor
 
Hello, Trev:

The page you mentioned could be http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vblr7/html/vagrpcomparison.asp

You can avoid the confusion with '=Nothing' and 'Is Nothing' if you take into account that String is a class, so a non initialized string variable contains a null reference, not a null string like in previous versions of Basic.
So, use 'Is Nothing' only to check if a string variable has been initialized, for example, before invoking a non shared method, like MyStringVariable.Length; and avoid using =Nothing for strings, as it is equivalent to ="" (or =string.empty) and these are clearer (in my opinion).
I think the better practice for beginners is always initialize string variables in its declaration statement, (dim sv as string="") so there is no problem invoking string members. Don't you think so?

Regards.


"Trev Hunter" <[email protected]> escribió en el mensaje | > Can you tell where in the helpfiles,
| > because I think this is completly nuts
|
| Look under the help for the "<>" or "=" operator. It's under the section
| about string comparison. I couldn't find the exact page in MSDN online, but
| it is in the help files installed with Visual Studio.
|
| > If string Is nothing means that there is a
| > declared String withouth a
| > pointer to a virtual memoryadres
|
| I agree, but the implementation of the "=" and "<>" operators for strings
| does an extra check and returns True (or false for <>) for "" = Nothing.
|
| Note I'm talking about "=" and not "is". i.e.
|
| "" = Nothing returns true
| "" is Nothing returns false
|
| Dim szTest as String
|
| szTest = Nothing returns true
| szTest is Nothing returns true
|
| ' Assign szTest to an empty string
| szTest = ""
|
| szTest = Nothing returns true
| szTest is Nothing returns false
|
| I admit, this is slightly confusing, that's why I don't use this method -
| it's harder to read.
|
| HTH,
|
| Trev.
 
The page you mentioned could be
Click to expand...
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vblr7/html/vagrpcomparison.asp

Nope. I seen that one and it isn't the same as the one in the help files
installed with Visual studio. Go to the Index and type in "<>" to get the
page on the said:
I think the better practice for beginners
is always initialize string variables in its
declaration statement, (dim sv as string="")
so there is no problem invoking string members.
Don't you think so?
Click to expand...

Yes, but IMHO, it's also important for beginners to learn the difference
too.

Best Regards,

Trev.

"José Manuel Agüero" <jmaguero_vodafone.es> wrote in message
Hello, Trev:

The page you mentioned could be
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vblr7/html/vagrpcomparison.asp

You can avoid the confusion with '=Nothing' and 'Is Nothing' if you take
into account that String is a class, so a non initialized string variable
contains a null reference, not a null string like in previous versions of
Basic.
So, use 'Is Nothing' only to check if a string variable has been
initialized, for example, before invoking a non shared method, like
MyStringVariable.Length; and avoid using =Nothing for strings, as it is
equivalent to ="" (or =string.empty) and these are clearer (in my opinion).
I think the better practice for beginners is always initialize string
variables in its declaration statement, (dim sv as string="") so there is no
problem invoking string members. Don't you think so?

Regards.


"Trev Hunter" <[email protected]> escribió en el mensaje
| > Can you tell where in the helpfiles,
| > because I think this is completly nuts
|
| Look under the help for the "<>" or "=" operator. It's under the section
| about string comparison. I couldn't find the exact page in MSDN online,
but
| it is in the help files installed with Visual Studio.
|
| > If string Is nothing means that there is a
| > declared String withouth a
| > pointer to a virtual memoryadres
|
| I agree, but the implementation of the "=" and "<>" operators for strings
| does an extra check and returns True (or false for <>) for "" = Nothing.
|
| Note I'm talking about "=" and not "is". i.e.
|
| "" = Nothing returns true
| "" is Nothing returns false
|
| Dim szTest as String
|
| szTest = Nothing returns true
| szTest is Nothing returns true
|
| ' Assign szTest to an empty string
| szTest = ""
|
| szTest = Nothing returns true
| szTest is Nothing returns false
|
| I admit, this is slightly confusing, that's why I don't use this method -
| it's harder to read.
|
| HTH,
|
| Trev.
 
Back
Top