False Positives From String Comparison using string.Equals()

  • Thread starter Thread starter Smithers
  • Start date Start date
S

Smithers

I am using the .Equals() method of the string class, as follows, to compare
string properties of two different instances of a given class.


if
(!valuesFromUI.Other_Description.Equals(valuesAsRetrievedFromDB.Other_Description,
StringComparison.OrdinalIgnoreCase))
{
return true;
}

What I am observing in testing is that the above .Equals() periodically
finds that the string values are different when they do not in fact appear
to be different in any way. By "appear to be different in any way" I mean
that I hover the mouse over each property and view in the "Text Visualizer",
then copy from there into Notepad and compare the values in notepad - they
appear to be the exact same string.

The bigger picture is this: I'm using Visual Studio Pro 2008. This is
happening in an ASP.NET Web application. I instantiate the class and
populate it with values from the underlying SQL Server 2005 database, then
load those properties into the UI, and then store the object instance in a
Session variable. Then, when the user navigates to a different page, I
instantiate a new instance of the class and populate it with values from the
UI. I then compare relevant properties of that instance - as in the sample
code above - against the same properties from the instance that was
previously stored in Session state. I do this to determine if the user
actually modified any values from the underlying database and therefore
whether to save any changes to the database.

How can I compare the strings in a way that would not result in such "false
positives"? I would think my approach, above, is perfectly fine. What can I
do differently?

Thanks.
 
Smithers said:
I am using the .Equals() method of the string class, as follows, to compare
string properties of two different instances of a given class.


if
(!valuesFromUI.Other_Description.Equals(valuesAsRetrievedFromDB.Other_Description,
StringComparison.OrdinalIgnoreCase))
{
return true;
}

What I am observing in testing is that the above .Equals() periodically
finds that the string values are different when they do not in fact appear
to be different in any way. By "appear to be different in any way" I mean
that I hover the mouse over each property and view in the "Text Visualizer",
then copy from there into Notepad and compare the values in notepad - they
appear to be the exact same string.
Click to expand...
But are they? Your test is not definitive. Whitespace is not distinguishable
visually and there are plenty of visible characters who share glyphs.

For starters, compare the string *lengths*. If those are the same, go into
the immediate window and check if Encoding.UTF8.GetBytes() yields the same
result on both strings. If that's the case and the strings still don't
compare equal then you might've found a genuine bug in the framework, but
that's very unlikely.
The bigger picture is this: I'm using Visual Studio Pro 2008. This is
happening in an ASP.NET Web application. I instantiate the class and
populate it with values from the underlying SQL Server 2005 database
Click to expand...

If you're using CHAR fields (fixed length, whitespace padding,
encoding-specific) or VARCHAR fields (encoding-specific) as opposed to
NVARCHAR fields, this could be the cause of your strings not round-tripping.
 
This is possible as the DataBase returns spaces.

However, simply testing (StringX == StringY) gives the same result and in my
opinion much easier then let it go trough an extra old derived C function.

Cor
 
However, simply testing (StringX == StringY) gives the same result
Click to expand...

That's equivalent to an ordinal comparison, not a case-insensitive ordinal
comparison like the OP is using.
 
It happens that Cor Ligthert[MVP] formulated :
This is possible as the DataBase returns spaces.

However, simply testing (StringX == StringY) gives the same result and in my
opinion much easier then let it go trough an extra old derived C function.

Cor
Click to expand...

No, that would not give the same results.
"A" == "a" would return false,
"A".Equals("a", StringComparison.OrdinalIgnoreCase) would return true.

You are right however to point to the possibility of extra spaces in
the "database" value. They would not be visible but would mess up the
comparison.
As a hint to the OP: that's why I don't just "Console.WriteLine" those
strings, but add some sort of brackets around them:
Console.WriteLine("[" + stringToDisplay + "]");
Then extra spaces would become apparent.

Hans Kesting
 
As a hint to the OP: that's why I don't just "Console.WriteLine" those
strings, but add some sort of brackets around them:
Console.WriteLine("[" + stringToDisplay + "]");
Then extra spaces would become apparent.
Click to expand...


Upon reading from the DB and UI, each string is trimmed (theString.Trim()).
So, I think that would resolve any extra spaces issue.

I have yet to try the suggestion by Mr. Covington. That looks like it might
shed some light on the situation.

-S
 
Hans,

Most people here are clever enough that they know that it is possible to
write, I take mostly the shortest notation when I want to show something.

("A".ToUpper() == "a".ToUpper())

Cor

Hans Kesting said:
It happens that Cor Ligthert[MVP] formulated :
This is possible as the DataBase returns spaces.

However, simply testing (StringX == StringY) gives the same result and in
my opinion much easier then let it go trough an extra old derived C
function.

Cor
Click to expand...

No, that would not give the same results.
"A" == "a" would return false,
"A".Equals("a", StringComparison.OrdinalIgnoreCase) would return true.

You are right however to point to the possibility of extra spaces in the
"database" value. They would not be visible but would mess up the
comparison.
As a hint to the OP: that's why I don't just "Console.WriteLine" those
strings, but add some sort of brackets around them:
Console.WriteLine("[" + stringToDisplay + "]");
Then extra spaces would become apparent.

Hans Kesting
Click to expand...
 
Cor said:
Hans,

Most people here are clever enough that they know that it is possible to
write, I take mostly the shortest notation when I want to show something.

("A".ToUpper() == "a".ToUpper())

Cor
Click to expand...

Why would you process the strings just to use one comparison, when there
is another comparison that does exactly what you want without first
processing the strings?
 
Goran,

Most people are clever enough to know what is done behind the scene.

Strings to Upper doesn't process the string, it only takes a subset of the
ascii code, it does not work by instance with the Turkish uppercase i.

Cor
 
Cor said:
Most people are clever enough to know what is done behind the scene.
Click to expand...
Those people shouldn't propagate techniques they should know are bad.
Strings to Upper doesn't process the string,
Click to expand...

What Göran meant is that .ToUpper() creates a new string, which is then
compared. Using .Equals() with a StringComparison saves you a string
creation (or two) because it compares character-by-character (and it skips
the comparison altogether if the lengths differ).

In short: never use .ToUpper() (or .ToLower()) to compare strings in a
case-insensitive manner. It's strictly worse than using .Equals() with the
appropriate StringComparison flag.

Yes, even if you think it doesn't look as nice. And even (or actually
*especially*) for demonstration code.
 
Cor said:
Goran,

Most people are clever enough to know what is done behind the scene.
Click to expand...

What do you mean by that?
Strings to Upper doesn't process the string,
Click to expand...

Of course it does. A method that just returns the same string without
doing anything would be pretty useless, would it not?
it only takes a subset of
the ascii code, it does not work by instance with the Turkish uppercase i.
Click to expand...

How is that relevant?
 
Göran Andersson said:
What do you mean by that?


Of course it does. A method that just returns the same string without
doing anything would be pretty useless, would it not?


How is that relevant?
Click to expand...

There exist strings a, b, c, and d for which

a.ToUpper() == b.ToUpper() but string.Equal(a,b,IgnoreCase) is false

c.ToUpper() != d.ToUpper() but string.Equal(c,d,IgnoreCase) is true
--
Göran Andersson
_____
http://www.guffa.com

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signature database 4221 (20090706) __________

The message was checked by ESET NOD32 Antivirus.

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Click to expand...

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