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False Positives From String Comparison using string.Equals()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. Is there any way you could capture and post an example of two strings
that compare different but do not appear different? Or unpack them into their component bytes and display them in hex? (Some other C# programmer who has the method at his fingertips will, I hope, chime in with an exact method of doing that.) Michael Covington wrote:
> Is there any way you could capture and post an example of two strings I'll chime in myself. Do this:> that compare different but do not appear different? > > Or unpack them into their component bytes and display them in hex? (Some > other C# programmer who has the method at his fingertips will, I hope, > chime in with an exact method of doing that.) string s = "Hello, world!"; // just an example foreach (char c in s) Console.WriteLine("{0:X4}", (int)c); That gives you 4 hex digits (sufficient for Unicode). 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 Show quoteHide quote "Smithers" <A@B.com> wrote in message news:uN9EGCX8JHA.5040@TK2MSFTNGP04.phx.gbl... >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. > It happens that Cor Ligthert[MVP] formulated :
> This is possible as the DataBase returns spaces. No, that would not give the same results.> > 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 > "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 Show quoteHide quote > > "Smithers" <A@B.com> wrote in message > news:uN9EGCX8JHA.5040@TK2MSFTNGP04.phx.gbl... >>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. >> << SNIP>>
> As a hint to the OP: that's why I don't just "Console.WriteLine" those Upon reading from the DB and UI, each string is trimmed (theString.Trim()). > strings, but add some sort of brackets around them: > Console.WriteLine("[" + stringToDisplay + "]"); > Then extra spaces would become apparent. 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 Show quoteHide quote "Hans Kesting" <news.hansdk@spamgourmet.com> wrote in message news:ObtqYBx8JHA.4444@TK2MSFTNGP05.phx.gbl... > 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 >> > > 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 > >> >> "Smithers" <A@B.com> wrote in message >> news:uN9EGCX8JHA.5040@TK2MSFTNGP04.phx.gbl... >>>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. >>> > > Cor Ligthert[MVP] wrote:
> Hans, Why would you process the strings just to use one comparison, when there > > 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 > 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 Show quoteHide quote "Göran Andersson" <gu***@guffa.com> wrote in message news:%23WtvSj98JHA.3544@TK2MSFTNGP04.phx.gbl... > Cor Ligthert[MVP] wrote: >> 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 >> > > 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? > > -- > Göran Andersson > _____ > http://www.guffa.com RE
> Most people are clever enough to know what is done behind the scene. No - they are not!"Most people" can't even, well, I better stop right there... Cor Ligthert[MVP] wrote:
> Goran, What do you mean by that?> > Most people are clever enough to know what is done behind the scene. > Strings to Upper doesn't process the string, 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 How is that relevant?> the ascii code, it does not work by instance with the Turkish uppercase i.
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"Göran Andersson" <gu***@guffa.com> wrote in message There exist strings a, b, c, and d for whichnews:uNUy5Fr9JHA.3244@TK2MSFTNGP03.phx.gbl... > Cor Ligthert[MVP] wrote: >> Goran, >> >> Most people are clever enough to know what is done behind the scene. > > What do you mean by that? > >> Strings to Upper doesn't process the string, > > 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. > > How is that relevant? a.ToUpper() == b.ToUpper() but string.Equal(a,b,IgnoreCase) is false c.ToUpper() != d.ToUpper() but string.Equal(c,d,IgnoreCase) is true Show quoteHide quote > __________ Information from ESET NOD32 Antivirus, version of virus signature database 4221 (20090706) __________> -- > Göran Andersson > _____ > http://www.guffa.com > > __________ Information from ESET NOD32 Antivirus, version of virus > signature database 4221 (20090706) __________ > > The message was checked by ESET NOD32 Antivirus. > > http://www.eset.com > > > The message was checked by ESET NOD32 Antivirus. http://www.eset.com  |
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