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Why does Replace return Nothing???I have the following very simple code ... Dim lStringA As String = "" Dim lStringB As String = "" .... lStringB = Replace(lStringA, "DUMMY", "", Compare:=CompareMethod.Text) lStringB.TrimEnd("\"c) According to the documenation, Replace will return a zero-length string ("") if lStringA is zero-length. So why is lStringB Nothing and causes an exception when calling the TrimEnd() function? Do I have to check whether lStringA is <> "" before I call Replace()? Thanks, Joe It does seem the documentation is wrong (a Nothing return is never
mentioned, http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vblr7/html/vafctreplace.asp unless one interprets "Copy of Expression with no occurrences of Find" as a possible Nothing, which anyway does not apply because Nothing is returned even with a non zero-length "Replace" ). I never noticed that (I usually manage to use stringbuilders when doing replacements ... ) -tom This is how the Replace function in the Microsoft.VisualBasic namespace is
implemented: Public Shared Function Replace(ByVal Expression As String, ByVal Find As String, ByVal Replacement As String, ByVal Start As Integer = 1, ByVal Count As Integer = -1, <OptionCompare> ByVal [Compare] As CompareMethod = 0) As String Dim text1 As String Try If (Count < -1) Then Throw New ArgumentException(Utils.GetResourceString("Argument_GEMinusOne1", "Count")) End If If (Start <= 0) Then Throw New ArgumentException(Utils.GetResourceString("Argument_GTZero1", "Start")) End If If ((Expression Is Nothing) OrElse (Start > Expression.Length)) Then Return Nothing End If If (Start <> 1) Then Expression = Expression.Substring((Start - 1)) End If If (((Find Is Nothing) OrElse (Find.Length = 0)) OrElse (Count = 0)) Then Return Expression End If If (Count = -1) Then Count = Expression.Length End If text1 = Strings.Join(Strings.Split(Expression, Find, (Count + 1), Compare), Replacement) Catch exception1 As Exception Throw exception1 End Try Return text1 End Function Note that the Expression parameter is defined as string and that the Start parameter defaults to 1. In your example, the test: If ((Expression Is Nothing) OrElse (Start > Expression.Length)) Then will be true and Nothing will be returned. If an empty string is interpreted as Nothing then the first part will be true otherwise the second part will be true because 1 is greater than the length of an empty string. So we are left with 2 scenarios: 1 - the documentation is incorrect or 2 - whoever wrote the Replace function expects that Nothing will be marshalled back as an empty string Either way, the behaviour is at odds with the documentation and, yes, you should either check to see if the result is Nothing before continuing. If you were to use the String.Replace method instead you would avoid this issue. Show quoteHide quote "Joe HM" <unixve***@yahoo.com> wrote in message news:1144880100.443887.40420@i39g2000cwa.googlegroups.com... > Hello - > > I have the following very simple code ... > > Dim lStringA As String = "" > Dim lStringB As String = "" > > ... > > lStringB = Replace(lStringA, "DUMMY", "", Compare:=CompareMethod.Text) > lStringB.TrimEnd("\"c) > > According to the documenation, Replace will return a zero-length string > ("") if lStringA is zero-length. So why is lStringB Nothing and causes > an exception when calling the TrimEnd() function? > > Do I have to check whether lStringA is <> "" before I call Replace()? > > Thanks, > Joe > Hi Stephany , interesting... where did you get that ? :)
-tom PS "If an empty string is interpreted as Nothing" that does not correspond to my experience (string.empty is not nothing) I had a look at it in Lutz Roeder's Reflector.
http://www.aisto.com/roeder/dotnet/ It is excellent for delving into the Framwork and seeing how things are implemented. Yes. You're right. I was wondering out loud if there might or might not be some mechanism in the framework that somehow modified the values that are marshalled to and from the methods (functions) in Microsoft.VisualBasic namespace, so that an empty string got interpreted as Nothing an/or vice versa. You may or may not recall that back in VB6 an number of the string functions returned variant containg strings rather than true strings. It was probably a spurious thought but it did cross my mind. <tommaso.gasta***@uniroma1.it> wrote in message Show quoteHide quote news:1144886721.229334.299490@t31g2000cwb.googlegroups.com... > > Hi Stephany , interesting... where did you get that ? :) > > -tom > > > PS > "If an empty string is interpreted as Nothing" > that does not correspond to my experience (string.empty is not nothing) > Sephany,
Very interesting and really odd what you found. I hope on a comment from Herfried, who loves the VB replace and methods like that. Cor Show quoteHide quote "Stephany Young" <noone@localhost> schreef in bericht news:u7fhQwoXGHA.3496@TK2MSFTNGP05.phx.gbl... > This is how the Replace function in the Microsoft.VisualBasic namespace is > implemented: > > Public Shared Function Replace(ByVal Expression As String, ByVal Find As > String, ByVal Replacement As String, ByVal Start As Integer = 1, ByVal > Count As Integer = -1, <OptionCompare> ByVal [Compare] As CompareMethod = > 0) As String > Dim text1 As String > Try > If (Count < -1) Then > Throw New > ArgumentException(Utils.GetResourceString("Argument_GEMinusOne1", > "Count")) > End If > If (Start <= 0) Then > Throw New > ArgumentException(Utils.GetResourceString("Argument_GTZero1", "Start")) > End If > If ((Expression Is Nothing) OrElse (Start > Expression.Length)) Then > Return Nothing > End If > If (Start <> 1) Then > Expression = Expression.Substring((Start - 1)) > End If > If (((Find Is Nothing) OrElse (Find.Length = 0)) OrElse (Count = 0)) > Then > Return Expression > End If > If (Count = -1) Then > Count = Expression.Length > End If > text1 = Strings.Join(Strings.Split(Expression, Find, (Count + 1), > Compare), Replacement) > Catch exception1 As Exception > Throw exception1 > End Try > Return text1 > End Function > > Note that the Expression parameter is defined as string and that the Start > parameter defaults to 1. > > In your example, the test: > > If ((Expression Is Nothing) OrElse (Start > Expression.Length)) Then > > will be true and Nothing will be returned. If an empty string is > interpreted as Nothing then the first part will be true otherwise the > second part will be true because 1 is greater than the length of an empty > string. > > So we are left with 2 scenarios: > > 1 - the documentation is incorrect > > or > > 2 - whoever wrote the Replace function expects that > Nothing will be marshalled back as an empty string > > Either way, the behaviour is at odds with the documentation and, yes, you > should either check to see if the result is Nothing before continuing. > > If you were to use the String.Replace method instead you would avoid this > issue. > > > > "Joe HM" <unixve***@yahoo.com> wrote in message > news:1144880100.443887.40420@i39g2000cwa.googlegroups.com... >> Hello - >> >> I have the following very simple code ... >> >> Dim lStringA As String = "" >> Dim lStringB As String = "" >> >> ... >> >> lStringB = Replace(lStringA, "DUMMY", "", Compare:=CompareMethod.Text) >> lStringB.TrimEnd("\"c) >> >> According to the documenation, Replace will return a zero-length string >> ("") if lStringA is zero-length. So why is lStringB Nothing and causes >> an exception when calling the TrimEnd() function? >> >> Do I have to check whether lStringA is <> "" before I call Replace()? >> >> Thanks, >> Joe >> > > "Cor Ligthert [MVP]" <notmyfirstn***@planet.nl> schrieb: The documentation for 'Microsoft.VisualBasic.Strings.Replace' in .NET 2.0 > Very interesting and really odd what you found. I hope on a comment from > Herfried, who loves the VB replace and methods like that. states: Show quoteHide quote | *Return Value* | [...] | 'Nothing' Expression is zero-length or 'Nothing', or 'Start' is | greater than length of 'Expression'. -- M S Herfried K. Wagner M V P <URL:http://dotnet.mvps.org/> V B <URL:http://classicvb.org/petition/> You are so right Herfried.
The link a poster gave earlier in the thread actually refers to .Net 1.0/1.1 so this is a change in .NET 2.0. Show quoteHide quote "Herfried K. Wagner [MVP]" <hirf-spam-me-here@gmx.at> wrote in message news:eN5aRTvXGHA.1200@TK2MSFTNGP03.phx.gbl... > "Cor Ligthert [MVP]" <notmyfirstn***@planet.nl> schrieb: >> Very interesting and really odd what you found. I hope on a comment from >> Herfried, who loves the VB replace and methods like that. > > The documentation for 'Microsoft.VisualBasic.Strings.Replace' in .NET 2.0 > states: > > | *Return Value* > | [...] > | 'Nothing' Expression is zero-length or 'Nothing', or 'Start' is > | greater than length of 'Expression'. > > -- > M S Herfried K. Wagner > M V P <URL:http://dotnet.mvps.org/> > V B <URL:http://classicvb.org/petition/> Stephany,
"Stephany Young" <noone@localhost> schrieb: You are right, the documentation on .NET 1.0/1.1's 'Replace' function > You are so right Herfried. > > The link a poster gave earlier in the thread actually refers to .Net > 1.0/1.1 so this is a change in .NET 2.0. doesn't state this case. Maybe it was a bug in the implementation and Microsoft decided to change the documentation to solve the problem. Personally I would have changed the implementation of the method to return an empty string instead of a 'Nothing' reference, because it would seem more intuitive to me, but I didn't spend time on thinking about the implications of such a change. -- M S Herfried K. Wagner M V P <URL:http://dotnet.mvps.org/> V B <URL:http://classicvb.org/petition/> I agree Herfried. Returning an empty string would certainly be more
intuitive from my point of view. Also, IIRC, the empty string has been the return value since Replace was introduced to VB (can't remember whether it was VB5 or VB6). Show quoteHide quote "Herfried K. Wagner [MVP]" <hirf-spam-me-here@gmx.at> wrote in message news:%23HiuWsvXGHA.5012@TK2MSFTNGP05.phx.gbl... > Stephany, > > "Stephany Young" <noone@localhost> schrieb: >> You are so right Herfried. >> >> The link a poster gave earlier in the thread actually refers to .Net >> 1.0/1.1 so this is a change in .NET 2.0. > > You are right, the documentation on .NET 1.0/1.1's 'Replace' function > doesn't state this case. Maybe it was a bug in the implementation and > Microsoft decided to change the documentation to solve the problem. > Personally I would have changed the implementation of the method to return > an empty string instead of a 'Nothing' reference, because it would seem > more intuitive to me, but I didn't spend time on thinking about the > implications of such a change. > > -- > M S Herfried K. Wagner > M V P <URL:http://dotnet.mvps.org/> > V B <URL:http://classicvb.org/petition/> Yeah ... I have NET 1.0/1.1 ... so I guess the documentation is not
right ... Thanks! Joe Hello -
Wow ... thanks for all the feedback! I have looked into using the String.Replace function but I cannot set the Compare:=CompareMethod.Text (case-insensitive) that way ... can I? Thanks, Joe 
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