Re: removing comments from xml strings - C# Discussion Boards

Re: Error-Object reference not set to an instance of an object

29-Nov-07 8:46

T4AMD wrote:
string[] ptu = cmdGetProfileFields.ExecuteScalar().ToString().Split(sep, StringSplitOptions.RemoveEmptyEntries);

seems like ExecuteScalar returns null.

if you want to be sure about this, split the line in two:

object obj=cmdGetProfileFields.ExecuteScalar();
string[] ptu = obj.ToString().Split(sep, StringSplitOptions.RemoveEmptyEntries);

and either add a null test, or use the debugger again.
If it is null, I can not tell you why...

Smile | :)

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T4AMD29-Nov-07 8:55

T4AMD

29-Nov-07 8:55

I keep getting the same error when i tried your code Luc, and I think that ExecuteScalar is returning a null value

removing comments from xml strings

AndrusM29-Nov-07 7:29

29-Nov-07 7:29

I use the code below to remove commants from xml strings.
The C# guru wrote me: that this code is wrong
since while iterating a collection it is not allowed to remove its element.
He wrote this is .NET bug: this code sholud throw exception.

Please confirm, is this .NET bug ?
How to remove comments from xml string (fix this code)?

using System;<br />
using System.Windows.Forms;<br />
using System.Xml;<br />
<br />
public class Test<br />
{<br />
    [STAThread]<br />
    static void Main()<br />
    {<br />
            XmlDocument xDoc = new XmlDocument();<br />
            xDoc.PreserveWhitespace = false;<br />
            xDoc.LoadXml(xml);<br />
            XmlNodeList list = xDoc.SelectNodes("//comment()");<br />
            foreach (XmlNode node in list)<br />
            {<br />
                node.ParentNode.RemoveChild(node);<br />
            }<br />
    }<br />
<br />
    const string xml = @"<?xml version=""1.0""><br />
<Report><!-- --></Report><br />
    ";<br />
<br />
}<br />

Andrus

Dave Kreskowiak29-Nov-07 9:27

29-Nov-07 9:27

No, it's not a bug. He's saying it should be, but that would involve changes to the compiler to add mind reading capability.

Enumerating a collection is valid only so long as the collection remains unchanged. Adding and removing items changes the collection. If either of these operations happen, the Enumerator is automatically invalid because the bounds of the enumeration are no longer known and it's also unknown if the current item even exists anymore. When this happens, the Enumerator is no longer is a known state and cannot be recovered. It's behavior is undefined.

Enumerators do do not exclusive access to the collection, so enumerating one is inherently not a thread-safe operation. The collection CAN be modified from outside the enumeration. If this happens, the enumerator is, again, invalid.

If you're going to modify a collection, you cannot use foreach. You must use a normal for statement with an index into the array of items.

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Dave Kreskowiak
Microsoft MVP
Visual Developer - Visual Basic
2006, 2007

AndrusM29-Nov-07 22:13

29-Nov-07 22:13

I did'nt understand your reply. You wrote that this code modifies same collection inside foreach loop which is not allowed.
.NET does not throw exception for this code.
So it is .NET bug, isn't it ?

Chris Shepherd explains:

A cloned IEnumerable instance is *not* the same IEnumerable, by nature. It may have references to the same data, but it
does not necessarily contain the same list of references. If you remove
items from the newly cloned instance, it won't affect its progenitor,
and vice versa. Kind of the point of clone, really.

This code is functionally working with two different IEnumerable objects. Below is an example of effectively what
original code is doing -- removing a subset of items in one list from
another list.

class General<br />
{<br />
     class ClassA<br />
     {<br />
         public string name;<br />
<br />
         public ClassA(string nm) { this.name = nm; }<br />
     }<br />
<br />
     static void Main(string[] args)<br />
     {<br />
         List<ClassA> listOne = new List<ClassA>();<br />
         List<ClassA> listTwo = new List<ClassA>();<br />
<br />
         ClassA a = new ClassA("A");<br />
         ClassA b = new ClassA("B");<br />
         ClassA c = new ClassA("C");<br />
         ClassA d = new ClassA("D");<br />
         ClassA e = new ClassA("E");<br />
<br />
         listOne.AddRange(new ClassA[] { a, b, c, d, e });<br />
         listTwo.AddRange(new ClassA[] { a, c, d });<br />
<br />
         string msg = "Start Length of listOne: " +<br />
             listOne.Count.ToString() +<br />
             "\nStart Length of listTwo: " +<br />
             listTwo.Count.ToString() +<br />
             "\nWhat is removed:\n";<br />
         foreach (ClassA z in listTwo)<br />
         {<br />
             msg += z.name + "\t";<br />
             listOne.Remove(z);<br />
         }<br />
<br />
         Console.WriteLine(msg + "\nLength of listOne: " +<br />
             listOne.Count.ToString() +<br />
             "\nLength of listTwo: " +<br />
             listTwo.Count.ToString());<br />
     }<br />
}<br />

Andrus

Dave Kreskowiak30-Nov-07 2:32

30-Nov-07 2:32

AndrusM wrote:
You wrote that this code modifies same collection inside foreach loop which is not allowed.
.NET does not throw exception for this code.
So it is .NET bug, isn't it ?

No, it's not a compile time error. There is no way for the compiler to know that the method called "Remove" removes items from the collection. You could call the very same method "Jaxtapose". The name of the method doesn't tell the compiler anything other than "this is a name". The compiler cannot tell what the code inside the method does.

It WILL throw a run-time exception though.

A guide to posting questions on CodeProject[^]

Dave Kreskowiak
Microsoft MVP
Visual Developer - Visual Basic
2006, 2007

AndrusM30-Nov-07 3:02

30-Nov-07 3:02

No, it will *not* cause run time exception in .NET 2 and 3.5.
It removes all comment from XML.

So my question was: Is this .NET but that this code does not cause run-time exception ?

Andrus

Dave Kreskowiak30-Nov-07 16:12

30-Nov-07 16:12

I wasn't talking about your code. I was talking about what the other person said about what should be a "bug". There are a few exceptions to the rule. It would appear that you have found one of them.

A guide to posting questions on CodeProject[^]

Dave Kreskowiak
Microsoft MVP
Visual Developer - Visual Basic
2006, 2007

AndrusM1-Dec-07 6:32

1-Dec-07 6:32

This cCode removes iterated item from collection inside foreach loop.
C# clearly does not allow to change iterated collection in foreach body.
This should cause exception.
So why do you think that this is exception of this rule and should be allowed ?

Andrus

PIEBALDconsult1-Dec-07 8:31

PIEBALDconsult

1-Dec-07 8:31

Not that last snippet you posted, it's enumerating one collection and removing from the other, so there's no problem.

Dave Kreskowiak1-Dec-07 12:41

Strange frequency readings

1-Dec-07 12:41

He's right. The snippet you posted isn't doing anything to the collection being enumerated. The exception to the rule...

A guide to posting questions on CodeProject[^]

Dave Kreskowiak
Microsoft MVP
Visual Developer - Visual Basic
2006, 2007

Guybrush8629-Nov-07 7:19

Guybrush86

29-Nov-07 7:19

Hi,
I have been making a tuning program for instruments, the idea being you play, a guitar say, into the mic and it tells you what note you are playing. It pretty much works, but I am getting strange readings. When nothing is being played, the mic is even muted, I get a reading of either 50 or 125 Hz. These values also pop up a lot when I am playing the guitar into it. I can't think where these phantom sounds are coming from and was wondering if anyone had an idea.

Cheers

Apply a frequency filter

Ennis Ray Lynch, Jr.29-Nov-07 8:01

Ennis Ray Lynch, Jr.

29-Nov-07 8:01

A/D are notorious for bad readings and the right combination of filters usually helps.

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Luc Pattyn29-Nov-07 9:12

29-Nov-07 9:12

The 50Hz is likely the mains frequency you are picking up somehow; I am not sure about
the 125Hz, are you using a CRT display?

try turning off everything you don't need, switch to a battery-powered notebook (no mains,
no CRT), go outside if possible, doing so you might be able to pinpoint the ghosts
that haunt you.

I guess your audio input is using some automatic volume control; if at all possible you
may want to switch it off or reduce its range (i.e. set a minimum required level).

BTW: I would not blame C# for this.

Smile | :)

Luc Pattyn [Forum Guidelines] [My Articles]

leppie29-Nov-07 9:54

leppie

29-Nov-07 9:54

Cosmic background microwave radiation, from the Big Bang.

xacc.ide
IronScheme a R5RS-compliant Scheme on the DLR

The rule of three: "The first time you notice something that might repeat, don't generalize it. The second time the situation occurs, develop in a similar fashion -- possibly even copy/paste -- but don't generalize yet. On the third time, look to generalize the approach."

Luc Pattyn29-Nov-07 9:59

29-Nov-07 9:59

Hertz was not even born then, was he?

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leppie29-Nov-07 10:17

leppie

29-Nov-07 10:17

Was meant to be a joke Smile | :)

Luc Pattyn29-Nov-07 11:16

29-Nov-07 11:16

Maybe I figured that Poke tongue | ;-P

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