Nuno Silva

Collection was modified

Many times i’m programming and i want to iterate a collection and modify it. Thsi usually happens when i want to remove a certain number of objects of a collection:

foreach( Element e in elements){
   if( ConditionIsSatisfied(e) ) {
       elements.Remove(e);
   }
}

The above method will give a collection modified exception because i’m modifying a collection while iterating it.

To resolve this problem without long lines of code, there is a simple thing i can do:

List<Element> temporayElements = new List<Element>(elements);
foreach( Element e in temporayElements){
   if( ConditionIsSatisfied(e) ) {
       elements.Remove(e);
   }
}

This way, i can iterate and check the conditions in one container and manipulate the original one without the possibility of throwing a “Collection was modified” exception.

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23 Jan 09 | Programming

11 Responses to “Collection was modified”

João Craveiro | January 23rd, 2009 at 10:16 am

Interestingly, I usually go for a different approach:

List toRemove = new List();
foreach( Element e in elements){
if( ConditionIsSatisfied(e) ) {
toRemove.Add(e);
}
}
foreach( Element e in toRemove){
elements.Remove(e)
}

From where I see it, this spares memory, and is probably not heavier on the computation side (your code may also have a hidden foreach cycle in that constructor ).
Luís Miranda | January 23rd, 2009 at 10:25 am

Alternatively, use Iterator.remove():

[code]
for (Iterator it = elements.iterator(); it.hasNext();) {
if (ConditionIsSatisfied(e) ) {
it.remove();
}
}
[/code]

This works even if you have an Iterable which is not a Collection.
Luís Miranda | January 23rd, 2009 at 10:31 am

The blog ate my generics! Let’s try again (underscore used for spacing):

for (Iteratorgt;g it = elements.iterator(); it.hasNext();) {
____if (ConditionIsSatisfied(e) ) {
________it.remove();
____}
}
Paulo Pires | January 23rd, 2009 at 10:53 am

I hate Luis Miranda.. always stealing my comments :-p
Bruno Silva | January 23rd, 2009 at 11:24 am

Using .NET 3.5 Linq
List ints =new List(new int[]{ 1,2,3,4,5,6,7});
ints = ints.Where((i) => (i % 2 == 0)).ToList();
Luís Miguel Silva | January 23rd, 2009 at 7:24 pm

By the way, how can i access a collection by index (just like i would in an array)?

I use some APIs that return collections and i really just need the first element.

I typically end up doing a iteration and breaking on the first result (which is pretty lame).

Thanks,
Luís
Nuno Silva | January 23rd, 2009 at 7:40 pm

@João Craveiro: Yes you’re right. It can be heavier, but is very elegant

@Luís Miranda: that is an interesting solution. i’m going to explore it better. It’s very practical.

@Bruno Silva: Link, that is a thing i have to study. Thanks for your contribution.

@Luís Miguel Silva: Which collection are you thinking of? Almost all the containers in C# access the index the same way has in an array.
A List, for instance, has the index ([]) operator.
Luís Miguel Silva | January 24th, 2009 at 6:56 pm

Hello Nuno!

Take this example:
//get Fixed disk stats
System.Management.ObjectQuery oQuery = new System.Management.ObjectQuery(”select FreePhysicalMemory from Win32_OperatingSystem”);
ManagementObjectSearcher oSearcher = new ManagementObjectSearcher(oMs, oQuery);
ManagementObjectCollection oReturnCollection = oSearcher.Get();

// Hmm…how do i access a single element? :oP Oh well..
foreach (ManagementObject oReturn in oReturnCollection)
{
fm = Convert.ToInt32(Convert.ToInt32(oReturn["FreePhysicalMemory"]) / mb);
break;
}
return fm;

How can i access a single element from the returned ManagementObjectCollection ?

Thanks,
Luís Silva
Luís Miguel Silva | January 25th, 2009 at 6:53 am

I think i just found a method to do what i want: ManagementObjectCollection.CopyTo()

Any alternatives / thoughts?

Thanks,
Luís
Nuno Silva | January 25th, 2009 at 10:14 am

Well, the object ManagementObjectCollection is really strange… They, at least, could have implemented an indexer.

And, as far as i found out, you’re not the only with this problem: http://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=94753

If it was i implementing that code i would do the same has you did. Is the simpler solution because, as far as i could found out, there is no better one.

Even the CopyTo implies the creation of a container. I think it doesn’t worth the effort.
Luís Miguel Silva | January 26th, 2009 at 12:25 am

Dratz :o\

This isn’t the first object i’ve had this exact same problem!

It’s really a performance hog…

Thanks anyway!
Luís

The Guy

My name is Nuno Silva and I am the author of this blog. I am a Computer Engineer and have a passion for game development.

So, in this blog you find especially things about the web, game development, software, my personal projects (Orion's belt for instance) and, sometimes, just crazy things that pass through my head.