c#: Changing a value in a dictionary in a foreach

I was trying to loop through a dictionary,

update it's value if a certain condition was met,

if not remove the item from the dictionary.

(Seems I have done something lke this before, but that's why I blog it, so I can remember next time… :))

Here's my first attempt:

        private void FindEditedFieldsNotWorking(Dictionary<string, string> dictionary, Control ctrl)

        {

            foreach (KeyValuePair<string,string> kvp in dictionary)

            {

                CheckBox c = ctrl.Controls["chk" + kvp.Key] as CheckBox;

                if (c != null && c.Checked)

                {

                    dictionary[kvp.Key] = ctrl.Controls[kvp.Key].Text;

                }

                else

                    dictionary.Remove(kvp.Key);

            }

        }

It compiles ok and seems ok.

Might have a misgiving with removing the item in a loop, but hey it's worth a shot..

When running this I got an exception something like

"Collection was modified; enumeration operation may not execute."

My first thought was that my dictionary.Remove was the culprit, so I put a breakpoint there.

But that was not the case (although that is not supported either in this loop)

It seems that I can't actually change the freakin' value within my foreach loop…!

So I tried the same but this time using just the keys in the loop:

         private void FindEditedFieldsNotWorkingEither(Dictionary<string, string> dictionary, Control ctrl)

        {

            foreach (string Key in dictionary.Keys)

            {

                CheckBox c = ctrl.Controls["chk" + Key] as CheckBox;

                if (c != null && c.Checked)

                {

                    dictionary[Key] = ctrl.Controls[Key].Text;

                }

                else

                    dictionary.Remove(Key);

            }

        }

Still no go, same error.

Eventually I read a little bit about this using google and found that I couldn't do any changes while enumerating.

Here are some links I used:

http://stackoverflow.com/questions/1070766/editing-dictionary-values-in-a-foreach-loop

http://stackoverflow.com/questions/1562729/why-cant-we-change-values-of-a-dictionary-while-enumerating-its-keys

Seems like a bad design decision has made this a problem for many ppl.

The incling is probably the implisit add functionality when doing dict[key]=value; when key is not existing.

Anyways we have to get around this somehow.

The solution I thought was nicest when I couldn't use the "logic" solution is:

.Net 2.0

        private void FindEditedFieldsWorking(Dictionary<string, string> dictionary, Control ctrl)

        {

            List<string> keys = new List<string>(dictionary.Keys);

            foreach (string key in keys)

            {

                CheckBox c = ctrl.Controls["chk" + key] as CheckBox;

                if (c != null && c.Checked)

                {

                    dictionary[key] = ctrl.Controls[key].Text;

                }

                else

                    dictionary.Remove(key);

            }

        }

Almost as nice as the first version, and with the added premium that dictionary.Remove won't be a problem.

Or if you're running .Net 3.0 or higher you could just do this:

        private void FindEditedFieldsWorking(Dictionary<string, string> dictionary, Control ctrl)

        {

          

            foreach (var key in dictionary.Keys.ToList())

            {

                CheckBox c = ctrl.Controls["chk" + key] as CheckBox;

                if (c != null && c.Checked)

                {

                    dictionary[key] = ctrl.Controls[key].Text;

                }

                else

                    dictionary.Remove(key);

            }

        }

where the ToList does the same as our 2.0 solution, namely creating a temp list to hold the keys.

Nice solution on a not so nice problem.

Hope this helps someone,

it will help me next time anyways :)

Regards

Henri Merkesdal

MERIT

Posted via email from Henris blogg