C# Collection was modified; enumeration operation may not execute

Question

List<String> employeeTimings;
    public string[] sTime = {"0000", "0030", "0100", "0130", "0200", "0230", "0300", "0330", "0400", "0430", "0500", "0530", "0600", "0630", "0700", "0730", "0800", "0830", "0900", "0930", "1000", "1030", "1100", "1130", "1200", 
                                 "1230", "1300", "1330", "1400", "1430", "1500", "1530", "1600", "1630", "1700", "1730", "1800", "1830", "1900", "1930", "2000", "2030", "2100", "2130", "2200", "2230", "2300", "2330"};

employeeTimings = new List<String>(sTime);
        int i=0;
        DataTable dt = new DataTable();
        dt.Columns.Add(new DataColumn("Location", typeof(string)));
        dt.Columns.Add(new DataColumn("Station", typeof(string)));

        foreach (DataColumn dc in dt.Columns)
        {
            dt.Columns.Add(employeeTimings[i], typeof(string));
            i++;
        }
        SchedulingTableDGV.DataSource = dt;

i get an error "Collection was modified; enumeration operation may not execute." how can i solve this?

score 3 · Accepted Answer · answered Jul 23 '12 at 08:36

3

You cannot add or remove from the collection you are iterating with a foreach. You need to perform adding or removing actions outside of the foreach.

    foreach (DataColumn dc in dt.Columns)
    {
        dt.Columns.Add(employeeTimings[i], typeof(string)); // Problem.
        i++;
    }

Try this:

foreach (var timing in employeeTimings)
{
    dt.Columns.Add(timing, typeof(string));
}

answered Jul 23 '12 at 08:36

Adam Houldsworth

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can u write me an example how in this case? – Kenneth Teo Jul 23 '12 at 08:38
You can use a for loop instead. `for(int i = 0; i < dt.Columns.Count; i++)` – Liron Jul 23 '12 at 08:39
@KennethTeo I've updated the answer with a possible alternative. – Adam Houldsworth Jul 23 '12 at 08:40
Without looking in the documentation; isn't `dt.Columns.AddRange(employeeTimings)` an opiton? Saves a `foreach`. – RobIII Jul 23 '12 at 08:40
@Robill The documentation states that `AddRange` takes a `DataColumn[]`, which wouldn't work in this instance without a projection to create the column from the string. – Adam Houldsworth Jul 23 '12 at 08:42

score 1 · Answer 2 · answered Jul 23 '12 at 08:39

You can't modify a collection you are looping over with foreach. The solution in your case is pretty simple: Since you do not use dc inside the loop, why not use a normal for loop, like:

int length = dt.Columns.Length;
for (int i = 0; i < length; i++)
{
    dt.Columns.Add(employeeTimings[i], typeof(string));
}

score 0 · Answer 3 · answered Jul 23 '12 at 08:38

0

With LINQ you can do:

    foreach (DataColumn dc in dt.Columns.OfType<DataColumn>().ToArray())
    {
        dt.Columns.Add(employeeTimings[i], typeof(string));
        i++;
    }

answered Jul 23 '12 at 08:38

TcKs

25,849
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score 0 · Answer 4 · answered Jul 23 '12 at 08:38

You are modifying the collection dt.Columns with the Add method while you are iterating over it:

foreach (DataColumn dc in dt.Columns)
{
    dt.Columns.Add(employeeTimings[i], typeof(string));
    i++;
}

Apart from not adding all timing strings this modifies the collection you are adding the columns to.

You should enumerate employeeTimings and add the columns like:

foreach(string s in employeeTimings)
{
    dt.Columns.Add(s, typeof(string));
}

score 0 · Answer 5 · answered Jul 23 '12 at 08:42

0

I guess your true intentions are to add the full employeeTimings list to the datatable columns
If this is right then you should loop on the list not on the columns

foreach (string s in employeeTimings) 
    dt.Columns.Add(s, typeof(string));

answered Jul 23 '12 at 08:42

Steve

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C# Collection was modified; enumeration operation may not execute

5 Answers5