collection was modified enumeration operation might not execute

Question

 string field = ViewState["Field"].ToString();

 DataTable dt = (DataTable)Session["Academic"];
 foreach (DataRow dr in dt.Rows)
 {
      if (dr["Degree"].ToString() == field)
      {
            dr.Delete();
            dt.AcceptChanges();
      }
 }
 Session["Academic"] = dt;
 gdvwAcademic1.DataSource = Session["Academic"] as DataTable;
 gdvwAcademic1.DataBind();

when this code executed raise error as "collection was modified enumeration operation might not execute." why this so..?

Have you seen the duplicates on SO according to this exception? — Tim Schmelter, Feb 20 '15 at 10:19
possible duplicate of [Collection was modified; enumeration operation may not execute](http://stackoverflow.com/questions/604831/collection-was-modified-enumeration-operation-may-not-execute) — Magnus, Feb 20 '15 at 10:39

David Watts · Accepted Answer · 2016-12-01T22:08:56.237

You cannot modify a collection in a foreach. Try this as an alternative to apomene's answer (Pretty much does the same thing, except using the remove method of a list instead of indexes.

List<DataRow> toDelete = new List<DataRow>();

foreach(DataRow dr in dt.Rows){
    if(dr["Degree"].ToString() == field){
        toDelete.Add(dr);
    }
}

foreach (DataRow dr in toDelete){
    dt.Rows.Remove(dr);
}

This should solve your problem.

Tim Schmelter · Answer 2 · 2015-02-20T10:26:39.713

6

You can't modify the collection in a foreach. So don't delete or add things in it.

You could prevent this error by creating a new collection with items you want to modify, for example with LINQ (the ToList() is essential):

var toDelete = dt.AsEnumerable().Where(r => r["Degree"].ToString() == field).ToList();

Now you can loop this without a problem.

foreach(DataRow rowToDelete in toDelete)
    rowToDelete.Delete();
dt.AcceptChanges(); // you could do that also in the loop now

edited Feb 20 '15 at 10:26

answered Feb 20 '15 at 10:20

Tim Schmelter

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@kls: then try my second approach, i'l delete the first. – Tim Schmelter Feb 20 '15 at 10:26

score 2 · Answer 3 · answered Feb 20 '15 at 10:24

you cant delete inside foreach use for loop or make a delete List:

List <int> toBeDel=new List<int>();

  foreach (DataRow dr in dt.Rows)
 {
      if (dr["Degree"].ToString() == field)
      {
            toBEDel.Add(indexOf(dr));
      }
 }

 foreach (int i in toBeDel)
 {
     dt.Rows[i].Delete();
 }

score 0 · Answer 4 · answered Oct 23 '19 at 08:56

for (int i = 0;i < dt.Rows.Count; i++)
 {
      DataRow dr = dt.Rows[i];
      if (dr["Degree"].ToString() == field)
      {
            dr.Delete();
            i-=1;
      }
 }
dt.AcceptChanges();

you can use this, one loop. For any people find this post.

collection was modified enumeration operation might not execute

4 Answers4