Winform Collection was modified; enumeration operation may not execute

Question

my code is:

    private void studentToolStripMenuItem_Click(object sender, EventArgs e)
    {

        foreach (Form frm in Application.OpenForms)
        {
            if (frm.Text == "student")
            {
                frm.Activate();
            }
            else
            {
                studetn obj = new studetn();
                obj.MdiParent = this;
                obj.Dock = DockStyle.Fill;
                obj.Show();
            }
        }                                                                     
    }

Have you tried to get a list of forms in loop and Activate them outside of the loop? — Mehrzad Chehraz, Feb 24 '16 at 13:21
You'll have to look at *all* the forms before you decide to create a new one. So this code is already borken by design. Add `return`, move the form creation code outside of the loop. And use the MdiChildren property. — Hans Passant, Feb 24 '16 at 13:30
here the scenario is change i am trying to check whether the mdi child is already open or not if open then activate it — kamran afridi, Feb 26 '16 at 11:34

score 2 · Accepted Answer · answered Feb 24 '16 at 13:32

Show() command modifies the Application.OpenForms collection which you are enumerating. It is not allowed. You should create and show the new form outside of the foreach loop:

private void studentToolStripMenuItem_Click(object sender, EventArgs e)
{
    // Find the student form
    Form frmStudent = null;
    foreach (Form frm in Application.OpenForms)
    {
        if (frm.Text == "student")
        {
            frmStudent = frm;
            break;
        }
    }                                                                     
    // if found - activate, if not found - create
    if(frmStudent!=null)
    {
        frmStudent.Activate();
    }
    else
    {
        studetn obj = new studetn();
        obj.MdiParent = this;
        obj.Dock = DockStyle.Fill;
        obj.Show();
    }
}

thanks all of u finally i solve it – kamran afridi Feb 24 '16 at 14:25 — kamran afridi, Feb 24 '16 at 14:25

Winform Collection was modified; enumeration operation may not execute

1 Answers1