C#: DataGridViewCheckBoxCell strange behaviour

Question

Possible Duplicate:
DataGridViewCheckBoxCell how to show checked when set during form load

I am having a problem with a DataGridView in a C#-WinForms project.

I fill it with data from a local database. Additionally, I have added a CheckBoxColumn that isn't bound to any data. I use this to filter a ListBox depending on the checked rows. That all is placed on the second page of a TabControl on a dialog form and is working perfectly.

Now I want to programatically select rows based on on a list of strings that I pass through when opening the form. I tried this code:

foreach (string pre in preselect)
{
    foreach (DataGridViewRow row in dgvProtFunc.Rows)
    {
        if ((string)row.Cells[2].Value == pre)
        {
            ((DataGridViewCheckBoxCell)row.Cells[0]).Value = true;
        }
    }
}

Setting the values and then the triggered filtering works on formload. However, the checkboxes are not checked. When checking one, all other values are lost. Thats bad, since I have to use such a pre-select. These are the other DataGridView event handler:

private void dgvProtFunc_CurrentCellDirtyStateChanged(object sender, EventArgs e)
{
    if (dgvProtFunc.IsCurrentCellDirty)
    {
        dgvProtFunc.CommitEdit(DataGridViewDataErrorContexts.Commit);
    }
}

private void dgvProtFunc_CellValueChanged(object sender, DataGridViewCellEventArgs e)
{
    if (!FormLoadCheck)
    {
        string data = string.Empty;
        ptSelectList.Clear();
        foreach (DataGridViewRow row in dgvProtFunc.Rows)
        {
           if (row.Cells[0].Value != null && Convert.ToBoolean(row.Cells[0].Value) == true)
           {
               ptSelectList.Add(Convert.ToInt32(row.Cells[1].Value));
           }
           data += row.Cells[0].Value;
        }
        ptSelectList.Sort();
        FilterRelais(ptSelectList.ToArray());
        MessageBox.Show(data);
   }
}

Can anyone explain this behaviour? Any solutions?