private void SelectAllItems()
    {
        foreach (DataGridViewRow row in itemsBySupplier.Rows)
        {
            DataGridViewCheckBoxCell check = (DataGridViewCheckBoxCell)row.Cells["selectItem"];
            check.Value = check.TrueValue;
        }
    }

foreach (DataGridViewRow row in itemsBySupplier.Rows)
    {
        DataGridViewCheckBoxCell check = (DataGridViewCheckBoxCell)row.Cells["selectItem"];

        string hid = row.Cells["Hid"].Value.ToString();

        if (Ws.ToCreate[_supplier].Count(i => i.Hid == hid) > 0)
        {
            check.Value = check.TrueValue;
        }
    }