DataGridview checkbox is checked when form loading

I open form,but the dataview ,the first cells ,can not checked. my code is below,i wanna all the checkboxcell is checked default.this code inside form_load. who have good idea. i have tested many ways.

DataGridView1.Refresh(); int count = Convert.ToInt32(DataGridView1.RowCount.ToString());

        for (int i = 0; i < count; i++)
        {

            DataGridViewCheckBoxCell checkCell = (DataGridViewCheckBoxCell)DataGridView1.Rows[i].Cells[0];
            checkCell.Value = true;
            Boolean flag = Convert.ToBoolean(checkCell.Value);
            if (flag == false)     
            {
                checkCell.Value = true;
            }
            else
                continue;
        }

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