How to get the value dynamically created checkbox value in asp.net gridivew

hi im adding checkbox controls dynamically in asp.net gridview like this

         '  CheckBox cb1 = new CheckBox();
            cb1.Text = row.Cells[3].Text;
            row.Cells[3].Controls.Add(cb1);

and i want to access that checkbox is checked or not on button click event....

Any one Help me...

 on button click i am try this 
            foreach (GridViewRow item in grdreport.Rows)
            {
                if (item.RowType == DataControlRowType.DataRow)
                {
                    CheckBox checkbox1 = (CheckBox)item.FindControl("cb1");
                   // cb1.Checked = true;
                    if (checkbox1.Checked)
                    {
                    }
                }
            }

but it gives me error Object reference not set to an instance of an object cb1 value is null