you should use my edited code it as
DataView dv = new DataView(table);
if (!String.IsNullOrEmpty(textBox1.Text))
{
dv.RowFilter = String.Format("vendor like '%{0}%'", textBox1.Text);
}
dv.RowFilter = dv.RowFilter == "" ? String.Format("model like '%{0}%'", textBox2.Text) : dv.RowFilter + String.Format("AND model like '%{0}%'", textBox2.Text);
purchase_mobile_DG.DataSource = dv;