Try To Increase Your DB Column Size's First
Later Try This..
cmd.Parameters.AddWithValue("@price", string.IsNullOrEmpty(textBox1.Text)?0:Convert.ToInt32(textBox1.Text));
cmd.Parameters.AddWithValue("@productcategory",
string.IsNullOrEmpty(cmdproduct.SelectedItem.ToString())?"":cmdproduct.SelectedItem.ToString());
cmd.Parameters.AddWithValue("@munits",
string.IsNullOrEmpty(cmdmunits.SelectedItem.ToString())?"":cmdmunits.SelectedItem.ToString());
cmd.Parameters.AddWithValue("@productname", string.IsNullOrEmpty(txtproductname.Text)?"":txtproductname.Text);
cmd.Parameters.AddWithValue("@productcode", string.IsNullOrEmpty(txtpc.Text)?"":txtpc.Text);
cmd.Parameters.AddWithValue("@price", string.IsNullOrEmpty(textBox1.Text)?"":textBox1.Text);
cmd.Parameters.AddWithValue("@date", string.IsNullOrEmpty(date1)? Convert.ToDateTime(Datetime.Now):date1);