输入的字符串是不正确format.how解决

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输入的字符串是不正确的format.how解决？

protected void totalprice()

    {

 

       int quantity = 0;

     int price = 0;

int rowTotal = 0;

int subTotal = 0;

int totalprice = 0;

foreach(GridViewRow row in GridView1.Rows)

{

   quantity = Convert.ToInt32(row.Cells[3].Text); //replace 0 with row cell index of qty

   price = Convert.ToInt32(row.Cells[4].Text); //replace 1 with row cell index of amount

   rowTotal = quantity * price;

   subTotal += rowTotal;

}

totalprice = subTotal;

lbltprice.Text = totalprice.ToString();

    }