textbox turn to zero (0)

public double numOfP1, numOfP2, piecePr1, piecePr2, prSub1, prSub2, totalSub;

       private void multiplyButton_Click(object sender, EventArgs e)
       {
           if ((price1.Text == null) || (numOfPiece1.Text == null))
           {
               numOfP1 = Convert.ToDouble(numOfPiece1.Text);
               numOfP1 = 0;
               piecePr1 = Convert.ToDouble(price1.Text);
               piecePr1=0; 
           }
           
           if ((price2.Text == null) || (numOfPiece2.Text == null))
           {
               numOfP2 = Convert.ToDouble(numOfPiece2.Text);
               numOfP2 = 0; 
               piecePr2 = Convert.ToDouble(price2.Text);
               piecePr2 =0; 
           }                  

           prSub2 = piecePr2 * numOfP2;
           priceSub2.Text = prSub2.ToString();
           prSub1 = piecePr1 * numOfP1;
           priceSub1.Text = prSub1.ToString();
       }
   }