Creating Product ID automatically

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C#2.0

C#

In my Product Details Windows Form, there is a column like Product ID and much more. I used textBox1 control for Product ID column. I want to create the Product ID automatically in the Load Event or Validating Event like this: 02112013/PID0001. When the user will submit the 1st Product ID(02112013/PID0001) and other details to SQL Server 2005 database. After submitting the 1st Product ID(02112013/PID0001) whenever the user will open the Product Details form, the Product ID should be created automatically in the Load Event or Validating Event like: 02112013/PID0002.
I wrote code in the Validating event of the textBox1 like this:

private void textBox1_Validating(object sender, CancelEventArgs e)
       {
           DateTime dt = DateTime.Now;
           int mon = dt.Month;
           int year = dt.Year;
           int day = dt.Day;
           string id1, id2;
           int id;
           SqlConnection con = new SqlConnection(Class1.cs);
           con.Open();
           string query = "select 'Max_No'=max(ProductID) from Product_Details";
           SqlCommand cm = new SqlCommand(query, con);
           SqlDataReader dr = cm.ExecuteReader();
           dr.Read();
           {
               if (dr[0].ToString() == "")
               {
                   id1 = "PID0000";
               }
               else
               {
                   id1 = dr[0].ToString();
               }
               id2 = id1.Substring(3, 4);
               id = Convert.ToInt32(id2);

               if ((id >= 0) && (id < 9))
               {
                   id = id + 1;
                   textBox1.Text = day.ToString() + mon.ToString() + year.ToString() + "/" + "PID000" + id;
               }
               else if ((id >= 9) && (id < 99))
               {
                   id = id + 1;
                   textBox1.Text = day.ToString() + mon.ToString() + year.ToString() + "/" + "PID00" + id;
               }
               else if ((id >= 99) && (id < 999))
               {
                   id = id + 1;
                   textBox1.Text = day.ToString() + mon.ToString() + year.ToString() + "/" + "PID0" + id;
               }
               dr.Close();
           }
           con.Close();
       }

The first Product ID is submitted successfully, when I start for the second it does not create the 2nd Product ID.
I am unable to find out the reason. Please someone help me.