ASP.NET

protected void Button4_Click(object sender, EventArgs e)
    {
        string str1 = "Select MAX(PID) From Product";
        OleDbCommand cmd = new OleDbCommand(str1, conn);
        OleDbDataReader dr = null;
        conn.Open();
        string max = cmd.ExecuteScalar().ToString();
        if (max != "")
        {
            max = "PID" + max;

        }
        else
        {
            max = "PID" + 1;
        }
        conn.Close();   
        string str = "Insert Into Product(ProductID,ProductName,ProductDescription,ProductPrice)Values('"+max+"','" + txtprdtname.Text + "','" + txtprdtdescrp.Text + "','" + txtprdtprice.Text + "')";
         cmd = new OleDbCommand(str, conn);
        conn.Open();
        cmd.ExecuteNonQuery();
        conn.Close();
        str = "Select * From Product";
        cmd = new OleDbCommand(str, conn);
        OleDbDataReader dr1 = null;
        conn.Open();
        dr1 = cmd.ExecuteReader();
        DropDownList2.DataSource = dr1;
        DropDownList2.DataTextField = "ProductID";
        DropDownList2.DataValueField = "ProductID";
        DropDownList2.DataBind();
        conn.Close();

        string str3 = "Select Max(PID) From Product";
        cmd = new OleDbCommand(str3, conn);
        conn.Open();
        string max1 = cmd.ExecuteScalar().ToString();
        string max2 = "PID" + max1;
        conn.Close();
        string str2 = "Update Product Set ProductID='" + max2 + "' Where(PID=" + max1 + ")";
        cmd = new OleDbCommand(str2, conn);
        conn.Open();
        cmd.ExecuteNonQuery();
        conn.Close();

    }