The answer is .IsPostBack
as suggested by @Kundan Singh Chouhan. Just to add to it, move your code into a separate method from Page_Load
private void PopulateFields()
{
using(SqlConnection con1 = new SqlConnection("Data Source=USER-PC;Initial Catalog=webservice_database;Integrated Security=True"))
{
DataTable dt = new DataTable();
con1.Open();
SqlDataReader myReader = null;
SqlCommand myCommand = new SqlCommand("select * from customer_registration where username='" + Session["username"] + "'", con1);
myReader = myCommand.ExecuteReader();
while (myReader.Read())
{
TextBoxPassword.Text = (myReader["password"].ToString());
TextBoxRPassword.Text = (myReader["retypepassword"].ToString());
DropDownListGender.SelectedItem.Text = (myReader["gender"].ToString());
DropDownListMonth.Text = (myReader["birth"].ToString());
DropDownListYear.Text = (myReader["birth"].ToString());
TextBoxAddress.Text = (myReader["address"].ToString());
TextBoxCity.Text = (myReader["city"].ToString());
DropDownListCountry.SelectedItem.Text = (myReader["country"].ToString());
TextBoxPostcode.Text = (myReader["postcode"].ToString());
TextBoxEmail.Text = (myReader["email"].ToString());
TextBoxCarno.Text = (myReader["carno"].ToString());
}
con1.Close();
}//end using
}
Then call it in your Page_Load
protected void Page_Load(object sender, EventArgs e)
{
if(!Page.IsPostBack)
{
PopulateFields();
}
}