After every postback dropdownlist items repeats

Question

I have bind my dropdownlist with database. But on each PostBack the items in dropdownlist gets repeat again.

e.g. OnPage Load I have this items in dropdownlist 1,2,3,4.. etc Now suppose page gets postBack then it looks like 1,2,3,4,1,2,3,4

Vb code

Private Sub hospitals_Load(sender As Object, e As EventArgs) Handles Me.Load
Me.PopulateAreas()
End Sub


Private Sub PopulateAreas()
        If IsPostBack Then
            Dim citySelector As DropDownList = Page.Master.FindControl("locationSelector")
            Using conn As New MySqlConnection()
                conn.ConnectionString = ConfigurationManager _
                    .ConnectionStrings("conio").ConnectionString()
                Using cmd As New MySqlCommand()
                    cmd.CommandText = "Select * from areas where areaCity Like '" + citySelector.SelectedItem.ToString + "%'"
                    cmd.Connection = conn
                    conn.Open()
                    Using sdr As MySqlDataReader = cmd.ExecuteReader()
                        While sdr.Read()
                            Dim item As New ListItem()
                            item.Text = sdr("areaName").ToString()
                            item.Value = sdr("areaID").ToString()
                            'item.Selected = Convert.ToBoolean(sdr("IsSelected"))
                            areasList.Items.Add(item)
                        End While
                    End Using
                    conn.Close()
                End Using
            End Using
        End If

UPDATE

I have master page which has dropdownlist of cities. I am using masterpage control to mycontent page like below.

Dim citySelector As DropDownList = Page.Master.FindControl("locationSelector")

Now in my PopulateArea() class there is query in which WHERE clause have cityselector. So according to city my area gets fetched.

Answer 1

Change your code to,

Private Sub PopulateAreas()
    If IsPostBack Then
        areasList.Items.clear()
        Dim citySelector As DropDownList = Page.Master.FindControl("locationSelector")
        Using conn As New MySqlConnection()
            conn.ConnectionString = ConfigurationManager _
                .ConnectionStrings("conio").ConnectionString()
            Using cmd As New MySqlCommand()
                cmd.CommandText = "Select * from areas where areaCity Like '" + citySelector.SelectedItem.ToString + "%'"
                cmd.Connection = conn
                conn.Open()
                Using sdr As MySqlDataReader = cmd.ExecuteReader()
                    While sdr.Read()
                        Dim item As New ListItem()
                        item.Text = sdr("areaName").ToString()
                        item.Value = sdr("areaID").ToString()
                        'item.Selected = Convert.ToBoolean(sdr("IsSelected"))
                        areasList.Items.Add(item)
                    End While
                End Using
                conn.Close()
            End Using
        End Using
    End If
End Sub