how to enter value in combo box after selecting values in asp.net

Place one dropdownlistbox and dont forget to give "autopostback=true". A button to set value and a label to show the value. Write the coding as below

protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
If IsPostBack=False then
           ddltest.Items.Add(New ListItem("Plz Select"))
            ddltest.Items.Add(New ListItem("One"))
            ddltest.Items.Add(New ListItem("two"))
            ddltest.SelectedIndex = 0
End if
Protected Sub ddltest_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles ddltest.SelectedIndexChanged
        lbltest1.Text = ddltest.SelectedValue
End Sub
Protected Sub btntest1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btntest1.Click
        ddltest.SelectedItem.Value = Val(ddltest.SelectedItem.Value) + 10
End Sub

You can set the value to combo box items like this... If you got it, plz add to my reputation and mark it as solved.