custom dropdownlist control

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custom-controls

Hi guys i have a custom dropdownlist control:

public void Bind<T>(IEnumerable<T> dataSource, string valueField, string textField)
       {
           DataSource = dataSource;
           DataValueField = valueField;
           DataTextField = textField;
           DataBind();

}

but when i use javascript to get the selectedindex of the control and set another custom dropdownlist's selectedindex to that of the first selected one, it does not change!

javascript code:

function test()
   {
       var drpCampaignType = document.getElementById('<%=drpCampaignType.ClientID %>');
       var drpCampaignTypeValue = drpCampaignType.selectedIndex       
var drpListCampaignDocketTemplate = document.getElementById('<%=drpListCampaignDocketTemplate.ClientID %>');
       var drpListCampaignDocketTemplateValue = drpListCampaignDocketTemplate.selectedIndex;
       var drpCampType = document.getElementById('<%=valueOffPurchRequiredFieldValidator.ClientID %>').selectedIndex= drpCampaignTypeValue;
       var drpCampDockTemplate = document.getElementById('<%=valueOffPurchRequiredFieldValidator.ClientID %>').selectedIndex= drpListCampaignDocketTemplateValue;

is there any way to overcome this, thanks in advance!