AutoComplete Textbox bind with Value and Text.

Here is my code,
UserId and UserName is retrieved and binding to List<useritem> collection:
Entity dynentity2 = collection.Entities[i];
if (dynentity2.Attributes.Count > 0 && dynentity2.Attributes.Contains("systemuserid"))
{

string strUserId = string.Empty;
string strUserName = string.Empty;
if (dynentity2.Attributes.Contains("systemuserid"))
strUserId = Convert.ToString((dynentity2.Attributes["systemuserid"]));

if (dynentity2.Attributes.Contains("fullname"))
strUserName = Convert.ToString((dynentity2.Attributes["fullname"]));

UserItem objItem = new UserItem();
objItem.UserId = strUserId;
objItem.UserName = strUserName;
lstUsers.Add(objItem);
}

Autocomplete textbox web method by typing username, returns List<string> collection:
[System.Web.Services.WebMethodAttribute(), System.Web.Script.Services.ScriptMethodAttribute()]
public static List<string> GetCompletionList(string prefixText, int count, string contextKey) {
List<string> objReturnList = new List<string>();
try
{
var products = lstUsers.Select(p => p.UserName.ToLower().StartsWith(prefixText.ToLower())).ToList();
foreach (UserItem objItem in lstUsers)
{
if (objItem.UserName.ToLower().StartsWith(prefixText.ToLower()))
{
objReturnList.Add(objItem.UserName);
}
}
return objReturnList.Distinct().ToList();

}
catch (Exception ex)
{
throw ex;
}
finally {
objReturnList = null;
}
}

when user selects the correct user, i have to pass the Userid(not UserName) to my DB to fetch his information.