ASP.Net Error: Cannot implicitly convert type System.Collections.Generic.List to string[]

I am getting the error here as cannot convert from System.collections.generic.list<string> to string[] for queryeventids(last line). I dont know what wrong in that.

List<string> queryEventIDList;
foreach (ListItem lstAssign in lstEvent.Items)
{
    if (lstAssign.Selected == true)
    {
        queryEventIDList.Add(lstAssign.Value);
        logfield = logfield + "," + lstEvent.SelectedItem.Text;
    }
}
string queryEventIDs = string.Join(",", queryEventIDList);