var myObject= new List<Core.MyObject>();
var myObject2= new List<Core.MyObject>();

if(!string.IsNullOrEmpty(txb.Text))
{
    var repository = ServiceLocator.Current.GetInstance<Core.RepositoryInterfaces.IRepository>();
    myObject= repository.GetWherePostCodeLike(txb.Text).ToList();
}

if(!string.IsNullOrWhiteSpace(ddl.SelectedValue))
{
    var repository = ServiceLocator.Current.GetInstance<Core.RepositoryInterfaces.IRepository>();
    myObject2= repository.GetNearTownX(ddlLocations.SelectedValue).ToList();
}

IEnumerable<MyObject> third = list.Concat(otherList);

var combined = dataSet.Concat(dataSet2).ToList();

dataSet.AddRange(dataSet2);

var merged = dataSet.Concat(dataSet2).ToList();