LINQ and Activator.CreateInstance() creates duplicates

Question

I have next LINQ query to read App.config to instantiate objects using fully qualified types stated inside:

var strategies = from strategy in section.Strategies
let indicators = (
    from indicator in strategy.Indicators
    select (IIndicatorReader)Activator.CreateInstance(Type.GetType(indicator.Type), bot.Mt, section.Symbol))
let orders = (
    from order in strategy.Orders
    select new OrderInfo(order.Id, order.Operation.Value, order.Amount))
select (IStrategy)Activator.CreateInstance(Type.GetType(strategy.Type), section.Symbol, strategy.Amount, strategy.Limit, indicators, orders);

So each time inside the strategy I call

indicatorList.Select(i => i.Operation)

this instantiation occurs:

(IIndicatorReader)Activator.CreateInstance(Type.GetType(indicator.Type), bot.Mt, section.Symbol))

and appropriate class's constructor is called.

But an indicator stated in App.config first is instantiated twice, all other - once. How can it be?? I will be happy to provide any additional information required.

---

My indicator collection:

public class IndicatorElementCollection : ConfigurationElementCollection, IEnumerable<IndicatorElement>
{
    ...

    public new IEnumerator<IndicatorElement> GetEnumerator()
    {
        return this.OfType<IndicatorElement>().GetEnumerator();
    }
}

Implementation of GetEnumerator() conversion from non-generic to generic is taken from this question on SO.

Another implementation:

foreach (OrderElement element in (System.Collections.IEnumerable)this)
{
    yield return element;
}

works in the same manner.

Answer 1

The LINQ expression indicators will be re-evaluated every time you call GetEnumerator. You need to force a single evaluation by invoking ToList or ToArray. This could of course result in memory space issues if you are expecting many indicators.