SelectMany.cs

public static IEnumerable<TResult> SelectMany<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, IEnumerable<TResult>> selector) { 
    if (source == null) throw Error.ArgumentNull("source");
    if (selector == null) throw Error.ArgumentNull("selector");
    return SelectManyIterator<TSource, TResult>(source, selector);
} 

static IEnumerable<TResult> SelectManyIterator<TSource, TResult>(IEnumerable<TSource> source, Func<TSource, IEnumerable<TResult>> selector) { 
    foreach (TSource element in source) { 
        foreach (TResult subElement in selector(element)) {
            yield return subElement; 
        }
    }
}

public static IEnumerable<TResult> SelectMany<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, int, IEnumerable<TResult>> selector) {
    if (source == null) throw Error.ArgumentNull("source"); 
    if (selector == null) throw Error.ArgumentNull("selector"); 
    return SelectManyIterator<TSource, TResult>(source, selector);
} 

static IEnumerable<TResult> SelectManyIterator<TSource, TResult>(IEnumerable<TSource> source, Func<TSource, int, IEnumerable<TResult>> selector) {
    int index = -1;
    foreach (TSource element in source) { 
        checked { index++; }
        foreach (TResult subElement in selector(element, index)) { 
            yield return subElement; 
        }
    } 
}
public static IEnumerable<TResult> SelectMany<TSource, TCollection, TResult>(this IEnumerable<TSource> source, Func<TSource, int, IEnumerable<TCollection>> collectionSelector, Func<TSource, TCollection, TResult> resultSelector)
{
    if (source == null) throw Error.ArgumentNull("source"); 
    if (collectionSelector == null) throw Error.ArgumentNull("collectionSelector");
    if (resultSelector == null) throw Error.ArgumentNull("resultSelector"); 
    return SelectManyIterator<TSource, TCollection, TResult>(source, collectionSelector, resultSelector); 
}

static IEnumerable<TResult> SelectManyIterator<TSource, TCollection, TResult>(IEnumerable<TSource> source, Func<TSource, int, IEnumerable<TCollection>> collectionSelector, Func<TSource, TCollection, TResult> resultSelector){
    int index = -1;
    foreach (TSource element in source){
        checked { index++; } 
        foreach (TCollection subElement in collectionSelector(element, index)){
            yield return resultSelector(element, subElement); 
        } 
    }
} 

public static IEnumerable<TResult> SelectMany<TSource, TCollection, TResult>(this IEnumerable<TSource> source, Func<TSource, IEnumerable<TCollection>> collectionSelector, Func<TSource, TCollection, TResult> resultSelector) {
    if (source == null) throw Error.ArgumentNull("source");
    if (collectionSelector == null) throw Error.ArgumentNull("collectionSelector"); 
    if (resultSelector == null) throw Error.ArgumentNull("resultSelector");
    return SelectManyIterator<TSource, TCollection, TResult>(source, collectionSelector, resultSelector); 
} 

static IEnumerable<TResult> SelectManyIterator<TSource, TCollection, TResult>(IEnumerable<TSource> source, Func<TSource, IEnumerable<TCollection>> collectionSelector, Func<TSource, TCollection, TResult> resultSelector) { 
    foreach (TSource element in source) {
        foreach (TCollection subElement in collectionSelector(element)) {
            yield return resultSelector(element, subElement);
        } 
    }
}