Question

What is Linq actually doing?

Answer 1

(I’m assuming this is for LINQ to Objects. Anything else will be implemented differently 🙂

It’s just returning everything from the first, and then everything from the second. All data is streamed. Something like this:

public static IEnumerable<T> Concat(this IEnumerable<T> source1,
    IEnumerable<T> source2)
{
    if (source1 == null)
    {
        throw new ArgumentNullException("source1");
    }
    if (source2 == null)
    {
        throw new ArgumentNullException("source1");
    }
    return ConcatImpl(source1, source2);
}


private static IEnumerable<T> ConcatImpl(this IEnumerable<T> source1,
    IEnumerable<T> source2)
{
    foreach (T item in source1)
    {
        yield return item;
    }
    foreach (T item in source2)
    {
        yield return item;
    }
}

I’ve split this into two methods so that the argument validation can be performed eagerly but I can still use an iterator block. (No code within an iterator block is executed until the first call to MoveNext() on the result.)