Does Enumerable.Concat always append at the end of the first collection?

Question

Does Enumerable.Concat always append at the end of the first collection?

Example:

object someObject = new object();
List<object> listA = new List<object>();
listA.Add(new int());
object item = listA.Concat(new object[] { (object)new float() }).FirstOrDefault();

Is it guaranteed that the item will be int not float after Concat on every use? That means:

[0] int
[1] float

The MSDN says nothing about element order in resulting collection, however examples show that the order is elements from first collection then elements from second collection.

Answer 1

Concat is a LINQ method. That means it's a query. It does not create a list or some other kind of collection but a sequence.

So what Concat actually does is combining the two source sequences. When you iterate through the result of Concat you first iterate through the first sequence and then through the second sequence. The sequence are thereby never changed.

So, yes

the item will be int not float after Concat on every use

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The MSDN says nothing about element order

Well it does say

Concatenates two sequences.

And to concatenate means to put one after the other, not to mix them up.

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From the reference source:

public static IEnumerable<TSource> Concat<TSource>(this IEnumerable<TSource> first, IEnumerable<TSource> second) {
    if (first == null) throw Error.ArgumentNull("first");
    if (second == null) throw Error.ArgumentNull("second");
    return ConcatIterator<TSource>(first, second);
}

static IEnumerable<TSource> ConcatIterator<TSource>(IEnumerable<TSource> first, IEnumerable<TSource> second) {
    foreach (TSource element in first) yield return element;
    foreach (TSource element in second) yield return element;
}

So you see the two consecutive foreach will first yield the elements of the first sequence, then the element of the second sequence.