Concatenate multiple IEnumerable<T>

Question

I'm trying to implement a method to concatenate multiple Lists e.g.

List<string> l1 = new List<string> { "1", "2" };
List<string> l2 = new List<string> { "1", "2" };
List<string> l3 = new List<string> { "1", "2" };
var result = Concatenate(l1, l2, l3);

but my method doesn't work:

public static IEnumerable<T> Concatenate<T>(params IEnumerable<T> List)
{
    var temp = List.First();
    for (int i = 1; i < List.Count(); i++)
    {
        temp = Enumerable.Concat(temp, List.ElementAt(i));
    }
    return temp;
}

Answer 1

Use SelectMany:

public static IEnumerable<T> Concatenate<T>(params IEnumerable<T>[] lists)
{
    return lists.SelectMany(x => x);
}

Answer 2

Just for completeness another imo noteworthy approach:

public static IEnumerable<T> Concatenate<T>(params IEnumerable<T>[] List)
{
    foreach (IEnumerable<T> element in List)
    {
        foreach (T subelement in element)
        {
            yield return subelement;
        }
    }
}

Answer 3

If you want to make your function work you need an array of IEnumerable:

public static IEnumerable<T> Concartenate<T>(params IEnumerable<T>[] List)
{
    var Temp = List.First();
    for (int i = 1; i < List.Count(); i++)
    {
        Temp = Enumerable.Concat(Temp, List.ElementAt(i));
    }
    return Temp;
}

Answer 4

All you have to do is to change:

public static IEnumerable<T> Concatenate<T>(params IEnumerable<T> lists)

to

public static IEnumerable<T> Concatenate<T>(params IEnumerable<T>[] lists)

Note the extra [].