Merging 2 Collection<T>

public IEnumerable<string> Combine(IEnumerable<string> col1, IEnumerable<string> col2)
{
    foreach(string item in col1)
        yield return item;

    foreach(string item in col2)
        yield return item;
}

class Program
{
	static void Main(string[] args)
	{
		Collection<string> myitems = new Collection<string>();
		myMthod(ref myitems);
		Console.WriteLine(myitems.Count.ToString());
		Console.ReadLine();
	}

	static void myMthod(ref Collection<string> myitems)
	{
		myitems.Add("string");
		if(myitems.Count <5)
			myMthod(ref myitems);
	}
}