ICollection does not offer a method for merging two collections. As merging collections was exactly what I needed, I wrote a bit of code for it. An extension method seemed to offer the most elegant solution:

public static ICollection<T> Add<T>(this IList<T> self, IList<T> input)
{
	foreach (T item in input)
	{
		self.Add(item);
	}
	return self;
}

Because I've overloaded the Add method, this will now work:

IList<string> list1 = new IList<string>();
IList<string> list2 = new IList<string>();
string name = 'J2';

list1.Add(list2);
list1.Add(name);

For the license to this code, see http://creativecommons.org/licenses/by/3.0/