In this particular case, yes:

List<double> listwant = new List<double>(toadd);

Whether it applies to your real life situation is a different matter,
however.

Bob,

This kind of questions depends mostly for what you mean with "fast".

If it is about looping, then it is mostly that in a method the looping is
replaced behind the scene and the processing time will be almost the same.
However, often it can with less row to write, which does not mean that it
goes faster.

Remember that looping in a good written loop is extremely fast, at that
point you will seldom find any processing time that is recognizable by the
user.

However if it is about coding, than you sometimes can find some less rows,
although with a foreach loop we are talking as well mostly about 2 or 3
rows.

Cor