Question

we all know the slow way:

foreach..

Answer 1

The List constructor is a good bet.

IEnumerable<T> enumerable = ...;
List<T> list = new List<T>(enumerable);

Answer 2

How about:

IEnumerable<T> sequence = GetSequenceFromSomewhere();
List<T> list = new List<T>(sequence);

Note that this is optimised for the situation where the sequence happens to be an IList<T> - it then uses IList<T>.CopyTo. It ll still be O(n) in most situations, but potentially a much faster O(n) than iterating :) (It also avoids any resizing as it creates it.)

Answer 3

You want to use the .AddRange method on generic List.

List<string> strings = new List<string>();
strings.AddRange(...);

.. or the constructor ..

new List<string>(...);

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