Why is .ForEach() on IList and not on IEnumerable? [duplicate]

Because ForEach(Action) existed before IEnumerable<T> existed.

Since it was not added with the other extension methods, one can assume that the C# designers felt it was a bad design and prefer the foreach construct.

Edit:

If you want you can create your own extension method, it won’t override the one for a List<T> but it will work for any other class which implements IEnumerable<T>.

public static class IEnumerableExtensions
{
  public static void ForEach<T>(this IEnumerable<T> source, Action<T> action)
  {
    foreach (T item in source)
      action(item);
  }
}