public static ReadOnlyCollection<T> AsReadOnlyCollection<T>(this IEnumerable<T> source)
{
    if(source == null)
      throw new ArgumentNulLException("source");

    IList<T> list = source as IList<T> ?? source.ToList();
    return new ReadOnlyCollection<T>(list);
}

请注意，在这种情况下，没有转换" IEnumerable<T>之类的事情(与LINQ堆栈中的所有其他方法一样)，您将获得与以前不同的对象.

Note that there is no such thing as "converting" an IEnumerable<T> in this case (as with all other methods in the LINQ stack), you will get back a different object than before.