Question

How to write extension methods for converting IQueryable<T> and IEnumerable<T> to ReadOnlyCollection<T>?

Thanks

Answer 1

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);
}

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.