What is ICollection ?

The ICollection<T> interface is the base interface for classes in the System.Collections.Generic namespace. The ICollection<T> interface extends IEnumerable<T> and is extended by IDictionary<TKey, TValue> and IList<T>. An IDictionary<TKey, TValue> implementation is a collection of key/value pairs, like the Dictionary<TKey, TValue> class. An IList<T> implementation is a collection of values, and its members can be accessed by index, like the List<T> class.

private void SendEmail(string host, int port,
        string username, string password,
        string from, string to,
        string subject, string body,
        ICollection<string> attachedFiles)