/// <summary>Retrieves a reference to the specified item, adding it if necessary.</summary>
/// <typeparam name="T">The type of the items in the set.</typeparam>
/// <param name="tset">The set to search, and add to if necessary.</param>
/// <param name="item">The item sought.</param>
/// <returns>A reference to a matching item if it is present in the set, using the item given if there isn’t
/// already a matching item.</returns>
/// <exception cref="System.InvalidOperationException"> An attempt was made to use this when the generic type of
/// the set is not a reference type (that is, a value or pointer type).</exception>
/// <remarks>This allows use of the set to restrain a group of objects to exclude duplicates, allowing for
/// reduced memory use, and reference-based equality checking, comparable with how
/// <see cref="string.Intern(string)"/> allows one to check for a copy of a string in the CLR intern pool, but
/// also allowing for removal, clearing and multiple pools. This is clearly only valid for reference types.
/// </remarks>
public static T FindOrStore <T>(this ThreadSafeSet <T> tset, T item) where T : class
{
ThreadSafeSet <T> .Box found = tset.PutIfMatch(new ThreadSafeSet <T> .Box(item));
return(found == null || found is ThreadSafeSet <T> .TombstoneBox ? item : found.Value);
}
