2 Why We Need Model-Based Testing

10.3.1 Sets
Systems with Complex State 159
The data fields of a value are not variable locations of memory like typical instances
of class types.
Note that compound values, although they may be tree-structured, are by construction
not allowed to contain circular references.
The CompoundValue base class provides the necessary support for structural equality,
hash coding, and several other services required by the modeling framework.
The NModel framework includes several families of compound values as built-in
types. These are documented in Appendix A and in the NModel package itself.
These types are used frequently in software models. In the following sections, we
will describe these built-in types for sets, maps, sequences, bags, pairs, and triples.
All of these built-in types are immutable value types, like the string type. If you
can program using strings, you should be able to master the modeling collection
types.
A set is an unordered collection of distinct elements. Sets are immutable values but
you can construct new sets by operations such as union and intersection, just as you
can construct .NET string values by concatenation. Appendix A.2.2 provides a list
of set operations provided by the NModel library.
You will find that sets, which are unordered, are preferable to ordered sequences
in many modeling examples because they reduce the number of states that must be
considered for design analysis and testing. States that use sets are more abstract than
states that use ordered data structures like sequences.
Two sets that contain the same elements are equal, regardless of the order in
which their elements were added. Sequences, on the other hand, are not equal if
their elements appear in different orders.
For example, consider a network model where there are some clients communicating
with a server. Clients may join and leave the session at any time. If the model
of the server state uses a set to keep track of active clients, then the order in which
clients arrive will not matter. This is a useful abstraction, especially if no operations
of the system depend on the order of client entry.
static Set clients = Set.EmptySet;
[Action]
static void AddClient(string client)
{
clients = clients.Add(client);
}
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