2 Why We Need Model-Based Testing

160 Modeling Systems with Structured State
[Action]
static void RemoveClient(string client)
{
clients = clients.Remove(client);
}
This model program shows the basic operation of sets. The expression Set.
EmptySet is a way to denote the empty set of strings. This is analogous to the
expression string.Empty provided by the .NET framework that returns the empty
string. We say “the” empty string because all empty strings are equal. 4 It is useful
to think that there is just one empty string value, just as there is only one value for
0 or 1. Similarly, there is just one empty set of strings.
We should point out that in the set implementation provided by NModel, the
values Set.EmptySet and Set.EmptySet are not equal. You can think
of each set as having a subscript based on the type parameter inside the angle
brackets. This is true even for subtypes: values of type Set and Set
are never equal, even if they contain the same elements.
Another thing to point out is that the Add and Remove operations return new set
values. This is because set values, like .NET strings, are immutable. There is no way
to change their values. However, we can update a state variable with the new value
that results from adding or removing an element:
clients = clients.Add(client);
If you were to explore the model program shown above for the client/server
system, you would notice that the following two traces result in the same end
state:
Trace 1 Trace 2
AddClient("alice") AddClient("bob")
AddClient("bob") AddClient("alice")
Modeling complex state often needs mathematical data structures like sets. Using
sets whenever order doesn’t matter can dramatically reduce the number of states you
will have to consider for design analysis and testing. Sets are the bread and butter
of experienced software modelers. They appear very often.
Creating sets
The easiest way to create a set is to list its elements in the constructor. The order in
which arguments appear doesn’t matter and duplicates will be ignored:
4 When we use the word “equals,” we mean equality in the sense of the Object.Equals method.
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