2 Why We Need Model-Based Testing

158 Modeling Systems with Structured State
Structural equality may be contrasted with reference equality where two objects
are equal if they occupy the same location of the computer’s memory. In objectoriented
programming languages, reference equality is the default. However, structual
equality may be explicitly provided by an implementation. For example, the
string type in .NET languages provides structural equality.
Sometimes we want to use data structures that go beyond the built-in value types. 3
We can do this with custom data types. But in order to preserve the desirable property
of state comparison, we need to use custom data types with structural equality.
The C# language provides a keyword struct that may be used to declare data types
with structural equality. There are some technical limitations here – for example, C#
structs must be of a fixed length in memory. The NModel framework uses classes
instead of structs for implementing structural data types.
In order for equality of states to be well defined, we use compound values.
Compound values are data types distinguished by their use of structural equality and
their ability to handle variable-length and tree-structured data in addition to records
of fixed size.
The NModel modeling framework includes a mechanism that makes it easy to
define custom structural types with a built-in base class called CompoundValue. This
data type is a useful building block for many kinds of data structured data values.
For example, a binary tree of integers could be defined in this way:
class IntTree : CompoundValue
{
public readonly int value;
public readonly IntTree left;
public readonly IntTree right;
}
public IntTree(int value, IntTree left, IntTree right)
{
this.value = value;
this.left = left;
this.right = right;
}
All fields of a type that derives from CompoundValue must be marked readonly.
This is because structural data types are immutable values, like strings and integers.
3 The string type isn’t technically a value type in .NET terminology, but for simplicity we will
use that term because strings are immutable and use structural equality. We ignore the issue of
whether a value is stack-allocated.
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