Ben d'état

Ben Scott


~/Class and method invariants and validation

24 Feb 2015

I was chatting to my man Tod and we came up with a cool pattern for validating both class and method invariants in a way that seems to scale.

Given a simple class Person, with some methods for updating from another Person and updating just the name:

public class Person
{
	public string Name { get; private set; }
	private int _age;

	public Person(string name, int age)
	{
		Name = name;
		_age = age;
	}

	public void UpdateFrom(Person person)
	{
		Name = person.Name;
		_age = person._age;
	}

	public void UpdateName(string name)
	{
		Name = name;
	}
}

The private _age is just to demonstrate a private field.

First add some validation on the constructor. I’m using my Check library but this could be implemented in any way that throws an exception if the rule isn’t set.

public Person(string name, int age)
{
	// method invariants
	Check.That(() => name).IsNotNullOrEmpty();
	Check.That(() => age >= 0);

	Name = name;
	_age = age;
}

This is what I’m calling a method invariant - a set of rules that validate the input to a method.

We also want to validate the state of a person at the end of the method.

static void Validate(Person person)
{
	Check.That(() => person.Name).IsNotNullOrEmpty();
	Check.That(() => person._ge >= 0);
}

This should happen at the end of each method that changes the state of the person. For example, the constructor becomes:

public Person(string name, int age)
{
	// method invariants
	Check.That(() => name).IsNotNullOrEmpty();
	Check.That(() => age >= 0);

	Name = name;
	_age = age;

	// validate myself
	Validate(this);
}

The UpdateName() method needs to do the same thing:

public void UpdateName(string name)
{
	Check.That(() => name).IsNotNullOrEmpty();

	Name = name;

	Validate(this);
}

The UpdateFrom() method can validate the entire incoming person:

public void UpdateFrom(Person person)
{
	Validate(person);

	Name = person.Name;
	_age = person._age;

	Validate(this);
}

Shweet.


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