Writing Validator Builders

Validator builders are the simplest & clearest way to define a validator when you wish to describe the validator using .NET code.

A validator builder is a class which implements the interface IBuildsValidator<TValidated>. Developers then use the ConfigureValidator method and the IConfiguresValidator<TValidated> helper to specify how the validator will function. Validator builders are type-safe and benefit from IDE autocomplete (aka Intellisense) functionality.

The basics: Adding rules to a builder

Rules may be added to either or both of the object being validated or values derived from that object. Here is an example of three ways to add rules to a validator, by using a builder.

public class PetValidatorBuilder : IBuildsValidator<Pet>
{
    public void ConfigureValidator(IConfiguresValidator<Pet> config)
    {
        config.AddRule<NotNull>();

        config.ForMember(x => x.Name, member =>
        {
            member.AddRule<NotNullOrEmpty>();
        });

        config.ForValue(x => GetAge(x.BirthDate), val =>
        {
            val.AddRule<IntegerInRange>(conf =>
            {
                conf.ConfigureRule(rule =>
                {
                    rule.Min = 0;
                    rule.Max = 200;
                })
            });
        });
    }

    static int GetAge(DateTime birthDate) { /* Implementation omitted */ }
}

In the first example, the AddRule method is used directly from the IConfiguresValidator<Pet> instance. This would add a rule that implements IRule<Pet>, to apply validation logic associated with the complete object instance. In this case it is the NotNull rule.

In the second example, the ForMember method is used to specify a member (in this case, a property) of the Pet class. Let's assume that Pet.Name is a property of type string. Rules added within the member lambda target that member's value. In this case the NotNullOrEmpty rule will validate this string value.

In the third example, the ForValue method is used. ForValue allows validation of a completely arbitrary value, not limited to only values returned-by/stored-in a member like a property. Developers are encouraged to prefer ForMember over ForValue where possible, as the ForValue method comes with some limitations which do not apply to ForMember.

Finally, the third example also demonstrates a rule that has configuration. If you look at the IntegerInRange rule, you will see that the rule class has two settable properties describing the minimum and maximum of the permitted range. The configuration syntax permits setting that state from the builder.

Further techniques

With the techniques above, we have only seen a part of what a validator builder can accomplish. The links below will describe the remaining techniques.