Question

I have several applications within my domain that accept similar inputs in text fields. Each application implements its own validation. I want to bring that functionality into a class library so that rather than re-inventing the wheel on each project, our developers can quickly implement the validation library, and move on.

I’m not the best when it comes to OO design. What I need is the ability for a user to enter an arbitrary string, and then for the validation library to check it against the known types to make sure that it matches one of them. Should I build an interface and make each type of string a class that implements that interface? (seems wrong since I won’t know the type when I read in the string). I could use some help identifying a pattern for this.

Thanks.

Answer 1

I’ve always been a fan of Fluent Validation for .Net. If it’s more robust then you need, it’s functionality is easy enough to mimic on your own.

If you’re interested, here’s a link to my very simple validation class. It’s similar in usage to Fluent Validation, but uses lambdas to create the validation assertions. Here’s a quick example of how to use it:

public class Person
{
    public Person(int age){ Age = age; }
    public int Age{ get; set;}
}

public class PersonValidator : AbstractValidator
{
    public PersonValidator()
    {
        RuleFor(p => p.Age >= 0,
            () => new ArgumentOutOfRangeException(
                "Age must be greater than or equal to zero."
        ));
    }
}

public class Example
{
    void exampleUsage()
    {
        var john = new Person(28);
        var jane = new Person(-29);

        var personValidator = new PersonValidator();

        var johnsResult = personValidator.Validate(john);
        var janesResult = personValidator.Validate(jane);

        displayResult(johnsResult);
        displayResult(janesResult);
    }

    void displayResult(ValidationResult result)
    {
        if(!result.IsValid)
            Console.WriteLine("Is valid");
        else
            Console.WriteLine(result.Exception.GetType());
    }
}

(see source code for a more thorough example).

Output:

Is valid
System.ArgumentOutOfRangeException