C# add validation on a setter method

Question

I have a a couple of variables that i define in C# by:

public String firstName { get; set; }
public String lastName { get; set; }
public String organization { get; set; }

What i want is to add validation to these methods when you try to set a value. Lets say your going to set a value for firstName, the i should pass thrue a regexp to actuelly be set, otherwise an exception should be thrown. Is this possible to build with this "short syntax" or should I go for standard (like in JAVA) getters and setters and in there validate the data?

Answer 1

If you want to validate when the property is set, you need to use non-auto properties (i.e., manually defined get and set methods).

But another way to validate is to have the validation logic separate from the domain object.

class Customer {
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Organization { get; set; }
}

interface IValidator<T> {
    bool Validate(T t);
}

class CustomerValidator : IValidator<Customer> {
    public bool Validate(Customer t) {
        // validation logic
    }
}

Then, you could say:

Customer customer = // populate customer
var validator = new CustomerValidator();
if(!validator.Validate(customer)) {
    // head splode
}

This is the approach I prefer:

1. A Customer should not responsible for validating its own data, that is another responsibility and therefore should live elsewhere.
2. Different situations call for different validation logic for the same domain object.

Answer 2

What you have now are called "auto-properties," and only perform a simple "get/set". In order to customize the behavior of the get or set, you will need to convert the properties to field-backed properties:

private string _firstName;
public string FirstName 
{ 
    get {return _firstName;} 
    set 
    {
       Validate(value); _firstName = value;
    }
}

Note that I changed String to string and capitalized the property name, in following accepted C# naming best practices.

Answer 3

It's best practice to apply SRP. Set the validation in a separate class.

You can use FluentValidation

       Install-Package FluentValidation

You would define a set of validation rules for Customer class by inheriting from AbstractValidator<Customer>:

Example:

  public class CustomerValidator : AbstractValidator<Customer> {
    public CustomerValidator() {
      RuleFor(x => x.Surname).NotEmpty();
      RuleFor(x => x.Forename).NotEmpty().WithMessage("Please specify a first name");
      RuleFor(x => x.Discount).NotEqual(0).When(x => x.HasDiscount);
      RuleFor(x => x.Address).Length(20, 250);
      RuleFor(x => x.Postcode).Must(BeAValidPostcode).WithMessage("Please specify a valid postcode");
    }

    private bool BeAValidPostcode(string postcode) {
      // custom postcode validating logic goes here
    }
  }

To run the validator, instantiate the validator object and call the Validate method, passing in the object to validate.

   Customer customer = new Customer();
   CustomerValidator validator = new CustomerValidator();

   ValidationResult result = validator.Validate(customer);

 if(! results.IsValid) {
   foreach(var failure in results.Errors) {
       Console.WriteLine("Property " + failure.PropertyName + " failed validation. Error was: " + failure.ErrorMessage);

} }