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?

Michael Haddad · Accepted Answer · 2016-05-13 08:52:45Z

31

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:

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

edited May 13, 2016 at 8:52

Michael Haddad

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answered May 25, 2011 at 15:55

jason

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10 Comments

Blindy Over a year ago

I don't agree, even if your validation fails, your customer data is invalid. What's to stop it from writing garbage to the database if you mess up external validation?

jason Over a year ago

@Blindy: Your persistence layer should be wired up with validators that don't persist if the validation fails.

Phil Over a year ago

@Blindy Customer shouldn't be responsible for writing to the database. That would be another responsibility that Customer shouldn't have.

Prisoner ZERO Over a year ago

I understand the "Single Responsibility Principle" (SRP) but I always wonder where the 'cut-off' should be. Meaning, you could take it too far...like creating a class to set properties (for you). I'm not saying anything about your example...I just seem to find myself asking 'How far should I go in applying SRP'? ...maybe I should ask this as a question?

StriplingWarrior Over a year ago

@Prisoner ZERO: I think that's the good basis for another question, but you'll need to come up with a specific example to make it better defined.

|

StriplingWarrior · Accepted Answer · 2014-01-23 16:17:04Z

16

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.

edited Jan 23, 2014 at 16:17

answered May 25, 2011 at 15:54

StriplingWarrior

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Comments

Prisoner ZERO · Accepted Answer · 2011-05-25 15:55:38Z

2

I wouldn't add validation in the setter at all. Rather, I would create a function called validate instead...that way all your validation code is in one spot rather scattered throughout your setters.

answered May 25, 2011 at 15:55

Prisoner ZERO

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Comments

M.Hassan · Accepted Answer · 2019-08-19 20:52:20Z

2

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);

} }

answered Aug 19, 2019 at 20:52

M.Hassan

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2 Comments

NoChance Over a year ago

Where do the message resulting from validation appear?

M.Hassan Over a year ago

Message can be part of the rule,e.g, RuleFor(x => x.Forename).NotEmpty().WithMessage("Please specify a first name");

Blindy · Accepted Answer · 2011-05-25 15:54:28Z

0

You have to use the full property syntax for this.

answered May 25, 2011 at 15:54

Blindy

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