Error message vs. throwing exception C# ASP.Net

Question

In many cases in web applications you would need to return an error message, rather than simple true/false result. One would use exceptions for that, but I consider exceptions to be an indication of, you know, exceptional behavior. Let's take a Register() function for a class User for instance. If it was successful we can simply return true, but if something went wrong we would like to know what exactly: "passwords don't match", "e-mail is in invalid format" and so on (could be an error code instead of a message, doesn't matter).

The question is what is the best practice for returning such error messages in C# and .Net? There might be a struct ready, something like:

public struct Result {
    public bool OK;
    public string Message;
}

Or perhaps I should just use a parameter in the function? Like Register(out string Message).

Update. This pretty much describes everything I need: http://blogs.msdn.com/kcwalina/archive/2005/03/16/396787.aspx

Answer 1

I think exceptions can and should be used here.

You can put your Register() call in a try/catch block therefore preventing application execution stop. In the catch block you will analyze what exactly got wrong and return an appropriate message.

Answer 2

If it's for validation purposes I would recommend you the excellent Fluent Validation library.

Quote from the site:

using FluentValidation;

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

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

Customer customer = new Customer();
CustomerValidator validator = new CustomerValidator();
ValidationResult results = validator.Validate(customer);

bool validationSucceeded = results.IsValid;
IList<ValidationFailure> failures = results.Errors;

Answer 3

Throwing exceptions creates CPU utilization while adding all call stack and setting other fields of exception class.

I prefer not to throw Exceptions. If it is really casual error like say Argument is null,i would go for struct with an ExceptionMessage field with it.

But it is better to type-value check before calling a method and expecting primitive types to return from the method since you need a boolean value return to.