Validating complex class in MVC - model-view-controller

Question

Consider the following code:
public class AccountNumber
{
[AccountNumber] //This validator confirms the format of the account number
public string Value {get; set;}
public int Format { get; set;}
public string ToString()
{
return Value + " is format " + Format;
}
}
public class MyViewModel
{
public MyViewModel()
{
SourceAccount = new AccountNumber();
DestinationAccount= new AccountNumber();
}
[Required]
AccountNumber SourceAccount {get; set;}
AccountNumber DestinationAccount {get; set;}
}
And then, in my View:
#Html.EditorFor(model => model.SourceAccount.Value)
#Html.EditorFor(model => model.DestinationAccount.Value)
Basically, I want to say that the user must enter a Source Account, and that they optionally enter a Destination Account. However, if they do enter a Destination Account it must conform to a certain format.
The problem with the code above is that the required validator on the SourceAccount will always return valid, as SourceAccount is never null. What would be a good approach for implementing what I am trying to achieve?
Please note that in real-life the setter for Value is more complex than shown, as it reformats the account number in a canonical format.
Edit Please note that we have to use inbuilt MVC validation, as that is what the rest of the project is currently using.

Answer 1

See Extending the Model Binder for Enhanced Validation.
This is fully compatible with built-in MVC validation.
You can - of course - customize this solution by using your own interface for validation.

Answer 2

A simple approach could be to add simple string properties for the SourceAccount and DestinationAccount numbers as follows:
public class MyViewModel
{
public MyViewModel()
{
}
[Required]
[AccountNumber]
public string SourceAccountNumber { get; set; }
[AccountNumber]
public string DestinationAccountNumber { get; set; }
public AccountNumber SourceAccount
{
get
{
return new AccountNumber
{
Value = SourceAccountNumber,
Format = 0 // Set Format appropriately
};
}
}
public AccountNumber DestinationAccount
{
get
{
return new AccountNumber
{
Value = DestinationAccountNumber,
Format = 0 // Set Format appropriately
};
}
}
}

Answer 3

Maybe you'd like to try FluentValidation, it's a model validation alternative to data annotation attributes, which allows you to add more complex model validation logic.
The code is still pretty concise and straightforward:
[Validator(typeof(PersonValidator))]
public class Person
{
public int Id { get; set; }
public string Name { get; set; }
public string Email { get; set; }
public int Age { get; set; }
}
public class PersonValidator : AbstractValidator<Person>
{
public PersonValidator()
{
RuleFor(x => x.Id).NotNull();
RuleFor(x => x.Name).Length(0, 10);
RuleFor(x => x.Email).EmailAddress();
RuleFor(x => x.Age).InclusiveBetween(18, 60);
}
}