Where's the best place for validation... constructor or leave to client to call?

Question

This is really a generic (and probably a more subjective too) question. I have some classes where I use an interface to define a standard approach to validating the object state. When I did this, I got to scratching my head... is it best to 1.) allow the constructor (or initializing method) to silently filter out the errant information automatically or... 2.) allow the client to instantiate the object however and let the client also call the interface's IsValid property or Validate() method before moving forward?

Basically one approach is silent but could be misleading in that the client may not be aware that certain pieces of information were filtered away due to it not meeting the validation criteria. The other approach then would be more straight forward, but also adds a step or two? What's typical here?

Okay, after a long day of trying to keep up with some other things, I finally did come up with an example. Please for me for it as it's not ideal and by no means something wonderful, but hopefully should serve well enough to get the point across. My current project is just too complicated to put something simple out for this, so I made something up... and trust me... totally made up.

Alright, the objects in the example are this:

Client: representing client-side code (Console App btw)

IValidationInfo: This is the actual interface I'm using in my current project. It allows me to create a validation framework for the "back-end" objects not necessarily intended for the Client to use since the business logic could be complicated enough. This also allowed me to separate validation code and call as-needed for the business logic.

OrderManager: This is an object the client-side code can use to manage their orders. It's client-friendly so-to-speak.

OrderSpecification: This is an object the client-side code can use to request an order. But if the business logic doesn't work out, an exception can be raised (or if necessary the order not added and exceptions ignored...) In my real-world example I actually have an object that's not quite so black-and-white as to which side of this fence it goes... thus my original question when I realized I could push validation request (calling IsValid or Validate()) to the cilent.

CustomerDescription: represents customers to which I've classified (pretending to have been read from a DB.

Product: Represents a particular product which is classified also.

OrderDescription: Represents the official order request.The business rule is that the Customer cannot order anything to which they've not been classified (I know.. that's not very real-world, but it gave me something to work with...)

Ok... I just realized I can't attach a file here, so here's the code. I apologize for it's lengthy appearance. That was the best I could do to create a client-friendly front-end and business logic back-end using my Validation interface:

public class Client { static OrderManager orderMgr = new OrderManager();

    static void Main(string[] args)
    {
        //Request a new order
        //Note:  Only the OrderManager and OrderSpecification are used by the Client as to keep the 
        //       Client from having to know and understand the framework beyond that point.
        OrderSpecification orderSpec = new OrderSpecification("Customer1", new Product(IndustryCategory.FoodServices, "Vending Items"));
        orderMgr.SubmitOrderRequest(orderSpec);
        Console.WriteLine("The OrderManager has {0} items for {1} customers.", orderMgr.ProductCount, orderMgr.CustomerCount);

        //Now add a second item proving that the business logic to add for an existing customer works
        Console.WriteLine("Adding another valid item for the same customer.");
        orderSpec = new OrderSpecification("Customer1", new Product(IndustryCategory.FoodServices, "Sodas"));
        orderMgr.SubmitOrderRequest(orderSpec);
        Console.WriteLine("The OrderManager now has {0} items for {1} customers.", orderMgr.ProductCount, orderMgr.CustomerCount);

        Console.WriteLine("Adding a new valid order for a new customer.");
        orderSpec = new OrderSpecification("Customer2", new Product(IndustryCategory.Residential, "Magazines"));
        orderMgr.SubmitOrderRequest(orderSpec);
        Console.WriteLine("The OrderManager now has {0} items for {1} customers.", orderMgr.ProductCount, orderMgr.CustomerCount);


        Console.WriteLine("Adding a invalid one will not work because the customer is not set up to receive these kinds of items.  Should get an exception with message...");
        try
        {
            orderSpec = new OrderSpecification("Customer3", new Product(IndustryCategory.Residential, "Magazines"));
            orderMgr.SubmitOrderRequest(orderSpec);
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.Message);
        }

        Console.ReadLine();
    }
}

public interface IValidationInfo
{
    string[] ValidationItems { get; }

    bool IsValid { get; }

    void Validate();

    List<string> GetValidationErrors();

    string GetValidationError(string itemName);
}

public class OrderManager
{
    private List<OrderDescription> _orders = new List<OrderDescription>();
    public List<OrderDescription> Orders
    {
        get { return new List<OrderDescription>(_orders); }
        private set { _orders = value; }
    }

    public int ProductCount
    {
        get
        {
            int itemCount = 0;
            this.Orders.ForEach(o => itemCount += o.Products.Count);
            return itemCount;
        }
    }

    public int CustomerCount
    {
        get
        {
            //since there's only one customer per order, just return the number of orders
            return this.Orders.Count;
        }
    }

    public void SubmitOrderRequest(OrderSpecification orderSpec)
    {
        if (orderSpec.IsValid)
        {
            List<OrderDescription> orders = this.Orders;

            //Since the particular customer may already have an order, we might as well add to an existing
            OrderDescription existingOrder = orders.FirstOrDefault(o => string.Compare(orderSpec.Order.Customer.Name, o.Customer.Name, true) == 0) as OrderDescription;
            if (existingOrder != null)
            {
                List<Product> existingProducts = orderSpec.Order.Products;
                orderSpec.Order.Products.ForEach(p => existingOrder.AddProduct(p));
            }
            else
            {
                orders.Add(orderSpec.Order);
            }
            this.Orders = orders;
        }
        else
            orderSpec.Validate(); //Let the OrderSpecification pass the business logic validation down the chain
    }
}


public enum IndustryCategory
{
    Residential,
    Textile,
    FoodServices,
    Something
}


public class OrderSpecification : IValidationInfo
{
    public OrderDescription Order { get; private set; }


    public OrderSpecification(string customerName, Product product)
    {
        //Should use a method in the class to search and retrieve Customer... pretending here
        CustomerDescription customer = null;
        switch (customerName)
        {
            case "Customer1":
                customer = new CustomerDescription() { Name = customerName, Category = IndustryCategory.FoodServices };
                break;
            case "Customer2":
                customer = new CustomerDescription() { Name = customerName, Category = IndustryCategory.Residential };
                break;
            case "Customer3":
                customer = new CustomerDescription() { Name = customerName, Category = IndustryCategory.Textile };
                break;
        }


        //Create an OrderDescription to potentially represent the order... valid or not since this is
        //a specification being used to request the order
        this.Order = new OrderDescription(new List<Product>() { product }, customer);

    }

    #region IValidationInfo Members
    private readonly string[] _validationItems =
    {
        "OrderDescription"
    };
    public string[] ValidationItems
    {
        get { return _validationItems; }
    }

    public bool IsValid
    {
        get
        {
            List<string> validationErrors = GetValidationErrors();
            if (validationErrors != null && validationErrors.Count > 0)
                return false;
            else
                return true;
        }
    }

    public void Validate()
    {
        List<string> errorMessages = GetValidationErrors();
        if (errorMessages != null && errorMessages.Count > 0)
        {
            StringBuilder errorMessageReported = new StringBuilder();
            errorMessages.ForEach(em => errorMessageReported.AppendLine(em));
            throw new Exception(errorMessageReported.ToString());
        }
    }

    public List<string> GetValidationErrors()
    {
        List<string> errorMessages = new List<string>();
        foreach (string item in this.ValidationItems)
        {
            string errorMessage = GetValidationError(item);
            if (!string.IsNullOrEmpty(errorMessage))
                errorMessages.Add(errorMessage);
        }

        return errorMessages;
    }

    public string GetValidationError(string itemName)
    {
        switch (itemName)
        {
            case "OrderDescription":
                return ValidateOrderDescription();
            default:
                return "Invalid item name.";
        }
    }

    #endregion

    private string ValidateOrderDescription()
    {
        string errorMessage = string.Empty;

        if (this.Order == null)
            errorMessage = "Order was not instantiated.";
        else
        {
            if (!this.Order.IsValid)
            {
                List<string> orderErrors = this.Order.GetValidationErrors();
                orderErrors.ForEach(ce => errorMessage += "\n" + ce);
            }
        }

        return errorMessage;
    }

}

public class CustomerDescription : IValidationInfo
{
    public string Name { get; set; }
    public string Street { get; set; }
    public string City { get; set; }
    public string State { get; set; }
    public int ZipCode { get; set; }
    public IndustryCategory Category { get; set; }

    #region IValidationInfo Members
    private readonly string[] _validationItems =
    {
        "Name",
        "Street",
        "City",
        "State",
        "ZipCode",
        "Category"
    };
    public string[] ValidationItems
    {
        get { return _validationItems; }
    }

    public bool IsValid
    {
        get
        {
            List<string> validationErrors = GetValidationErrors();
            if (validationErrors != null && validationErrors.Count > 0)
                return false;
            else
                return true;
        }
    }

    public void Validate()
    {
        List<string> errorMessages = GetValidationErrors();
        if (errorMessages != null && errorMessages.Count > 0)
        {
            StringBuilder errorMessageReported = new StringBuilder();
            errorMessages.ForEach(em => errorMessageReported.AppendLine(em));
            throw new Exception(errorMessageReported.ToString());
        }
    }

    public List<string> GetValidationErrors()
    {
        List<string> errorMessages = new List<string>();
        foreach (string item in this.ValidationItems)
        {
            string errorMessage = GetValidationError(item);
            if (!string.IsNullOrEmpty(errorMessage))
                errorMessages.Add(errorMessage);
        }

        return errorMessages;
    }

    public string GetValidationError(string itemName)
    {
        //Validation methods should be called here... pretending nothings wrong for sake of discussion & simplicity
        switch (itemName)
        {
            case "Name":
                return string.Empty;
            case "Street":
                return string.Empty;
            case "City":
                return string.Empty;
            case "State":
                return string.Empty;
            case "ZipCode":
                return string.Empty;
            case "Category":
                return string.Empty;
            default:
                return "Invalid item name.";
        }
    }

    #endregion
}


public class Product
{
    public IndustryCategory Category { get; private set; }
    public string Description { get; private set; }

    public Product(IndustryCategory category, string description)
    {
        this.Category = category;
        this.Description = description;
    }
}







public class OrderDescription : IValidationInfo
{
    public CustomerDescription Customer { get; private set; }

    private List<Product> _products = new List<Product>();
    public List<Product> Products
    {
        get { return new List<Product>(_products); }
        private set { _products = value; }
    }

    public OrderDescription(List<Product> products, CustomerDescription customer)
    {
        this.Products = products;
        this.Customer = customer;
    }

    public void PlaceOrder()
    {
        //If order valid, place
        if (this.IsValid)
        {
            //Do stuff to place order
        }
        else
            Validate(); //cause the exceptions to be raised with the validate because business rules were broken
    }

    public void AddProduct(Product product)
    {
        List<Product> productsToEvaluate = this.Products;
        //some special read, validation, quantity check, pre-existing, etc here
        // doing other stuff... 
        productsToEvaluate.Add(product);
        this.Products = productsToEvaluate;
    }

    #region IValidationInfo Members

    private readonly string[] _validationItems =
    {
        "Customer",
        "Products"
    };
    public string[] ValidationItems
    {
        get { return _validationItems; }
    }

    public bool IsValid
    {
        get
        {
            List<string> validationErrors = GetValidationErrors();
            if (validationErrors != null && validationErrors.Count > 0)
                return false;
            else
                return true;
        }
    }

    public void Validate()
    {
        List<string> errorMessages = GetValidationErrors();
        if (errorMessages != null && errorMessages.Count > 0)
        {
            StringBuilder errorMessageReported = new StringBuilder();
            errorMessages.ForEach(em => errorMessageReported.AppendLine(em));
            throw new Exception(errorMessageReported.ToString());
        }
    }

    public List<string> GetValidationErrors()
    {
        List<string> errorMessages = new List<string>();
        foreach (string item in this.ValidationItems)
        {
            string errorMessage = GetValidationError(item);
            if (!string.IsNullOrEmpty(errorMessage))
                errorMessages.Add(errorMessage);
        }

        return errorMessages;
    }

    public string GetValidationError(string itemName)
    {
        switch (itemName)
        {
            case "Customer":
                return ValidateCustomer();
            case "Products":
                return ValidateProducts();
            default:
                return "Invalid item name.";
        }
    }

    #endregion

    #region Validation Methods

    private string ValidateCustomer()
    {
        string errorMessage = string.Empty;

        if (this.Customer == null)
            errorMessage = "CustomerDescription is missing a valid value.";
        else
        {
            if (!this.Customer.IsValid)
            {
                List<string> customerErrors = this.Customer.GetValidationErrors();
                customerErrors.ForEach(ce => errorMessage += "\n" + ce);
            }
        }

        return errorMessage;
    }

    private string ValidateProducts()
    {
        string errorMessage = string.Empty;

        if (this.Products == null || this.Products.Count <= 0)
            errorMessage = "Invalid Order. Missing Products.";
        else
        {
            foreach (Product product in this.Products)
            {
                if (product.Category != Customer.Category)
                {
                    errorMessage += string.Format("\nThe Product, {0}, category does not match the required Customer category for {1}", product.Description, Customer.Name);
                }
            }
        }
        return errorMessage;
    }
    #endregion
}

Answer 1

Well if I correctly understood, there are basically two question - whether you should fail right away or later and whether you should omit/assume certain information.

1) I always prefer failing as soon as possible - good example is failing at compile time vs failing at run time - you always want to fail at compile time. So if something is wrong with the state of some object, as Jon said - throw exception right away as loudly as you can and deal with it - do not introduce additional complexity down the road as you'll be heading for if/elseif/elseif/elseif/else mumbo jumbo.

2) When it comes to user input, if you are in position to simply filter out errors automatically - just do it. For example, I almost never ask users for country - if I really need it, I automatically detect it from IP and display it in the form. It's way easier if user just needs to confirm/change the data - and I don't need to deal with null situation.

Now, in case we are talking about the data generated by code during some processing - for me situation is drastically different - I always want to know an much as possible (for easier debugging down the road) and ideally you never should destroy any piece of information.

To wrap up, in your case I would recommend that you keep IsValid as simple yes/no (not yes/no/maybe/kindaok/etc). If you can fix some problems automatically - do it, but consider that they keep object in IsValid yes. For everything else, you throw exception and go to IsValid=no.

Answer 2

It's completely depends on the client. There's a trade-off as you already mentioned. By default approach number 1 is my favorite. Creating smart classes with good encapsulation and hiding details from client. The level of smartness depends who is going to use the object. If client is business aware you can reveal details according to the level of this awareness. This is a dichotomy and should not be treated as black or white.

Answer 3

Any reason you wouldn't want the constructor to noisily throw an exception if the information is valid? It's best to avoid ever creating an object in an invalid state, in my experience.