Data Transfer Objects

Data Transfer Objects Pattern

Data Transfer Objects (DTOs) are a necessary time-sink when working with SOA.  Proper SOA dictates that you must not pass domain objects across service boundaries.  In many cases, such as when using an ORM, it is extremely difficult to pass your domain objects across the service boundaries even if you choose to disregard the SOA guidance. Also, there may be good reasons why you do not want to make your domain objects serializable.

DTOs Defined

So what exactly are DTOs?  They are objects that have no behavior outside of accessors and mutators of their properties. Often times DTOs line up one-to-one with domain objects as in the following example.

///

/// Domain Object
///

 

public class Company{
private string name;
private string taxId;
public Company(string name, string taxId) {
this.name = name;
this.taxId = taxId;
}

public string Name { get { return name; } }
public string TaxId { get { return taxId; } }}

///

/// Data Transfer Object
///

 

public class CompanyDTO{
public string Name { get; set; }
public string TaxId { get; set; }}

Other times DTOs may need to carry data for a request that has no direct representation as a single object in the domain. Consider a service that needs to expose data about a company and its owner. Obviously the clients shouldn’t have to request data about the company and then data about the owner in a separate request. Doing so would incur the costs of two round trips, and the owner data is a part of a single logical unit as exposed by the service.


///

/// A company
///

 

public class Company{
private string name;
private string taxId;

public Company(string name, string taxId)
{
this.name = name;
this.taxId = taxId;
}

public string Name { get { return name; } }
public string TaxId { get { return taxId; } }

public Owner Owner { get; set; }}

///

/// Owner of a company
///

 

public class Owner{
public Company Company { get; set; }

public string FirstName { get; set; }
public string LastName { get; set; }}

///

/// Data Transfer Object
///

 

public class CompanyDTO{
public string Name { get; set; }
public string TaxId { get; set; }

public string OwnerName { get; set; }
}

From Domain Object to DTO and Back Again

The assembler pattern, a subset of the mapping pattern, is used to translate DTOs to and from domain objects.

It is possible to create a reusable conventions based assembler. I will talk more about this in the future. For small projects, the cost of creating assemblers for the DTOs is trivial. For larger ones, it is still worth the effort for the sake of encapsulation.

///

/// Assembler for Company and CompanyDTO.
///

 

public interface ICompanyAssembler{
CompanyDTO Convert(Company company);
Company Convert(CompanyDTO DTO);}

What is so useful about an object that has no behavior?

DTOs may seem like weak objects because of their lack of behavior; but this is what makes them the one object that is safe to pass across a service boundary. A client that interacts with your service may, and often will, have a different domain from the service itself. In fact, many different clients with many different domains may interact with your service. It is difficult and undesirable to share the same domain model between a service and all its potential clients.

Consider again the Company example. Let us assume that the service we are writing is a better business bureau catalog of companies. The domain object, Company, may include such methods as “FileComplaint” or “RevokeLicense.”

This service may be consumed by an auction site that wants to use the companies names and ratings but has no need to file complaints and does not have the ability to revoke a license.  The company domain object for the auction site will include methods such as “AddAuction” or “AssociateWithReview.”

Sharing the domain objects between the auction site and the better business bureau service would pollute both domains; not to mention the fact that they may, and in this example certainly would, have different owners who do not wish to share code bases.

Sharing DTOs, because of their lack of behavior, works fine though. The DTOs serve as a simple, abstracted definition for the data that the service and its clients must exchange.

DTOs also insulate your service clients from changes to the domain of the service and vice versa. You are free to change your domain objects as much as you want and leave the DTOs alone. The assembler can simply be updated as the domain objects are updated.

If the domain objects were shared by the clients, every change to a domain object would directly impact clients, greatly increasing the costs of maintaining the service.

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MVC Forms With Validation Open-Sourced

I’ve set up a Google code repository here for the ASP.NET MVC Forms With Validation framework I discussed in this post.

I have made some alterations to it since the post that I will review in a later posting.

Briefly, the major changes are as follows:

  1. Multiple fieldsets are now supported in the forms.
  2. I have taken the first steps towards supporting objects that contain child objects.  A 1-1 relationship is supported in the current revision. A new fieldset is generated for each child object.
  3. Strategies have been revamped so that they no longer are mapped to types.  Now a strategy is allowed to return null if it does not want to handle a property descriptor passed to it. If it returns null, the next strategy in the list will be tried.

Check it out. Tell me what you think. I am accepting patches and if you want to be a contributor, contact me through the contact form on this blog.

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Forms Framework with Validation for ASP.NET MVC

Update: This project has been open-sourced.  See details in this post.

Another Update: This article was written using asp.net MVC preview 4. The better option now is to use MVCContrib’s validators or the MVCContrib fluent html project in general with IDataErrorInfo. I leave this post here as a reference. Also, if you wish to have a more Django style forms framework, this would be a great starting point. The code is freely available for anyone who wishes to use it.

One thing that is notably missing from ASP.NET MVC is a good way to handle forms and their validation.  To resolve this issue, I started on a simple forms framework this weekend.

The end goal

I don’t particularly like the action filter to handle the insertion of the model as a parameter.  I would prefer it be done via windsor and interceptors; however, for the first go round I have decided to keep the castle stack out of this. The technique could easily be adapted to use MVC Contrib’s WindsorControllerFactory and interceptors so that attributes do not have to be on every action you wish to use a form helper with. More on that later.

For those of you who want to skip the reading and get straight to the code, download the sample project. Look at the /Home/Contact page. Note: the sample project depends on MVC Preview 4.

The Components

  • Field – A field is the smallest unit of input and validation in a form.
  • Widget – A widget is an abstraction of HTML template text for input.
  • FormBase – Base class for form helpers.
  • ModelForm – A form auto created from a POCO model.

Widget

A widget has a name, a value, and some attributes. The name and value properties are by default shortcuts to the name and value attributes of the widget’s attribute collection.  However, this behavior is overridable in subclasses. A widget also has a way of rendering itself as XHTML.

Field

A field has a name, a value, and a widget. By default a field is required, and it has a publicly exposed validate method so that it can be asked to validate its value. It also has the ability to output itself as XHTML.

FormBase

FormBase, the base class for forms contains a collection of fields, a method to validate the fields, and a method to load the values of fields from a name value collection. The latter facilitates the loading of data from a browser request.

The first concrete implementation of FormBase I created was ModelForm. A ModelForm accepts a generic type argument and in its constructor takes an object of that type. It uses this object to generate fields for the form. The generation logic is implemented using the strategy pattern so that it is easily customizable.

Here is the strategy interface

The ModelForm registers default strategies

The constructor allows you to pass in your own strategies

With that, we have everything we need to make a form from a POCO.

Model Action Filter

One of the neatest things ASP.NET MVC does is allow you to make controllers with parameters that will be filled in from the request.

I wanted to be able to do this with my form classes as well.

For the first go round, I decided not to use what I would prefer: Windsor and Interceptors.  Instead I integrated the MVC way by using their action filters.

The filters give us everything we need to set the values for a parameter.  We have access to the parameters through the ActionMethod.GetParmaeters() method and we can set the parameters via the ActionParameters dictionary.

Here is the action filter

FormFactory is a simple helper class that uses a strategy pattern to create forms based upon the type passed in and a NameValueCollection.

Settings filterContext.Action

Now, this action will work!

Rendering the Form in a View

I provided three canonical methods for rendering the form:

AsDiv – renders the form with each field wrapped in a div
AsTable – render the form with each field as a table row
AsList – render the form with each field as a list item of an unordered list

I also provided a AsCustom method that allows you to specify an XElement to wrap the form fields inside of and an XElement to use as the parent for the children generated by the field instances.

If you want even more flexibility, the rendering is completely overridable by subclassing.

Rendering the form is as simple as passing it to a view and calling AsDiv() or your preferred alternative.

All output is valid XHTML.  Invalid fields receive an error class.

Rendered

Validation

Download the Sample Project
Note: the sample project depends on MVC Preview 4.

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Building NHibernate From Source

Recently, on nhusers, the NHibernate users mailing list, I’ve seen questions from people wishing to get started building NHibernate from source. Some people are unfamiliar with open source in the .NET world and the tools used for it, so I made this beginner level screencast about how to get started building NHibernate from source.

You can download the screencast here.

Here is a list of the prerequisites you will need to follow along:

For reference, you can find the official NHibernate document about getting started from source here.

Note: I realized after I posted this that I forgot to provide a link to the free codec needed to view this screencast.  You will need to d/l the free camstudio lossless codec (here) to view this video.  Extract the files, right click on the .inf, and select install.  You will receive a warning, but it’s ok.  This is a widely used screencasting codec and you can find a lot of info about it via Google.
In the future, I’m going to convert my videos to flash and stream them to make things easier all-around.
Sorry for the inconvenience.