Builder pattern and dependency injection

Question

I'm working with the unit of work pattern to glue together my application logic. To create such a unit of work object I use the builder pattern.

But my unit of work classes need access to the service collection of the dependency injection container but as the builder pattern requires you to know how to new the object this won't work. That's why I wrote the Build<T> method on MyUnitOfWork which uses reflection and the service container to get the parameters to construct the unit of work.

I know that I could remove MyUnitOfWorkConfiguration from the unit of work constructors but in the code I'm using this MyUnitOfWorkConfiguration class is quite big and the change would mean I need to duplicate the properties into the builder and the unit of work class.

public class MyUnitOfWorkConfiguration
{
    public bool RunSynchronously { get; set; }
}
public class SpecialUnitOfWorkConfiguration : MyUnitOfWorkConfiguration
{
    public bool IgnoreSpecialError { get; set; }
}

public abstract class MyUnitOfWork<TConfiguration> where TConfiguration : MyUnitOfWorkConfiguration
{
    protected readonly TConfiguration Configuration;

    protected MyUnitOfWork(IMyDependencyIncectedService myDependencyIncectedService, TConfiguration configuration)
    {
        // do something with the myDependencyIncectedService
        Configuration = configuration;
    }

    protected void ExecuteInner(Action action)
    {
        if (Configuration.RunSynchronously)
            action();
        else
            Task.Run(action);
    }
}
public class LoginUnitOfWork : MyUnitOfWork<MyUnitOfWorkConfiguration>
{
    public LoginUnitOfWork(IMyOtherDependencyIncectedService myOtherDependencyIncectedService, IMyDependencyIncectedService myDependencyIncectedService, MyUnitOfWorkConfiguration configuration)
        : base(myDependencyIncectedService, configuration)
    {
        // do something with the myOtherDependencyIncectedService
    }

    public void Login(string username, string password)
    {
        ExecuteInner(() =>
        {
            // do the login
        });
    }
}
public class SpecialUnitOfWork : MyUnitOfWork<SpecialUnitOfWorkConfiguration>
{
    public SpecialUnitOfWork(IMyDependencyIncectedService myDependencyIncectedService, SpecialUnitOfWorkConfiguration configuration) 
        : base(myDependencyIncectedService, configuration) { }

    public void Execute()
    {
        ExecuteInner(() =>
        {
            try { /* do special action */ }
            catch (SpecialException)
            {
                if (!Configuration.IgnoreSpecialError) throw;
            }
        });
    }
}

public abstract class MyUnitOfWorkBuilder<TConfiguration> where TConfiguration : MyUnitOfWorkConfiguration, new()
{
    protected readonly TConfiguration Configuration = new TConfiguration();

    public MyUnitOfWorkBuilder<TConfiguration> RunSynchronously()
    {
        Configuration.RunSynchronously = true;
        return this;
    }

    public T Build<T>() where T : MyUnitOfWork<TConfiguration>
    {
        var constructor = typeof(T).GetConstructors().First();
        var parameters = constructor.GetParameters();
        var parameterValues =
            parameters.Select(
                parameter =>
                    parameter.ParameterType.IsAssignableFrom(typeof(MyUnitOfWorkConfiguration))
                        ? Configuration
                        : ServiceLocator.Current.GetInstance(parameter.ParameterType));

        return (T)constructor.Invoke(parameterValues.ToArray());
    }
}
public class MyUnitOfWorkBuilder : MyUnitOfWorkBuilder<MyUnitOfWorkConfiguration> { }
public class SpecialUnitOfWorkBuilder : MyUnitOfWorkBuilder<SpecialUnitOfWorkConfiguration>
{
    public SpecialUnitOfWorkBuilder IgnoreSpecialError()
    {
        Configuration.IgnoreSpecialError = true;
        return this;
    }
}

var loginUnitOfWork = new MyUnitOfWorkBuilder()
    .RunSynchronously()
    .Build<LoginUnitOfWork>();
loginUnitOfWork.Login("myUserName", "myPassword");

var specialUnitOfWork = new SpecialUnitOfWorkBuilder()
   .IgnoreSpecialError()
   .RunSynchronously()
   .Build<SpecialUnitOfWork>();
specialUnitOfWork.Execute();

Examples for such a unit of work are:

• ExitApplicationUnitOfWork (logs out users, cleans up, asks if the user is sure etc.)
• LoginUserUnitOfWork (logs the user in)
• LogoutUserUnitOfWork (logs the user out)
• BackupUnitOfWork (backs up stuff)

At the moment there are about 8 of those unit of work types and after the implementation of the new feature set there will be around 15.

My questions are

• Is what I do bad practice?
• Do I follow the rules of the builder, unit of work and dependency injection pattern? Or is what I did here something strange I came up with?^^
• Could I solve this better?

P.S.: If you have any other inputs to my code please just let me know.

Answer 1

As t3chb0t mentioned in a comment, the code is hard to review because it contains only the the structural code without any functional stuff.

Maybe that is the reason that it looks a little bit over-engineered to me. I'll try it anyway.. however probably the review does not fit to your real code ;)

In my eyes, the builder has no functionality but providing a nice fluent API for initializing the configuration object and creating the UnitOfWork object. The second part requires the access to a service locator (with has actually nothing to do with dependency injection).

So, first of all I would drop the builder and create the configuration object manually (alternatively you could use the builder only for building the the configuration object and pass it to the unit of work in a separate step):

var config = new ConfigurationObjectA();
config.PropertyA = "Value";
...

var unitOfWork = new UnitOfWorkA(config, dependenyA, ....);

However, there is still the problem with the dependencies that must be injected manually. Depending on the DI Framework, you can use a factory for creating the UnitOfWorks with all it's dependencies. Ninject for example has an excellent Extensions providing exactly that by creating a simple interface:

public interface IUnitOfWorkFactory
{
    UnitOfWorkA CreateUnitOfWorkA(ConfigurationA config);
    UnitOfWorkB CreateUnitOfWorkB(ConfigurationB config);
    ....
}

Just register the interface as factory and you can use it for creating the unit of work objects. If the unit of work has additional constructor dependencies (which have to be registered in the DI container of course), Ninject injects these dependencies automatically.

That solution looks a) simpler with b) less code and c) without using a service locator ;)

Answer 2

I am unable to leave comments due to the amount of reputation I have, so I have to resort to leaving a post.

Is what I do bad practice?

Yes. Service location is a well established anti-pattern. If you have to resort to using service location there is something wrong with the underlying design of your application.