How do I correctly use Unity to pass a ConnectionS

Question

I've literally just started using the Unity Application Blocks Dependency Injection library from Microsoft, and I've come unstuck.

This is my IoC class that'll handle the instantiation of my concrete classes to their interface types (so I don't have to keep called Resolve on the IoC container each time I want a repository in my controller):

public class IoC
{
    public static void Intialise(UnityConfigurationSection section, string connectionString)
    {
        _connectionString = connectionString;
        _container = new UnityContainer();
        section.Configure(_container);
    }

    private static IUnityContainer _container;
    private static string _connectionString;

    public static IMovementRepository MovementRepository
    {
        get { return _container.Resolve<IMovementRepository>(); }
    }
}

So, the idea is that from my Controller, I can just do the following:

_repository = IoC.MovementRepository;

I am currently getting the error:

Exception is: InvalidOperationException - The type String cannot be constructed. You must configure the container to supply this value.

Now, I'm assuming this is because my mapped concrete implementation requires a single string parameter for its constructor. The concrete class is as follows:

public sealed class MovementRepository : Repository, IMovementRepository
{
    public MovementRepository(string connectionString) : base(connectionString) { }
}

Which inherits from:

public abstract class Repository
{
    public Repository(string connectionString)
    {
        _connectionString = connectionString;
    }

    public virtual string ConnectionString
    {
        get { return _connectionString; }
    }
    private readonly string _connectionString;
}

Now, am I doing this the correct way? Should I not have a constructor in my concrete implementation of a loosely coupled type? I.e. should I remove the constructor and just make the ConnectionString property a Get/Set so I can do the following:

public static IMovementRepository MovementRepository
{
   get
   {
      return _container.Resolve<IMovementRepository>(
         new ParameterOverrides
         {
            { 
               "ConnectionString", _connectionString 
            }
         }.OnType<IMovementRepository>() );
   }
}

So, I basically wish to know how to get my connection string to my concrete type in the correct way that matches the IoC rules and keeps my Controller and concrete repositories loosely coupled so I can easily change the DataSource at a later date.

EDIT 09:52:

Just to re-iterate what I'm after. I want to know the correct way to pass the ConnectionString or an IRepositoryConfiguration object (prefer that idea, thanks Mc) to a concrete class from Unity. I'm not too fussed on what I pass, just how I pass it whilst maintaining loose coupling.

Answer 1

I didnt use Unity but I have used configuration objects in these situations. For example you can write your code like this

class Configuration:IRepositoryConfiguration,IMailConfiguration
{
    private string connectionString;

    //IRepository configurations
    public string ConnectionString
    {
        //read connection string from somewhere
        get { return connectionString; }
    }

    //EMail configurations
    public string Smtp
    {
        get { return smpt; }
    }

}

interface IRepositoryConfiguration
{
    string ConnectionString { get;}
}



public abstract class Repository
    {
        public Repository(IRepositoryConfiguration configuration)
        {
            _connectionString = configuration.ConnectionString;
        }

        public virtual string ConnectionString
        {
            get { return _connectionString; }
        }
        private readonly string _connectionString;
    }

So you can register IRepositoryConfiguration and Unity will resolve your configuration objects. Also you can add extra parameters in this approach easily.

Update

I think it is ok to have a constructor that accepts IRepositoryConfiguration object in your concrete classes(abstract repository and MovementRepository). Becase they are implementations details and concrete implementations of IMovementRepository.so they need to know connection string.

Setter or Constructor Injection

I prefer constructor injection over setter injection. I think constructor injection leads more discoverable APIs. in Constructor injection as soon as when you want to instantiate object, you see what an object needs to work but in Setter injection you must learn which property to set to use API. For detailed information you can read Constructor ve Setter Injection

Answer 2

You can configure the unity container for this:

IUnityContainer container = new UnityContainer()
  .RegisterType<IMovementRepository, MovementRepository>(
    new InjectionConstructor("connectionstring goes here"));

in XLM that would probably be something like this:

<type type="foo.IMovementRepository,foo" mapTo="foo.MovementRepository,foo">
  <typeConfig extensionType="Microsoft.Practices.Unity.Configuration.TypeInjectionElement, Microsoft.Practices.Unity.Configuration">     
   <constructor>
     <param name="connectionString" parameterType="System.String" >
       <value value="conectionstring goes here" type="System.String"/>
     </param>           
   </constructor>
  </typeConfig>
</type>

or wrap the connectionstring as mcaaltuntas points out.

Answer 3

I ll add another way :)

You can use Injection parameters that get passed into the constructor when you register a type. This way you can still use constructor injection.

class Repository : IRepository {
  readonly string m_ConnectionString;
  public Repository(string connectionString) {
    ConnectionString = connectionString;
  }
}

//When registering
container.RegisterType<IRepository, Repository>(new InjectionConstructor("connectionstring"));

Answer 4

Probably the most straight forward way to do this is to set up a constructor section for your mapped type in the unity configuration and inside the constructor section have a parameter element for the connection string that passes in a name value for a connection string you have defined in the connectionStrings section of your web configuration.

Inside your constructor code for the Repository class, have some code that uses the name value of the connection string to get the full connection string from the connectionStrings section.

EDIT:

Here's an example for you using Unity 2.0

In your web.config, specify the connection string and a mapping for unity to map an IRepository<T> to a SqlRepository<T>. Based on your other questions, we'll assume that IRepository<T> is in your model project and SqlRepository<T> is in your DAL project.

<?xml version="1.0"?>
<configuration>
    <configSections>
        <section name="unity" type="Microsoft.Practices.Unity.Configuration.UnityConfigurationSection, Microsoft.Practices.Unity.Configuration" />
    </configSections>
    <connectionStrings>
        <add name="SqlConnection" connectionString="data source=(local)\SQLEXPRESS;Integrated Security= SSPI; Initial Catalog= DatabaseName;" providerName="System.Data.SqlClient"/>
    </connectionStrings>
    <unity>
        <containers>
            <container>
                <types>
                    <type type="ModelProject.IRepository`1, ModelProject" mapTo="DALProject.SqlRepository`1, DALProject">
                        <constructor>
                            <param name="connectionString">
                                <value value="SqlConnection" />
                            </param>
                        </constructor>
                    </type>
                </types>
            </container>
        </containers>
  </unity>
</configuration>

Now for the IRepository<T> interface in the model project. In this example, I'm also going to be using LINQ to SQL to return objects from the SQL Database

namespace ModelProject
{
    /// <summary>
    /// Interface implemented by a Repository to return
    /// <see cref="IQueryable`T"/> collections of objects
    /// </summary>
    /// <typeparam name="T">Object type to return</typeparam>
    public interface IRepository<T>
    {
        IQueryable<T> Items { get; }
    }
}

And the SQLRepository<T> class in the DAL project

namespace DALProject
{
    /// <summary>
    /// Generic class for returning an <see cref="IQueryable`T"/>
    /// collection of types
    /// </summary>
    /// <typeparam name="T">object type</typeparam>
    public class SqlRepository<T> : IRepository<T> where T : class
    {
        private Table<T> _table;

        public SqlRepository(string connectionString)
        {
            // use the connectionString argument value to get the
            // connection string from the <connectionStrings> section
            // in web.config
            string connection = ConfigurationManager.ConnectionStrings[connectionString].ConnectionString;

            _table = (new DataContext(connection)).GetTable<T>();
        }

        /// <summary>
        /// Gets an <see cref="IQueryable`T"/> collection of objects
        /// </summary>
        public IQueryable<T> Items
        {
            get { return _table; }
        }
    }
}

Let's also use a custom controller factory to allow unity to return controllers for us. This way, unity will inject any dependencies that the controllers have

In global.asax

namespace WebApplicationProject
{
    public class MvcApplication : System.Web.HttpApplication
    {
        public static void RegisterRoutes(RouteCollection routes)
        {
            // your routes
        }

        protected void Application_Start()
        {
            RegisterRoutes(RouteTable.Routes);
            ControllerBuilder.Current.SetControllerFactory(new UnityControllerFactory());
        }
    }

    public class UnityControllerFactory : DefaultControllerFactory
    {
        private IUnityContainer _container;

        public UnityControllerFactory()
        {
            _container = new UnityContainer();

            var controllerTypes = from t in Assembly.GetExecutingAssembly().GetTypes()
                                  where typeof(IController).IsAssignableFrom(t)
                                  select t;

            foreach (Type t in controllerTypes)
                _container.RegisterType(t, t.FullName);

            UnityConfigurationSection section = (UnityConfigurationSection)ConfigurationManager.GetSection("unity");

            section.Configure(_container);
        }

        protected override IController GetControllerInstance(RequestContext requestContext, Type controllerType)
        {
            // see http://stackoverflow.com/questions/1357485/asp-net-mvc2-preview-1-are-there-any-breaking-changes/1601706#1601706
            if (controllerType == null) { return null; }

            return (IController)_container.Resolve(controllerType);
        }
    }

}

And Here's a controller example. PageSize might be defined on a base controller or on the controller as a property.

namespace WebApplicationProject.Controllers
{
    public class CustomersController : Controller
    {
        private IRepository<Customer> _customerRepository;
        public int PageSize { get; set; }

        public CustomersController() { }

        public CustomersController(IRepository<Customer> customerRepository)
        {
            this._customerRepository = customerRepository;
            // let's set it to 10 items per page.
            this.PageSize = 10; 
        }

        public ViewResult List(string customerType, int page)
        {
            var customerByType = (customerType == null) ?
                customerRepository.Items : customerRepository.Items.Where(x => x.CustomerType == customerType);

            int totalCustomers = customerByType.Count();
            ViewData["TotalPages"] = (int)Math.Ceiling((double)totalCustomers/ PageSize);
            ViewData["CurrentPage"] = page;
            ViewData["CustomerType"] = customerType;

            // get the right customers from the collection
            // based on page number and customer type.    
            return View(customerByType
                .Skip((page - 1) * PageSize)
                .Take(PageSize)
                .ToList()
            );
        }

    }
}

When the customers list controller action is invoked, unity will correctly instantiate an instance of SqlRepository<Customer> for the controller and inject this into the constructor. the connectionString string used for SqlRepository<T> is set in the unity configuration and is passed into the constructor for a typed SqlRepository<T>.