使用Unity创建实现给定接口的类的实例时如何设置给定属性

Question

Given this simplified scenario:

public interface IRepository
{
    string ConnectionString { get; set; }
}

public abstract class Repository : IRepository
{
    public string ConnectionString { get; set; }
}

public class CustomerRepository : Repository, ICustomerRepository
{
}

public class OrderRepository : Repository, IOrderRepository
{
}

and the following Unity registrations:

IUnityContainer container = new UnityContainer();
container.RegisterType<ICustomerRepository, CustomerRepository>();
container.RegisterType<IOrderRepository, OrderRepository>();

What is the recommended way to intercept the creation of any IRepository instance in order to set the value for the ConnectionString property (and by extension any other property).

UPDATE 1: This post is not about design patters or an alternate solution involving re-factoring of the original classes. It's about Unity and the best way to set a property by intercepting the creation of instances implementing a given interface.

Answer 1

If you make the connectionString a constructor parameter the configuration looks like this

container.RegisterType<ICustomerRepository, CustomerRepository>(new InjectionConstructor("myConnStr1");

For property injection the configuration is

container.RegisterType<IOrderRepository, OrderRepository>(new InjectionProperty("ConnectionString", "myConnStr2");

---

[Update] The TecX project on codeplex contains a port of the Castle Windsor Typed Factory Facilities . It allows you to register a factory interface that takes parameters.

public interface ICustomerRepositoryFactory
{
  ICustomerRepository Create(string connectionString);
}

container.RegisterType<ICustomerRepositoryFactory>(new TypedFactory());
container.RegisterType<ICustomerRepository, CustomerRepository>(new InjectionProperty("ConnectionString");

If you resolve the factory interface and call the Create method it will map the method parameter to the according constructor parameter. [Update2] It does not yet support property injection (yet. ask again tomorrow) . It also supports property injection but you will have to mark the property for injection when you register your repository types. [/Update2] You will not have to implement the factory. The extension does that for you.

Answer 2

Since you need connection strings coming from differnt providers (as you mentioned in your comment). I would suggest a slight change to the design, you can add a new dependency such as IConnectionStringProvider

public interface IConnectionStringProvider 
{
    string GetConnectionString(); 
}

Then update IRepository

public interface IRepository  
{    
        IConnectionStringProvider ConnectionStringProvider  {get; set;}
        string ConnectionString { get; } // no set is required 
 }

public abstract class Repository : IRepository     
   {       
         IConnectionStringProvider ConnectionStringProvider  {get; set;}
         public string ConnectionString 
           { 
              get 
                 {
                    return ConnectionStringProvider.GetConnectionString(); 
                 }
            }
   }    

Then you can use unity to inject the right connection string provider per your need

container.RegisterType<IConnectionStringProvider , ConnectionStringProvider>();

Answer 3

Your repositories should not be responsible of creating the connection.

1. You are breaking Single Responsibility Principle
2. You got duplicated code = harder to maintain the code in the future.
3. What do you do if you want to share a connection between your repositories?

Solution:

Create an IConnectionFactory (which implementation you can register as an instance/singleton) and use that as an dependency in your repositories.

Update 1

You can use InjectionProperty . Lookup the connection string on the line before you configure it.

Update 2:

Use factories to configure the repositories: http://www.pnpguidance.net/post/RegisteringFactoryMethodCreateObjectsUnityStaticFactoryExtension.aspx

Something like:

container.Configure<IStaticFactoryConfiguration>()
    .RegisterFactory<ICustomerRepository>
    (new FactoryDelegate(c => 
       { 
          var repos = new CustomerRepository();
          repos.ConnectionString = yourComPlus.ConnectionString;
          return repos;
       }
    ));