将多个EF上下文注册到DI容器中

Question

I'm starting to develop an application using Domain Driven Design concepts and applying n-layer patterns for the architecture. My problem is related to this question : because I need to create one or more database contexts.

Instead of to instantiate my contexts using the new keyword, I'm using Ninject to create a DI container and resolve all dependencies automatically, but here starts the confusion!

This is basically what I have here:

public interface IDataContext : IDisposable 
{ 
}

public abstract class DataContext : DbContext, IDataContext 
{
}

public class ContextA : DataContext
{
}

public class ContextB : DataContext
{
}

The first question is how to register the contexts, when they are implementing the same interface?

And the second question is how should to call the repositories, eg:

public class MyClass(IUserRepository userRepository, IBankRepository bankRepository)
{
  // IUserRepository is inside ContextA
  // IBankRepository is inside ContextB
}

In the sample code above, I'd like to use the same UnitOfWork. Is that possible?

Answer 1

The first question is how to register the contexts, when they are implementing the same interface?

This is where your problem is. According to the Liskov Substitution Principle implementations of an abstraction should be interchangable without causing the consumer to break. Since each DbContext will have its own model, a consumer will typically expect one particular data model and will break (at runtime) when you supply it with a different model.

And this violation directly causes you trouble with registration. So the solution is to give each context its own abstraction and register those abstractions separately.

Answer 2

If you have users in ContextA and banks in ContextB you should reflect it in your interfaces so the repository is constructed with the context containing the expected entities :

public interface IDataContext : IDisposable 
{ 
}

public interface IContextA : IDataContext
{
   public DbSet<User> Users { get; set; } 
}

public interface IContextB : IDataContext
{
   public DbSet<Bank> Banks { get; set; } 
}

public abstract class DataContext : DbContext, IDataContext 
{
}

public class ContextA : DataContext, IContextA
{
}

public class ContextB : DataContext, IContextB
{
}