Generic query handler that looks up entity by primary key

[andregizero]

I'm using your IQueryHandler pattern with good results. Command and query handlers are basically my data access layer and I would like to always use a query handler when querying the database for something. Even if it is for simply looking up an object by its primary key. I have however been unable to get rid of the use of repositories for one use case: looking up a domain object by its primary key.

I can of course create a specific class that implements IQueryHandler for each domain class I want to be able to look up by primary key, but that seems unnecessary since I can do it in a generic way using a repository.

This is the generic repository I would like to replace:

private readonly MyDbContext context;
private readonly DbSet<T> dbSet;

public EfRepository(MyDbContext dbContext)
{
    this.context = dbContext;
    this.dbSet = dbContext.Set<T>();
}

public async Task<T> GetByIdAsync(params object[] id)
{
    return await dbSet.FindAsync(id);
 }

To replace this I defined the following query class:

public class EntityByPrimaryKeyQuery<T> : IQuery<T> where T : class
{
    public EntityByPrimaryKeyQuery(int id)
    {
        Id = id;
    }

    public int Id { get; set; }
}

I then defined the following query handler class:

public class EntityByPrimaryKeyQueryHandler<T>
    : IQueryHandler<EntityByPrimaryKeyQuery<T>, T>
    where T : class
{
    private readonly MyDbContext dbContext;

    public EntityByPrimaryKeyQueryHandler(MyDbContext dbContext)
    {
        this.dbContext = dbContext;
    }

    public async Task<T> HandleAsync(EntityByPrimaryKeyQuery<T> query)
    {
        var dbSet = dbContext.Set<T>();

        return await dbSet.FindAsync(query.Id);
    }
 }

I use Simple Injector and I normally register all my IQueryHandler implementations with one line:

container.Register(typeof(IQueryHandler<,>), AppDomain.CurrentDomain.GetAssemblies());

I then try to inject an instance of the IQueryHandler in my service MyService for a specific domain class like this:

IQueryHandler<EntityByPrimaryKeyQuery<MyDomainClass>, MyDomainClass> queryHandler

When I do this, Simple Injector gives me the following error:

The constructor of type MyService contains the parameter with name 'queryHandler' and type IQueryHandler<EntityByPrimaryKeyQuery<MyDomainClass>, MyDomainClass> that is not registered. Please ensure IQueryHandler<EntityByPrimaryKeyQuery<MyDomainClass>, MyDomainClass> is registered, or change the constructor of MyService . Note that there exists a registration for a different type IQueryHandler<TQuery, TResult> while the requested type is IQueryHandler`2[[EntityByPrimaryKeyQuery<EntityByPrimaryKeyQuery<MyDomainClass>,MyDomainClass>.'

Is what I am trying to achieve possible? If so, what am I doing wrong?

[dotnetjunkie]

This method:

container.Register(typeof(IQueryHandler<,>), AppDomain.CurrentDomain.GetAssemblies());

Has the following behavior (copied from the API documentation):

Registers all concrete, non-generic, public and internal types in the given set of assemblies that implement the given openGenericServiceType

In other words, this method only registers non-generic types, but EntityByPrimaryKeyQueryHandler<T> is generic.

There are multiple ways around this, such as using the Register overload that takes in an IEnumerable<Type> will using Container.GetTypesToRegister to retrieve the types, while supplying a TypesToRegisterOptions with IncludeGenericTypeDefinitions set to true.

The easiest solution, however, is to add the following registration to your configuration:

container.Register(typeof(IQueryHandler<,>), typeof(EntityByPrimaryKeyQueryHandler<>));

This will add an open-generic registration next to all the closed registrations made.

[andregizero]

Thank you Steven, that works great!