Using repository pattern to eager load entities using ThenIclude

You can change it to something like this:

public virtual TEntity GetById<TEntity>(int id, Func<IQueryable<TEntity>, IQueryable<TEntity>> func) 
{
    DbSet<TEntity> result = this.Set<TEntity>();

    IQueryable<TEntity> resultWithEagerLoading = func(result);

    return resultWithEagerLoading.FirstOrDefault(e => e.Id == id);
}

And you can use it like this:

productRepository.GetById(2, x => x.Include(p => p.Orders)
                                   .ThenInclude(o => o.LineItems)
                                   .Include(p => p.Parts))

This is a bit of an old question, but since it doesn't have an accepted answer I thought I'd post my solution to this.

I'm using EF Core and wanted to do exactly this, access eager loading from outside my repository class so I can specify the navigation properties to load each time I call a repository method. Since I have a large number of tables and data I didn't want a standard set of eagerly loading entities since some of my queries only needed the parent entity and some needed the whole tree.

My current implementation only supports IQueryable method (ie. FirstOrDefault, Where, basically the standard lambda functions) but I'm sure you could use it to pass through to your specific repository methods.

I started with the source code for EF Core's EntityFrameworkQueryableExtensions.cs which is where the Include and ThenInclude extension methods are defined. Unfortunately, EF uses an internal class IncludableQueryable to hold the tree of previous properties to allow for strongly type later includes. However, the implementation for this is nothing more than IQueryable with an extra generic type for the previous entity.

I created my own version I called IncludableJoin that takes an IIncludableQueryable as a constructor parameter and stores it in a private field for later access:

public interface IIncludableJoin<out TEntity, out TProperty> : IQueryable<TEntity>
{
}

public class IncludableJoin<TEntity, TPreviousProperty> : IIncludableJoin<TEntity, TPreviousProperty>
{
    private readonly IIncludableQueryable<TEntity, TPreviousProperty> _query;

    public IncludableJoin(IIncludableQueryable<TEntity, TPreviousProperty> query)
    {
        _query = query;
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }

    public IEnumerator<TEntity> GetEnumerator()
    {
        return _query.GetEnumerator();
    }

    public Expression Expression => _query.Expression;
    public Type ElementType => _query.ElementType;
    public IQueryProvider Provider => _query.Provider;

    internal IIncludableQueryable<TEntity, TPreviousProperty> GetQuery()
    {
        return _query;
    }
}

Note the internal GetQuery method. This will be important later.

Next, in my generic IRepository interface, I defined the starting point for eager loading:

public interface IRepository<TEntity> where TEntity : class
{
    IIncludableJoin<TEntity, TProperty> Join<TProperty>(Expression<Func<TEntity, TProperty>> navigationProperty);
    ...
}

The TEntity generic type is the interface of my EF entity. The implmentation of the Join method in my generic repository is like so:

public abstract class SecureRepository<TInterface, TEntity> : IRepository<TInterface>
    where TEntity : class, new()
    where TInterface : class
{
    protected DbSet<TEntity> DbSet;
    protected SecureRepository(DataContext dataContext)
    {
        DbSet = dataContext.Set<TEntity>();
    }

    public virtual IIncludableJoin<TInterface, TProperty> Join<TProperty>(Expression<Func<TInterface, TProperty>> navigationProperty)
    {
        return ((IQueryable<TInterface>)DbSet).Join(navigationProperty);
    }
    ...
}

Now for the part that actually allows for multiple Include and ThenInclude. I have several extension methods that take and return and IIncludableJoin to allow for method chaining. Inside which I call the EF Include and ThenInclude methods on the DbSet:

public static class RepositoryExtensions
{
    public static IIncludableJoin<TEntity, TProperty> Join<TEntity, TProperty>(
        this IQueryable<TEntity> query,
        Expression<Func<TEntity, TProperty>> propToExpand)
        where TEntity : class
    {
        return new IncludableJoin<TEntity, TProperty>(query.Include(propToExpand));
    }

    public static IIncludableJoin<TEntity, TProperty> ThenJoin<TEntity, TPreviousProperty, TProperty>(
         this IIncludableJoin<TEntity, TPreviousProperty> query,
         Expression<Func<TPreviousProperty, TProperty>> propToExpand)
        where TEntity : class
    {
        IIncludableQueryable<TEntity, TPreviousProperty> queryable = ((IncludableJoin<TEntity, TPreviousProperty>)query).GetQuery();
        return new IncludableJoin<TEntity, TProperty>(queryable.ThenInclude(propToExpand));
    }

    public static IIncludableJoin<TEntity, TProperty> ThenJoin<TEntity, TPreviousProperty, TProperty>(
        this IIncludableJoin<TEntity, IEnumerable<TPreviousProperty>> query,
        Expression<Func<TPreviousProperty, TProperty>> propToExpand)
        where TEntity : class
    {
        var queryable = ((IncludableJoin<TEntity, IEnumerable<TPreviousProperty>>)query).GetQuery();
        var include = queryable.ThenInclude(propToExpand);
        return new IncludableJoin<TEntity, TProperty>(include);
    }
}

In these methods I am getting the internal IIncludableQueryable property using the aforementioned GetQuery method, calling the relevant Include or ThenInclude method, then returning a new IncludableJoin object to support the method chaining.

And that's it. The usage of this is like so:

IAccount account = _accountRepository.Join(x=>x.Subscription).Join(x=>x.Addresses).ThenJoin(x=>x.Address).FirstOrDefault(x => x.UserId == userId);

The above would load the base Account entity, it's one-to-one child Subscription, it's one-to-many child list Addresses and it's child Address. Each lambda function along the way is strongly typed and is supported by intellisense to show the properties available on each entity.