如何使用匿名投影重构此方法以使其更通用？

Question

What I have is the following method. I use anonymous projection to filter the includes EF does. I learned this method from this blogpost: http://thedatafarm.com/data-access/use-projections-and-a-repository-to-fake-a-filtered-eager-load/

public IEnumerable<Entities.Nutrient> FindAllForSpecificLanguage(bool overridePossibleLogicalDelete)
{
    using (var context = CreateObjectContext())
    {
        context.ContextOptions.LazyLoadingEnabled = false;
        Entities.Nutrient[] result;

        var list = context.Nutrients
            .Select(nut => new
            {
                Entity = nut,
                Descriptions = nut.Descriptions.Where(desc => desc.LanguageCode.Equals(DataLanguageContext.Current.DataLanguageCode))
            }).ToList(); //perform query
        var resultList = list
            .Select(entity => entity.Entity);

        return resultList;
    }
}

This method should be built into all services (the api supports about 30 languages, at the moment we have a lot of DB overhead...). I'm trying to build it in a generic way, but I'm horribly inexperienced in expression trees. I thought I completely recreated the function, but I'm missing something because it isn't working. This is what I have so far:

public virtual IEnumerable<TEntity> FindAllForSpecificLanguage(bool overridePossibleLogicalDelete, Expression<Func<TEntity, IEnumerable<object>>> selectEntityDescriptions)
{
    using (var context = CreateObjectContext())
    {
        context.ContextOptions.LazyLoadingEnabled = false;
        ObjectQuery<TEntity> queryObjectSet = GetObjectSet(context);
        TEntity[] result;

        Type anonType = new {Entity = default(TEntity), Descriptions = Enumerable.Empty<object>()}.GetType();

        // (entityManagerBaseEntity) => new { Entity = entityManagerBaseEntity, Descriptions = selectEntityDescriptions(entityManagerBaseEntity) }
        // 1) "(entityManagerBaseEntity) =>"
        var pe = Expression.Parameter(typeof(TEntity), "entityManagerBaseEntity");
        // 2) "selectEntityDescriptions(entityManagerBaseEntity)"
        var exprFunc = Expression.Invoke(selectEntityDescriptions, pe);
        // get constructor for anonymous type
        var constructorInfo = anonType.GetConstructor(new[] { typeof(TEntity), typeof(IEnumerable<object>) });
        // 3) "new AnonType(entityManagerBaseEntity, exprFunc(entityManagerBaseEntity))"
        var constructAnonType = Expression.New(constructorInfo, pe, exprFunc);
        // 4) combine all to a lambda
        // {entity => new <>f__AnonymousType0`2(entity, Invoke(entity => entity.Descriptions.Where(desc => desc.LanguageCode.Equals(DataLanguageContext.Current.DataLanguageCode)), entity))}
        var cooleExpression = Expression.Lambda<Func<TEntity, dynamic>>(constructAnonType, pe);
        //var bla = cooleExpression.Compile();
        //var list = queryObjectSet.AsQueryable().Provider.CreateQuery<dynamic>(cooleExpression).ToList();
        var list = queryObjectSet.Select(cooleExpression).ToList(); //perform query
        var resultList = list
            .Select(entity => entity.Entity as TEntity);

        return resultList;
    }
}

(note: CreateObjectContext and GetObjectSet are perfectly working methods)

Which should be called this way:

_nutrientManager.FindAllForSpecificLanguage(true, (entity) => entity.Descriptions.Where(desc => desc.LanguageCode.Equals(DataLanguageContext.Current.DataLanguageCode)))

The expression that gets built is typed in the comments. It looks fine I guess, but the join is never performed. If I debug I get the following stacktrace:

System.NotSupportedException: Only parameterless constructors and initializers are supported in LINQ to Entities. at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.NewTranslator.TypedTranslate(ExpressionConverter parent, NewExpression linq) at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TypedTranslator`1.Translate(ExpressionConverter parent, Expression linq)

Answer 1

So .. a few things:

1. You can't invoke functions in e L2E expression
2. You can only use constructors without parameters
3. You can build the whole where clause yourself in the expression

the whole solution can be found here: https://github.com/YoeriVD/entity-framework-filter-on-include/blob/master/ExpressionTrees/Program.cs

the important piece is:

    private static void Main(string[] args)
    {
        FilterOnInclude<Car, Wheel>(car => car.Wheels, wheel => wheel.SizeInInches == 14)
            .ForEach(car => Console.WriteLine($"car : {car.Name} wheels: {car.Wheels.Count}"));
    }

    private static IEnumerable<TEntity> FilterOnInclude<TEntity, TChildEntity>(
        Expression<Func<TEntity, IEnumerable<TChildEntity>>> propertyExpression,
        Expression<Func<TChildEntity, bool>> predicateExpression)
        where TEntity : class
    {
        using (var context = new CarContext())
        {
            context.Configuration.LazyLoadingEnabled = false;

            var selector = CreateSelector(propertyExpression, predicateExpression);
            return
                context.Set<TEntity>().Select(
                    selector
                    ).ToList().Select(e => e.Entity).ToArray();
        }
    }

    private static Expression<Func<TEntity, EntityWithFilteredChildren<TEntity, TChildEntity>>> CreateSelector
        <TEntity, TChildEntity>(
        Expression<Func<TEntity, IEnumerable<TChildEntity>>> propertyExpression,
        Expression<Func<TChildEntity, bool>> predicateExpression)
    {
        var selectType = typeof (EntityWithFilteredChildren<TEntity, TChildEntity>);

        //bind entity
        var entityValueParam = Expression.Parameter(typeof (TEntity), "entityValue");
        var entityProp = selectType.GetProperty("Entity");
        var entityValueAssignment = Expression.Bind(
            entityProp, entityValueParam);
        //bind collection
        var childrenProp = selectType.GetProperty("Children");
        var descriptionsMemberExpression = (propertyExpression.Body as MemberExpression);
        var descriptionsPropertyInfo = (PropertyInfo) descriptionsMemberExpression.Member;
        var descriptionsProperty = Expression.Property(entityValueParam, descriptionsPropertyInfo);
        //perform where call
        var whereCall = Expression.Call(typeof (Enumerable), "Where", new[] {typeof (TChildEntity)}, descriptionsProperty,
            predicateExpression);

        var descriptionValueAssignment = Expression.Bind(
            childrenProp, whereCall);

        var ctor = Expression.New(selectType);
        var memberInit = Expression.MemberInit(ctor, entityValueAssignment, descriptionValueAssignment);
        var selector = Expression.Lambda<Func<TEntity, EntityWithFilteredChildren<TEntity, TChildEntity>>>(memberInit,
            entityValueParam);

        return selector;
    }

    public class EntityWithFilteredChildren<T, TChild>
    {
        public T Entity { get; set; }
        public IEnumerable<TChild> Children { get; set; }
    }

Answer 2

Obviously, I don't know the larger structure of your project, but can you just make your first example more generic by replacing Entities.Nutrient with TEntity ? This is the sort of thing I'm thinking of:

public IEnumerable<TEntity> FindAllForSpecificLanguage<TEntity>(
    Func<DbContext, IEnumerable<TEntity>> selectEntities,
    Func<TEntity, IEnumerable<object>> selectEntityDescriptions)
{
    using (var context = CreateObjectContext())
    {
        context.ContextOptions.LazyLoadingEnabled = false;

        var list = selectEntities(context)
            .Select(nut => new
            {
                Entity = nut,
                Descriptions = selectEntityDescriptions
            }).ToList(); //perform query
        var resultList = list
            .Select(entity => entity.Entity);

        return resultList;
    }
}