Initializing strongly typed objects in LINQ to Entities

Question

I have a plain old CLR object which is essentially a wrapper for two entity framework objects, I'm doing this so I can pass this wrapper object to a strongly typed view in the MVC framework. My foo wrapper class is very simple:

public class FooWrapper
{
    public FooWrapper(Foo f, Bar b)
    {
        this.FooObject = f;
        this.BarObject = b;
    }

    public Foo FooObject { get; private set; }
    public Bar BarObject { get; private set; }
}

What I have so far for my ListFoosWithBars function is as follows:

public IEnumerable<FooWrapper> ListFoosWithBars(int userID)
{
    IEnumerable<Bar> tempBar = ListBarsByUserID(userID);
    IEnumerable<FooWrapper> results = (from f in _entities.FooSet
                                       join b in tempBar on f.ID equals b.foos.ID
                                       select new FooWrapper(f, b));
    return results;
}

This doesn't work because evidently LINQ to Entities doesn't support parametrized initialization, an exception is thrown that says just that: "Only parameterless constructors and initializers are supported in LINQ to Entities." I was wondering if there is another way to achieve this same result?

Answer 1

IF you add a parameterless constructor to your FooWrapper and then use object initialization instead, like so:

public IEnumerable<FooWrapper> ListFoosWithBars(int userID)
{
    IEnumerable<Bar> tempBar = ListBarsByUserID(userID);

    IEnumerable<FooWrapper> results = (
        from f in _entities.FooSet
        join b in tempBar on f.ID equals b.foos.ID
        select new FooWrapper()
        {
            FooObject = f, 
            BarObject = b
        });

    return results;
}

Answer 2

Ok, but what if you want FooObject and BarObject to be readonly? It seems a bit backwards to me that they negate the ability to use a constructor on the object.

I can see a lot of people breaking good encapsulation practices in order to utilize object initialization in this scenario.

Answer 3

Why aren't you using the .AsEnumerable()? In that way, you won't need to create a parameterless constructor and that is what you want.

Your code was almost good. Change it to this:

public IEnumerable<FooWrapper> ListFoosWithBars(int userID)
{
    IEnumerable<Bar> tempBar = ListBarsByUserID(userID);
    IEnumerable<FooWrapper> results = (from f in _entities.FooSet.AsEnumerable()
                                       join b in tempBar on f.ID equals b.foos.ID
                                       select new FooWrapper(f, b));
    return results;
}

I had the same problem today. I had a class with one parameter constructor. This constructor filled a private readonly field which was returned by a property only with a get and not with a set.

Answer 4

Try a different initialization:

public class FooWrapper
{
    public FooWrapper() { }

    public Foo FooObject { get; set; }
    public Bar BarObject { get; set; }
}


public IEnumerable<FooWrapper> ListFoosWithBars(int userID)
{
    IEnumerable<Bar> tempBar = ListBarsByUserID(userID);

    IEnumerable<FooWrapper> results = (
        from f in _entities.FooSet
        join b in tempBar on f.ID equals b.foos.ID
        select new FooWrapper 
        {
            FooObject = f,
            BarObject = b
        });

    return results;
}

Answer 5

I found this useful what possible workarounds are there for "only parameterless constructors are support in Linq to Entites"