ASP.Net MVC Web API Return list of objects

Question

How do I correctly return a list of "CarTypes" objects (from the second method), where the TyreID that is passed in, is not the primary key of the CarType class - so for example, I want to return a list of all CarTypes, where the TyreID is 5:

// GET api/CarTypes
public IEnumerable<CarTypes> GetCarTypes()
{
    return db.CarTypes.AsEnumerable();  //This works fineCar
}

// GET api/CarTypes/5
public IEnumerable<CarTypes> GetCarTypes(long id)
{
    CarTypes cartypes = db.CarTypes.Select(t => t.TyreID == id).AsEnumerable();
    if (roomtypes == null)
    {
        throw new HttpResponseException(Request
            .CreateResponse(HttpStatusCode.NotFound));
    }

    return cartypes;
}

It currently shows the error:

Cannot implicitly convert type 'System.Collections.Generic.IEnumerable' to 'MvcApplication4.Models.CarTypes'. An explicit conversion exists (are you missing a cast?)

And does it matter if I use Select/SelectMany/Where in the query?

WestDiscGolf · Accepted Answer · 2012-06-08 13:56:33Z

9

Firstly you need to use Where instead of Select; secondly you don't need to use AsEnumerable() after you've changed it to Where but you might have to call ToList() so that the Linq2Sql/EntityFramework executes the query before returning the values to the view.

 // GET api/CarTypes/5
    public IEnumerable<CarTypes> GetCarTypes(long id)
    {
        var cartypes = db.CarTypes.Where(t => t.TyreID == id).ToList();
        if (cartypes == null || !cartypes.Any())
        {
            throw new HttpResponseException(Request.CreateResponse(HttpStatusCode.NotFound));
        }

        return cartypes;
    }

I've also added in an additional check after the query has executed but you might not need this depending on how you want to handle an empty collection.

edited Jun 8, 2012 at 13:56

answered Jun 8, 2012 at 10:31

WestDiscGolf

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4 Comments

Mark Over a year ago

Hi @WestDiscGolf - VS (2012 RC) shows an error on the db.CarTypes... Cannot implicitly convert type 'System.Collections.Generic.List<MvcApplication4.Models.CarTypes>' to 'MvcApplication4.Models.cartypes

TechQuery Over a year ago

Hi @WestDiscGolf, it might be too forum to comment on, but have a question on the above implementation, if you look at the API, it returns a list but we have IEnumerable as the return type for the method. The above implementation will succeed.. No doubt about it. My questions are, 1) is it fine to use both interchangeably 2) For sure , we are returning a list, then whats the harm in having the return type as List than IEnumerable

WestDiscGolf Over a year ago

Hi @TechQuery, So it's not so much "interchangeable" but the fact that List<T> implements IEnumerable<T> (msdn.microsoft.com/en-us/library/6sh2ey19(v=vs.110).aspx) but so does a lot of things. If you return an interface then anything which implements it can be used to consume it however if a more concrete implementation eg List<T> is used then it limits what can be consumed.

ADM-IT Over a year ago

It is fun but your "cartypes" will never be a null. It is always a list with zero or more elements.

PanJanek · Accepted Answer · 2012-06-08 10:26:50Z

1

You should use "Where" Instead of "Select".

CarTypes cartypes = db.CarTypes.Where(t => t.TyreID == id).AsEnumerable();

"Select" is for specifying which data should be returned for each record, not for filtering the records. Your query with "Select" returns boolean values: false for records with TyreID != id and true for one record where TyreID = id :)

edited Jun 8, 2012 at 10:26

answered Jun 8, 2012 at 10:08

PanJanek

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2 Comments

Mike Simmons Over a year ago

also, don't you want to return cartypes.AsEnumerable()

Mark Over a year ago

Hi - I do have .AsEnumerable() at the end of the Linq statement - but whether I add cartypes.AsEnumerable() or not, the error still remains on the linq statement. Thanks again, Mark.

user1429080 · Accepted Answer · 2012-06-08 10:28:01Z

1

Shouldn't you have:

IEnumerable<CarTypes> cartypes = db.CarTypes.Where(t => t.TyreID == id).AsEnumerable();

Instead of:

CarTypes cartypes = db.CarTypes.Select(t => t.TyreID == id).AsEnumerable();

Note: I would have made this a comment under PanJanek's answer but I'm not currenlty allowed beacuse of my low reputation...

answered Jun 8, 2012 at 10:28

user1429080

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1 Comment

Mark Over a year ago

Thanks a lot - that's what I was missing (IEnumerable at the start of the line) - PanJanek/Mike - thank you also for helping - cheers, Mark

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