mvc querying an ienumerable property of a class

Question

So I have this model setup

public class ListOfThings
{
    public int Id {get;set;}
    public List<int> DefaultRanks {get;set;}

    static public IEnumerable<ListOfThings> ListOfDefault{
       get {
            listOfDefault = new List<ListOfDefault>();
            listOfDefault.Add(N1);
            listOfDefault.Add(N2);
            listOfDefault.Add(N3);
            return listOfDefault.AsReadOnly();
       }
    }

    static public ListOfThings N1{ get { return new ListOfThings { Id = 1, Addend = new List<int> { 1 }}; } }  
    static public ListOfThings N2{ get { return new ListOfThings { Id = 2, Addend = new List<int> { 1,2 }}; } }
    static public ListOfThings N3{ get { return new ListOfThings { Id = 3, Addend = new List<int> { 1,2,3 }}; } }

    static private ListOfThings n1;
    static private ListOfThings n2;
    static private ListOfThings n3;
    static private List<ListOfThings> listOfDefault;
}

And whenever I try to query that static Ienumerable, it'll give me a null value.

var test = ListOfThings.ListOfDefault.Where(a => a.Id == 2);

And if I try to do this, it'll give me a cast warning.

ListOfThings test = ListOfThings.ListOfDefault.Where(a => a.Id == 2);

Any advice is appreciated..

Thanks!! -G

What does any of this have to do with ASP.NET, MVC or Entity Framework? It's just LINQ to Objects as far as I can see... — Jon Skeet
– Jon Skeet, Commented Jan 26, 2012 at 9:46

Jon Skeet · Accepted Answer · 2012-01-26 09:45:59Z

2

It's not clear what you mean by "it'll give me a null value" - I would dispute that, with the code you've given. (A short but complete program demonstrating the problem would really help.)

The second line will definitely give a compile time failure, on the grounds that Where returns an IEnumerable<ListOfThings>, not a single ListOfThings. You may want something like:

ListOfThings test = ListOfThings.ListOfDefault.SingleOrDefault(a => a.Id == 2);

Note that you're never initializing or using n1, n2 or n3 in the code you've given. I'd also suggest that an iterator block would make your ListOfDefault property simpler:

static public IEnumerable<ListOfThings> ListOfDefault {
   get {
        yield return N1;
        yield return N2;
        yield return N3;
   }
}

answered Jan 26, 2012 at 9:45

Jon Skeet

1.5m893 gold badges9.3k silver badges9.3k bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

gdubs Over a year ago

Thanks for the reply, I think that was what I was missing but instead of "SingleOrDefault" the "FirstOrDefault" worked. Well, the "null exception error" I was getting was pointin to the "a.Id==2" area whenever I debug. wasn't really sure on what was happening until you mentioned the SingleOrDefault! Thanks!

Jon Skeet Over a year ago

@gdubs: You'd get a NullReferenceException if you used n1 instead of N1 etc... is it possible that that's what was happening?

gdubs Over a year ago

hmm..what do you mean? like "yield return n1"? well, I took out the private statics (eg n1, n2) and just left the public ones and tried the old line and it's still doing it. What is the difference between "yield return" and just using the private static list that i have and adding the values to it? i'm kinda curious as i saw this on the "validate" method of the Ivalidatable attribute, the way it adds values is by using "yield return new ValidationResult"..

Jon Skeet Over a year ago

@gdubs: Yes - if you were using n1, that would have given you a null value, because it was never initialized. Again, if you're having problems then a short but complete program would be very useful. As for using yield return instead of using a static variable: currently your code isn't thread-safe, for one thing...

gdubs Over a year ago

no i think i got the whole thing working. was just really curious about that yield return. so is it safer than the way i was doing it? "return listOfDefault.AsReadOnly();" i was just reading that it saves up memory or something better than the one i was using..

|

Michal B. · Accepted Answer · 2012-01-26 10:00:02Z

1

There are some problems in your code. The changes I made are:

// in a getter
listOfDefault = new List<ListOfDefault>(); 
// should be:
listOfDefault = new List<ListOfThings>();

// static getters
static public ListOfThings N1{ get { return new ListOfThings { Id = 1, Addend = new List<int> { 1 }}; } }  
static public ListOfThings N2{ get { return new ListOfThings { Id = 2, Addend = new List<int> { 1,2 }}; } }
static public ListOfThings N3{ get { return new ListOfThings { Id = 3, Addend = new List<int> { 1,2,3 }}; } }
// should be:

static public ListOfThings N1 { get { return new ListOfThings { Id = 1, DefaultRanks = new List<int> { 1 } }; } }
static public ListOfThings N2 { get { return new ListOfThings { Id = 2, DefaultRanks = new List<int> { 1, 2 } }; } }
static public ListOfThings N3 { get { return new ListOfThings { Id = 3, DefaultRanks = new List<int> { 1, 2, 3 } }; } }

And the most important is you don't call the FirstOrDefault() on your LINQ (if you want to get more objects use ToList() ):

var test = ListOfThings.ListOfDefault.Where(a => a.Id == 2).FirstOrDefault();

This way it should work.

answered Jan 26, 2012 at 10:00

Michal B.

5,7177 gold badges45 silver badges71 bronze badges

2 Comments

gdubs Over a year ago

Yeah I saw what I was missing.. Thanks! Btw, what's the difference between using ".Where(x => x...).FirstOrDefault()" and ".FirstOrDefault(x => x....)" ? sorry kind of a noob on this..

Jon Skeet Over a year ago

@gdubs: There's no difference - the latter is simpler IMO.

Amit · Accepted Answer · 2012-01-26 10:00:17Z

I tried recreating the issue there were few problems. I have pasted the code below

public class ListOfThings
{
    public int Id { get; set; }
    public List<int> DefaultRanks { get; set; }

    static public IEnumerable<ListOfThings> ListOfDefault
    {
        get
        {
            listOfDefault = new List<ListOfThings>();
            listOfDefault.Add(N1);
            listOfDefault.Add(N2);
            listOfDefault.Add(N3);
            return listOfDefault.AsReadOnly();
        }
    }

    static public ListOfThings N1 { get { return new ListOfThings { Id = 1, DefaultRanks = new List<int> { 1 } }; } }
    static public ListOfThings N2 { get { return new ListOfThings { Id = 2, DefaultRanks = new List<int> { 1, 2 } }; } }
    static public ListOfThings N3 { get { return new ListOfThings { Id = 3, DefaultRanks = new List<int> { 1, 2, 3 } }; } }

    static private ListOfThings n1;
    static private ListOfThings n2;
    static private ListOfThings n3;
    static private List<ListOfThings> listOfDefault;
}



class Program
{
    static void Main(string[] args)
    {
        var test = ListOfThings.ListOfDefault.Where(a => a.Id == 2);
        IEnumerable<ListOfThings> ds = ListOfThings.ListOfDefault.Where(a => a.Id == 2);

    }
}


ListOfThings test = ListOfThings.ListOfDefault.Where(a => a.Id == 2);

is returning Ienumerable not the single objectif you want single or first object do something like

 ListOfThings ds = ListOfThings.ListOfDefault.Where(a => a.Id == 2).FirstOrDefault();

Collectives™ on Stack Overflow

mvc querying an ienumerable property of a class

3 Answers 3

6 Comments

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

6 Comments

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related