Linq in from ObservableCollection in ObservableCollection

Question

I am tring to get the value "thisValueIwant". Is there any possibility to get this value so easy? Or Maybe there is another solution for these 2 ObservableCollection

public class Foo
{
    public int number { get; set; }
    public ObservableCollection<FooInFoo> Details { get; set; }
}

public class FooInFoo
{
    public string thisValueIwant { get; set; }
}

public class TestClass
{
    public void Test()
    {
        ObservableCollection<Foo> FooCollection = new ObservableCollection<Foo>();

        FooCollection.Add(new Foo{
            number =1,
            Details = new ObservableCollection<FooInFoo>{ new FooInFoo {thisValueIwant = "very important string"}}
        });

        string x = (from f in FooCollection
                    where f.number == 1
                    select ???)
    }
}

Answer 1

Since ObservableCollection<FooInFoo> Details is a collection, you have to decide which details you want: the first, last, any or all.

Assuming you want the first:

var d = FooCollection.Where(f => f.Number == 1).FirstOrDefault()?.Details.FirstOrDefault()?.thisValueIwant;

Or the last:

var d = FooCollection.Where(f => f.Number == 1).FirstOrDefault()?.Details.LastOrDefault()?.thisValueIwant;

Or all (materialized as an array):

var ds = FooCollection.Where(f => f.Number == 1).FirstOrDefault()?.Details.Select(d => d.thisValueIwant).ToArray();

Answer 2

An ObservableCollection<T> is a Collection<T> which implements IEnumerable<T> . Therefore the fact that your FooCollection is an observable collection is not important, you can regard it as a sequence of Foo , an IEnumerable<Foo> and equally an IEnumerable<FooInFoo>

Your code will be like (sorry, I only know how to write in Method format) In baby steps:

IEnumerable<Foo> AllFooWithNumber1 = FooCollection
   .Where(foo => foo.Number == 1);

If you are certain there is exactly one continue with:

Foo fooWithNumber1 = AllFooWithNumber1.Single();

Consider using SingleOrDefault if you are not certain that there is one.

Once you have the Foo that you want, you can select the Details:

IEnumerable<FooInFoo> detailsOfFooWithNumber1 = fooWithNumber1.Details;
FooInFoo detailIWant = detailsOfFooWithNumber1
   .Where(detail => some expression that uses detail...)
   .SingleOrDefault();

string thisValueIWant = defailtIWant.thisValueIWant;

Or in one statement:

string thisValueIWant = FooCollection
   .Where(foo => foo.Number == 1)
   .Single()
   .Details
   .Where(detail => ...)
   .Single()
   .thisValueIWant;

Problems might arise if you are not certain there is one Single element.

If you want to check foo.Number for a given value AND all details for some predicate, consider using Enumerable.SelectMany . This is used whenever you have sequences of sequences (arrays within arrays). With SelectMany you enumerate over all these inner arrays as if it was one array:

IEnumerable<string> valuesIWant = FooCollection
    .Where(foo => foo.Number == 1)
    .SelectMany(foo => foo.Details)
    // now you have one sequence of all FooInFoo that are Details within
    // Foo objects with Number 1
    .Where(detail => expression that selects the FooInFoo you want)
    .Select(detail => detail.thisValueIWant);

Answer 3

You need my ObservableComputations library maybe. Using this library you can code like this:

Expression<Func<string>> expr = () => 
   FooCollection
      .Filtering(а => f.number == 1)
      .FirstComputing().Value
      .Using(f => f != null 
           ? f.Details.FirstComputing().Using(fif => 
               fif.Value != null ? fif.Value.thisValueIwant : null).Value
           : null).Value;

Computing<string> x = expr.Computing();

// x.Value is what you want

x is INotifyPropertyChanged and notifies you about changes of computing result of expr. Do not forget make all properties mentioned in the code above notify of changes through the INotifyPropertyChanged interface.