Question

I have two classes – MyBaseClass and BaseClassContainer – that are declared like such:

public class MyBaseClass
{
    private Guid id;
    public Guid ID
    {
        if (id == Guid.Empty)
        {
            id = Guid.NewGuid();
        }
        return id;
    }
    //...Other Properties omitted for brevity
}

and

public class BaseClassContainer : INotifyPropertyChanged
{
    private ObservableCollection<MyBaseClass> baseClasses;
    public ObservableCollection<MyBaseClass> BaseClasses
    {
    //...Omitted for brevity...
    }
}

Then in my code I have an ObservableCollection of type BaseClassContainer (BaseClassContainerCollection). What I’m trying to figure out is how can I use LINQ to select a single BaseClassContainer from the ObservableCollection where one of its MyBaseClass.ID matches a specific Guid. The reason I’m using the Single() method is because I know they’re all going to be unique.

I’ve tried the following but it doesn’t work:

var result = BaseClassContainerCollection.Single(container => container.BaseClasses.Single(baseClass => baseClass.ID == specificGuid));

I get an error saying: Cannot implicitly convert type ‘MyBaseClass’ to ‘bool’. What am I missing?

Answer 1

Lets break apart your query:

BaseClassContainerCollection.Single(yourPredicate);

Single, as it is used here, basically says “filter BaseClassContainerCollection on this predicate” (a “filter” function that evaluates to true or false for whether or not to include it in the results). Instead of a function that returns true/false, you’re saying you want it to evaluate to a MyBaseClass, which doesn’t make sense. Your inner call to Single makes sense, because x => x.Id == guid is a function that returns true/false and filters to only those elements that meet the criteria (then states that you know there will only be one of them in the results or else throw an exception).

What you want to do is Select the single MyBaseClass result from the inner query, then call Single on the result (without a predicate) since you know the result should only have one item returned. I believe you’re looking for:

BaseClassContainerCollection.Select(container => container.BaseClasses.Single(baseClass => baseClass.ID == specificGuid)).Single();