C# LINQ subquery (On entities)

Question

Hi all,

I am trying to use LINQ to query a list of objects wherever appropriate. Currently I am stuck on the syntax of a nested query which I hope you can help me with.

Classes:

public class FooType
{
    public int Id { get; set; }

    public IList<Foo> Foos { get; set; }

}

public class Foo
{
    public int FooTypeId { get; set; }

    public string CorrelationId { get; set; }
}

public class Bar
{
    public string FooCorrelationId { get; set; }
}

Usage:

IList < FooType > fooTypes = new List < FooType >();
// ... add a lot of FooTypes, each enriched with some Foos
Bar bar = new Bar(){FooCorrelationId = "abcdef"};
Foo foo = fooTypes.Where( ft => ft.Foos.Where( f => f.CorrelationId == bar.FooCorrelationId ) ).First<Foo>();

This fails because the outer WHERE-expression is fed with something that does not provide a boolean return value. As you might guess, I intend to something like the following in LINQ:

 foreach (FooType fooType in fooTypes)
 {
     Foo foo = fooType.Foos
                      .Where(f => f.CorrelationId == bar.FooCorrelationId)
                      .First();
 }

Do you have any idea how to get that Foo I'm looking for, the one with CorrelationId "abcdef"?

And one more question for the benchmarkers : What about performance? Does LINQ optimize it's queries? Or is the result the same as an iteration over the objects like I did in my "something like" block?

Thanks a lot in advance!

Answer 1

Just replace the outer where with a SelectMany and get the first

Foo foo = fooTypes
    .SelectMany(ft => ft.Foos.Where(f => f.CorrelationId == bar.FooCorrelationId))
    .First();

If you an to get the first Foo across all FooType, what has CorrelationId == FooCorrelationId.