LINQ to Objects: Querying Nested Collections

I love LINQ.  LINQ is just fun to use.  I love the fact that I can query the same way across databases, XML  documents and objects (alright XML is slightly different, but...)

Today I had the task of querying down a complex set of objects that each own a collection data member.  I needed to get to some data 4-layers deep and thought the LINQ query would get pretty nasty.  That is until I learned that I can use the "from" keyword multiple times in the same LINQ query.

My example will be very contrived to save having to look at a lot of extra code for different collections, so I'll go ahead and just nest the same collection several layers down, just so we get the concept.

I have a Person class:

public class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }

    public int IQ { get; set; }

    private List<Person> _children;
    public List<Person> Children
    {
        get{ return _children ?? (_children = new List<Person>()); } 
        set{ _children = value; }
    }
}

If I set up a new List<Person> like so:

  List<Person> grandpas = new List<Person>
  {
    new Person{
      FirstName = "John", LastName = "Doe", IQ = 91,
      Children = new List<Person>{ // children
        new Person{ FirstName = "Meh", LastName = "Doe", IQ = 103 },
        new Person{
          FirstName = "Tae Kwon", LastName = "Doe", IQ = 125,
          Children = new List<Person>{ // grandchildren
            new Person{ FirstName = "Karate", LastName = "Kid", IQ = 68, },
            new Person{ FirstName = "Karate", LastName = "Kid2", IQ = 117 },
          }
        },
      }
    },
 
    new Person{
      FirstName = "John", LastName = "Doh", IQ = 135,
      Children = new List<Person>{ // children
        new Person{ FirstName = "Meh", LastName = "Doh", IQ = 42 },
        new Person{
          FirstName = "Tae Kwon", LastName = "Doh", IQ = 140,
          Children = new List<Person>{ // grandchildren
            new Person{ FirstName = "Doh", LastName = "Boy", IQ = 111 },
            new Person{ FirstName = "Doh", LastName = "Nut", IQ = 98 },
          }
        },
      }
    },
  };


Check out how easy it is to get to data that is below the top layer:

IEnumerable<Person> grandChildrenWithHighIQ = from grandpa in grandpas from parent in grandpa.Children from grandkid in parent.Children where grandkid.IQ > 100 select grandkid;

EDIT: I was asked to provide a version with the fluent API as well:

IEnumerable grandChildrenWithHighIQ = grandpas .SelectMany(grandpa => grandpa.Children.SelectMany(parent => parent.Children)) .Where(grandchild => grandchild.IQ > 100);

Now we know longer need be afraid of querying complex objects and getting the data we want.  Write your queries in the bLINQ of an eye, and get your data back even faster!

And I'll put a shameless plug out there for LINQPad since I took a screenshot from its results above.  It is a fantastic tool for crafting LINQ queries and is a wonderful sandbox for small .NET proof-of-concept projects because you can write small programs without creating a Visual Studio project or solution.

Enjoy!