C# linq in Dictionary<>

Question

I have an object allStudents = Dictionary<ClassRoom, List<Student>>()

In Linq how would I get a list of all the students who are male? (student.Gender=="m") from all the Classrooms?

Ian

Answer 1

Try the following

var maleStudents = allStudents
  .SelectMany(x => x.Values)
  .Where(x => x.Gender=="m");

The trick to this is the SelectMany operation. It has the effect of flattening a collection of List<Student> into a single collection of Student. The resulting list is the same as if you'd lined up each list front to back.

Answer 2

You can use nested from clause. The first from selects all classes together with their students (an item from the dictionary), which is represented as a KeyValuePair<ClassRoom, List<Student>>. Then you can select all students from the class using the Value property and filter them:

var q = from cls in allStudents
        from s in cls.Value
        where s.Gender == "M" select s;

Under the cover, the nested from clause is translated to the SelectMany method call.