Linq- Difference between Select and SelectMany

Suppose we have a “Student” class and a “Subject” Class.

Student class has a  public List<string> Subjects { get; set; }   property.

new Student

            {

                Name = "Tom",

                Gender = "Male",

                Subjects = new List<string> { "ASP.NET", "C#" }

            },

Now, to print all the subjects using the “Select” method

The “Select” method returns a List of List<string>, so we have to use two nested foreach loops to print the subjects.

IEnumerable<List<string>> result = Student.GetAllStudetns().Select(s => s.Subjects);

foreach (List<string> stringList in result)

{

    foreach (string str in stringList)

    {

        Console.WriteLine(str);

    }

}

The “SelectMany()” method on the other hand, flattens the queries that return lists of lists into a single list.

So, for above example, If we use SelectMany() to print all the subjects of students, then we have to use just one foreach loop. i.e.

IEnumerable<string> result = Student.GetAllStudetns().SelectMany(s => s.Subjects);

  foreach (string str in result)

     {

        Console.WriteLine(str);

     }

Posted in: LINQ