HI WELCOME TO Sirees

Part 9 - Difference between Select and SelectMany in LINQ

Let us understand the difference between Select and SelectMany with an example.

We will be using the following Student class in this demo. Subjects property in this class is a collection of strings.

public class Student
{
    public string Name { get; set; }
    public string Gender { get; set; }
    public List<string> Subjects { get; set; }
    public static List<Student> GetAllStudetns()
    {
        List<Student> listStudents = new List<Student>
        {
            new Student
            {
                Name = "Tom",
                Gender = "Male",
                Subjects = new List<string> { "ASP.NET", "C#" }
            },
            new Student
            {
                Name = "Mike",
                Gender = "Male",
                Subjects = new List<string> { "ADO.NET", "C#", "AJAX" }
            },
            new Student
            {
                Name = "Pam",
                Gender = "Female",
                Subjects = new List<string> { "WCF", "SQL Server", "C#" }
            },
            new Student
            {
                Name = "Mary",
                Gender = "Female",
                Subjects = new List<string> { "WPF", "LINQ", "ASP.NET" }
            },
        };
        return listStudents;
    }
}

In this example, the Select() method returns List of List<string>. To print all the subjects we will have to use 2 nested foreach loops.
IEnumerable<List<string>> result = Student.GetAllStudetns().Select(s => s.Subjects);
foreach (List<string> stringList in result)
{
    foreach (string str in stringList)
    {
        Console.WriteLine(str);
    }
}

SelectMany() on the other hand, flattens queries that return lists of lists into a single list. So in this case to print all the subjects we have to use just one foreach loop.
IEnumerable<string> result = Student.GetAllStudetns().SelectMany(s => s.Subjects);
foreach (string str in result)
{
    Console.WriteLine(str);
}

Output:
difference between select and selectmany in linq