Difference Between Select and SelectMany

Question

I\'ve been searching the difference between Select and SelectMany but I haven\'t been able to find a suitable answer. I need to learn the differenc

Answer 1

The SelectMany() method is used to flatten a sequence in which each of the elements of the sequence is a separate.

I have class user same like this

class User
    {
        public string UserName { get; set; }
        public List Roles { get; set; }
    }

main:

var users = new List
            {
                new User { UserName = "Reza" , Roles = new List{"Superadmin" } },
                new User { UserName = "Amin" , Roles = new List{"Guest","Reseption" } },
                new User { UserName = "Nima" , Roles = new List{"Nurse","Guest" } },
            };

var query = users.SelectMany(user => user.Roles, (user, role) => new { user.UserName, role });

foreach (var obj in query)
{
    Console.WriteLine(obj);
}
//output

//{ UserName = Reza, role = Superadmin }
//{ UserName = Amin, role = Guest }
//{ UserName = Amin, role = Reseption }
//{ UserName = Nima, role = Nurse }
//{ UserName = Nima, role = Guest }

You can use operations on any item of sequence

int[][] numbers = {
                new[] {1, 2, 3},
                new[] {4},
                new[] {5, 6 , 6 , 2 , 7, 8},
                new[] {12, 14}
            };

IEnumerable result = numbers
                .SelectMany(array => array.Distinct())
                .OrderBy(x => x);

//output

//{ 1, 2 , 2 , 3, 4, 5, 6, 7, 8, 12, 14 }

 List> numbers = new List> {
                new List {1, 2, 3},
                new List {12},
                new List {5, 6, 5, 7},
                new List {10, 10, 10, 12}
            };

 IEnumerable result = numbers
                .SelectMany(list => list)
                .Distinct()
                .OrderBy(x=>x);

//output

// { 1, 2, 3, 5, 6, 7, 10, 12 }