async await in C#

The async await comes in C# 5.0. This keywords is used in same function.
Suppose you have login task taht takes more time to response, it will stop the UI for this time.
So in this case you need to write asynchronous code.
The await keyword is used to stop the current method execution untill task is completed/ return result.
we can not use await if we not apply async keyword in function.

groupby multiple colunm

Step1: Suppose you have classes:

 public class Teacher
    {
        public string School { get; set; }
        public string Friend { get; set; }
        public string Book { get; set; }
        public List<Student> Children { get; set; }
    }

    public class Student
    {
        public string School { get; set; }
        public string Name { get; set; }
        public string Address { get; set; }
        public string Friend { get; set; }
        public string Mother { get; set; }
        public string Book { get; set; }
    }

Step2: In step1 you have created class, now on your controller action method you can set some mock data as example given below:  List<Student> student = GetList();

Step3: after that we have apply query group by:-
 public class TestController : ControllerBase
    {

        [HttpGet]
        [Route("groupby")]
        public async Task<IActionResult> TestGroupBy()
        {
            List<Student> student = GetList();
            var result =
            from c in student
            group c by new { c.School, c.Friend, c.Book, } into gcs
            select new Teacher()
            {
                School = gcs.Key.School,
                Friend = gcs.Key.Friend,
                Book = gcs.Key.Book,
                Children = gcs.ToList(),
            };
            return Ok(result);
        }

        private static List<Student> GetList()
        {
            return new List<Student>()
                {
                    new Student()
                        {School = "S1", Book = "Book1", Friend = "Hamid", Name = "Stev"},
                    new Student()
                        {School = "S2", Book = "Book1", Friend = "Hamid", Name = "Bill"},
                    new Student()
                        {School = "S3", Book = "Book1", Friend = "Smith", Name = "Root"},
                    new Student()
                        {School = "S2", Book = "Book1", Friend = "Finch", Name = "Root"},
                };
        }
    } Ok(result);
        }

Step4: Result:

[
  {
    "school": "S1",
    "friend": "Hamid",
    "book": "Book1",
    "children": [
      {
        "school": "S1",
        "name": "Stev",
        "address": null,
        "friend": "Hamid",
        "mother": null,
        "book": "Book1"
      },
      {
        "school": "S1",
        "name": "Stev",
        "address": null,
        "friend": "Hamid",
        "mother": null,
        "book": "Book1"
      }
    ]
  },
  {
    "school": "S2",
    "friend": "Smith",
    "book": "Book1",
    "children": [
      {
        "school": "S2",
        "name": "Root",
        "address": null,
        "friend": "Smith",
        "mother": null,
        "book": "Book1"
      },
      {
        "school": "S2",
        "name": "Root",
        "address": null,
        "friend": "Smith",
        "mother": null,
        "book": "Book1"
      }
    ]
  }
]