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The GroupBy() method in LINQ is used to group elements in a sequence based on a common property. The GroupBy() method returns a IGrouping<TKey, TResult> object, which is a collection of elements that have the same key.

To use the GroupBy() method, you need to specify the property that you want to group the elements by. You can also specify a lambda expression to customize the grouping.

The following code groups a list of Person objects by their age:

C#

var people = new List<Person>();
people.Add(new Person { Age = 20, Name = "John" });
people.Add(new Person { Age = 30, Name = "Jane" });
people.Add(new Person { Age = 10, Name = "Peter" });

var groupedPeople = people.GroupBy(p => p.Age);

In this code, the GroupBy() method groups the people list by the Age property of the Person objects. The groupedPeople variable is a IGrouping<int, Person> object, which is a collection of Person objects that have the same age.

To fetch the list of records, you can use the ToList() method. The ToList() method returns a list of all the elements in the IGrouping object.

The following code fetches the list of records from the groupedPeople variable:

C#

var records = groupedPeople.ToList();

In this code, the records variable is a list of all the Person objects in the groupedPeople object.