[C#] LINQ to Objects에서 작동하지 않는 구별

class Program
{
    static void Main(string[] args)
    {
        List<Book> books = new List<Book>
        {
            new Book
            {
                Name="C# in Depth",
                Authors = new List<Author>
                {
                    new Author
                    {
                        FirstName = "Jon", LastName="Skeet"
                    },
                     new Author
                    {
                        FirstName = "Jon", LastName="Skeet"
                    },
                }
            },
            new Book
            {
                Name="LINQ in Action",
                Authors = new List<Author>
                {
                    new Author
                    {
                        FirstName = "Fabrice", LastName="Marguerie"
                    },
                     new Author
                    {
                        FirstName = "Steve", LastName="Eichert"
                    },
                     new Author
                    {
                        FirstName = "Jim", LastName="Wooley"
                    },
                }
            },
        };


        var temp = books.SelectMany(book => book.Authors).Distinct();
        foreach (var author in temp)
        {
            Console.WriteLine(author.FirstName + " " + author.LastName);
        }

        Console.Read();
    }

}
public class Book
{
    public string Name { get; set; }
    public List<Author> Authors { get; set; }
}
public class Author
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public override bool Equals(object obj)
    {
        return true;
        //if (obj.GetType() != typeof(Author)) return false;
        //else return ((Author)obj).FirstName == this.FirstName && ((Author)obj).FirstName == this.LastName;
    }

}

이것은 “LINQ in Action”의 예를 기반으로합니다. 목록 4.16.

이것은 Jon Skeet을 두 번 인쇄합니다. 왜? Author 클래스에서 Equals 메서드를 재정의하려고 시도했습니다. 여전히 Distinct가 작동하지 않는 것 같습니다. 내가 무엇을 놓치고 있습니까?

편집 : 나는 == 및! = 연산자 과부하도 추가했습니다. 여전히 도움이되지 않습니다.

 public static bool operator ==(Author a, Author b)
    {
        return true;
    }
    public static bool operator !=(Author a, Author b)
    {
        return false;
    }

답변

public class Author : IEquatable<Author>
{
    public string FirstName { get; set; }
    public string LastName { get; set; }

    public bool Equals(Author other)
    {
        if (FirstName == other.FirstName && LastName == other.LastName)
            return true;

        return false;
    }

    public override int GetHashCode()
    {
        int hashFirstName = FirstName == null ? 0 : FirstName.GetHashCode();
        int hashLastName = LastName == null ? 0 : LastName.GetHashCode();

        return hashFirstName ^ hashLastName;
    }
}

public class AuthorEquals : IEqualityComparer<Author>
{
    public bool Equals(Author left, Author right)
    {
        if((object)left == null && (object)right == null)
        {
            return true;
        }
        if((object)left == null || (object)right == null)
        {
            return false;
        }
        return left.FirstName == right.FirstName && left.LastName == right.LastName;
    }

    public int GetHashCode(Author author)
    {
        return (author.FirstName + author.LastName).GetHashCode();
    }
}

var temp = books.SelectMany(book => book.Authors)
                .GroupBy (y => y.FirstName + y.LastName )
                .Select (y => y.First ());

foreach (var author in temp){
  Console.WriteLine(author.FirstName + " " + author.LastName);
}

YourList.GroupBy(i => i.Id).Select(i => i.FirstOrDefault()).ToList();