Distinct not working with LINQ to Objects

Question

class Program
{
    static void Main(string[] args)
    {
        List books = new List 
        {
            new Book
            {
                


        

Answer 1

The Distinct() method checks reference equality for reference types. This means it is looking for literally the same object duplicated, not different objects which contain the same values.

There is an overload which takes an IEqualityComparer, so you can specify different logic for determining whether a given object equals another.

If you want Author to normally behave like a normal object (i.e. only reference equality), but for the purposes of Distinct check equality by name values, use an IEqualityComparer. If you always want Author objects to be compared based on the name values, then override GetHashCode and Equals, or implement IEquatable.

The two members on the IEqualityComparer interface are Equals and GetHashCode. Your logic for determining whether two Author objects are equal appears to be if the First and Last name strings are the same.

public class AuthorEquals : IEqualityComparer
{
    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();
    }
}