Distinct not working with LINQ to Objects

Question

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


        

Answer 1

You can use extension method on list which checks uniqueness based on computed Hash. You can also change extension method to support IEnumerable.

Example:

public class Employee{
public string Name{get;set;}
public int Age{get;set;}
}

List<Employee> employees = new List<Employee>();
employees.Add(new Employee{Name="XYZ", Age=30});
employees.Add(new Employee{Name="XYZ", Age=30});

employees = employees.Unique(); //Gives list which contains unique objects. 

Extension Method:

    public static class LinqExtension
        {
            public static List<T> Unique<T>(this List<T> input)
            {
                HashSet<string> uniqueHashes = new HashSet<string>();
                List<T> uniqueItems = new List<T>();

                input.ForEach(x =>
                {
                    string hashCode = ComputeHash(x);

                    if (uniqueHashes.Contains(hashCode))
                    {
                        return;
                    }

                    uniqueHashes.Add(hashCode);
                    uniqueItems.Add(x);
                });

                return uniqueItems;
            }

            private static string ComputeHash<T>(T entity)
            {
                System.Security.Cryptography.SHA1CryptoServiceProvider sh = new System.Security.Cryptography.SHA1CryptoServiceProvider();
                string input = JsonConvert.SerializeObject(entity);

                byte[] originalBytes = ASCIIEncoding.Default.GetBytes(input);
                byte[] encodedBytes = sh.ComputeHash(originalBytes);

                return BitConverter.ToString(encodedBytes).Replace("-", "");
            }

Answer 2

The Above answers are wrong!!! Distinct as stated on MSDN returns the default Equator which as stated The Default property checks whether type T implements the System.IEquatable interface and, if so, returns an EqualityComparer that uses that implementation. Otherwise, it returns an EqualityComparer that uses the overrides of Object.Equals and Object.GetHashCode provided by T

Which means as long as you overide Equals you are fine.

The reason you're code is not working is because you check firstname==lastname.

see https://msdn.microsoft.com/library/bb348436(v=vs.100).aspx and https://msdn.microsoft.com/en-us/library/ms224763(v=vs.100).aspx

Answer 3

Another solution without implementing IEquatable, Equals and GetHashCode is to use the LINQs GroupBy method and to select the first item from the IGrouping.

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);
}