using System;

using System.Linq;

using System.Collections.Generic;

using System.Runtime.InteropServices;

​

public static class Program

{

    public static void Main()

    {

        List<MyClass> list = new List<MyClass>();

        list.Add(new MyClass() { BillId = "123" });

        list.Add(new MyClass() { BillId = "777" });

        list.Add(new MyClass() { BillId = "999" });

        list.Add(new MyClass() { BillId = "123" });

        List<MyClass> unique = list.UniqueBy(x => x.BillId).ToList();

        Console.WriteLine(string.Join(", ", unique.Select(x => x.BillId)));

    }

    public class MyClass

    {        

        public object classObj {get;set;}         

        public string BillId {get;set;}         

    }

    /// <summary>

    /// Returns a sequence of elements that appear exactly once in the source sequence,

    /// according to a specified key selector function.

    /// </summary>

    public static IEnumerable<TSource> UniqueBy<TSource, TKey>(

        this IEnumerable<TSource> source,

        Func<TSource, TKey> keySelector,

        IEqualityComparer<TKey> comparer = default)

    {

        ArgumentNullException.ThrowIfNull(source);

        ArgumentNullException.ThrowIfNull(keySelector);

​

        Dictionary<TKey, (TSource Item, bool Unique)> dictionary = new(comparer);

        if (source.TryGetNonEnumeratedCount(out int count))

            dictionary.EnsureCapacity(count); // Assume that most items are unique

        foreach (TSource item in source)

            CollectionsMarshal.GetValueRefOrAddDefault(dictionary, keySelector(item),

                out bool exists) = exists ? default : (item, true);

​

        foreach ((TSource item, bool unique) in dictionary.Values)

            if (unique)

                yield return item;

    }

}

​

​