using System;

using System.Collections.Generic;

using System.Linq;

​

public class Message

{

    public int MsgID { get; set; }

    public string Content { get; set; }

    public override string ToString() => $"{MsgID}  |  {Content}";

}

​

public class Sent

{

    public int MsgID { get; set; }

    public string Content { get; set; }

    public int Status { get; set; }

}

​

public class Program

{

    public static void Main()

    {

        List<Message> messages = new()

        {

            new() { MsgID = 1, Content = "aaa" },

            new() { MsgID = 2, Content = "bbb" },

            new() { MsgID = 3, Content = "ccc" },

            new() { MsgID = 4, Content = "ddd" },

            new() { MsgID = 5, Content = "eee" }

        };

        List<Sent> sentList = new()

        {

            new() { MsgID = 1, Content = "aaa", Status = 0 },

            new() { MsgID = 3, Content = "ccc", Status = 0 }

        };

        List<Message> result = messages

            .ExceptBy(sentList.Select(msg => msg.MsgID), msg => msg.MsgID)

            .ToList();

        Console.WriteLine(string.Join(Environment.NewLine, result));

    }

}