using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        // Student collection

        IList<A> Alist = new List<A>() 

        { 

            new A() { AccNo = "Ac1", AcName = "ABA1"} ,

                new A() { AccNo = "Ac2", AcName = "ABA1"} ,

                new A() { AccNo = "Ac3", AcName = "ABA1"},

                new A() { AccNo = "Ac4", AcName = "ABA2"},

                new A() { AccNo = "Ac5", AcName = "ABA3"}

        };

        var strAc = "Ac1,Ac2,Ac7,Ac9";

        var str = strAc.Split(',');

        var test = (from s in str join K in Alist on s equals K.AccNo into ps 

                    from p in ps.DefaultIfEmpty() where p == null select s).ToList();

    var result = str.Where(x=>! Alist.Any(n=>n.AccNo==x)).ToList();

        foreach(var gp in test)

        {

            Console.WriteLine(gp);

        }

        Console.WriteLine();

        foreach(var gp in result)

        {

            Console.WriteLine(gp);

        }

    }

}

​

public class A{

​

    public string AccNo { get; set; }

    public string AcName { get; set; }

}

​