using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var result = GetDemoAccounts(new int[]{ 2,3,4,5 });

        Console.WriteLine(result.Count());

    }

    public static List<SomeClass> GetDemoAccounts(int[] accountIds)

        {      

            return accountIds.SelectMany(id => SomeClassProvider.GetDemoAccounts().Where(x => x.Id == id)).ToList();

        }

}

​

public static class SomeClassProvider

{

    public static List<SomeClass> GetDemoAccounts()

    {

        Console.WriteLine("GetDemoAccount executed");

        return new List<SomeClass> {

            new SomeClass { Id = 1 },

            new SomeClass { Id = 2 },

            new SomeClass { Id = 3 },

            new SomeClass { Id = 4 },

            new SomeClass { Id = 5 },

            new SomeClass { Id = 6 },

            new SomeClass { Id = 7 },

            new SomeClass { Id = 8 }

        };

    }

}

​

    public class SomeClass 

    {

        public int Id {get;set;}

    }   

​