using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Diagnostics;

using System.Globalization;

using System.Data;

using System.Collections;

using System.IO;                    

​

// https://stackoverflow.com/questions/46602044/de-aggregate-a-list-in-c-sharp-linq

​

class MyClass 

{

  public int Id { get; set; }

  public int Value { get; set; } 

}

​

public class Program

{

    public static void Main()

    {

        var list = new List<MyClass>() {

          new MyClass { Id = 1, Value = 3 },

          new MyClass { Id = 2, Value = 4 },

        };

​

        var array = list

            .SelectMany(c => Enumerable.Range(0, c.Value).Select(i => c.Id))

            .ToArray();

        array.Dump();

    }

}