using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<Foo> foos = new List<Foo> {

           new Foo { Id=1, Name="A", Value=1200, Date=new DateTime(2016,12,31) },

           new Foo { Id=2, Name="B", Value=5200, Date=new DateTime(2016,12,31) },

           new Foo { Id=3, Name="C", Value=3500, Date=new DateTime(2016,12,31) },

           new Foo { Id=4, Name="D", Value=100, Date=new DateTime(2016,12,31) },

        };

​

        List<Bar> bars = new List<Bar> {

           new Bar { Id=44, Value=859, Date=new DateTime(2016,12,8) },

           new Bar { Id=23, Value=851, Date=new DateTime(2016,12,7) },

           new Bar { Id=31, Value=785, Date=new DateTime(2016,12,9) }

        };

        var results = foos.Zip(bars, (f, b) => new Result{ 

            FooId = f.Id, Name = f.Name, Value = f.Value + b.Value, BarId =  b.Id, Date = b.Date

        }).Concat(foos.Skip(bars.Count).Select(f => new Result {

            FooId = f.Id, Name = f.Name, Value = f.Value

        }));

        foreach(var item in results)

        {

            Console.Write("{0} {1} {2} {3} {4}", item.FooId, item.Name, item.Value, item.BarId, item.Date);

            Console.WriteLine();    

        }

    }

}

​

public class Foo

{

   public int Id { get; set; }

   public string Name { get; set; }

   public int Value { get; set; }

   public DateTime Date { get; set; }

}

​

public class Bar

{

   public int Id { get; set; }

   public int Value { get; set; }

   public DateTime Date { get; set; }

}

​

public class Result

{

    public int FooId {get; set; }

    public string Name {get; set; }

    public int Value {get;set;}

    public int? BarId {get;set;}

    public DateTime? Date {get; set;}

}