using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var _db = new DB();

        var data = (from a in _db.Table_A()    

        join b in _db.Table_B() on a.ID_A equals b.ID_A    

        join c in _db.Table_C() on new { a.ID_A,b.ID_B} equals new { c.ID_A,c.ID_B }    

        select a);

        foreach(var detail in data)

        {

            Console.WriteLine("ID_A = {0}, data_A = {1}",detail.ID_A,detail.data_A);

        }

    }   

}

​

public class DB {

    public List<TableA> Table_A()

    {

        var list = new List<TableA>();

        list.Add(new TableA(){ID_A = 1, data_A="a"});

        list.Add(new TableA(){ID_A = 2, data_A="b"});

        return list;

    }

    public List<TableB> Table_B()

    {

        var list = new List<TableB>();

        list.Add(new TableB(){ID_A = 1, data_B=2,ID_B=1});

        list.Add(new TableB(){ID_A = 2, data_B=3,ID_B=2});

        return list;

    }

    public List<TableC> Table_C()

    {

        var list = new List<TableC>();

        list.Add(new TableC(){ID_C = 1, data_C=1, ID_A = 1, ID_B =1});

        list.Add(new TableC(){ID_C = 1, data_C=1, ID_A = 1, ID_B =null});

        return list;

    }

}

​

public class TableA

{

    public int ID_A{ get;set;}

    public string data_A {get;set;}

}

​

​

​

public class TableB

{

    public int ID_B{ get;set;}

    public int data_B {get;set;}

    public int ID_A{get;set;}

}

​

​

​

public class TableC

{

    public int ID_C{ get;set;}

    public int data_C {get;set;}

    public int ID_A{get;set;}

    public int? ID_B{get;set;}

}

​

​