Linq result from many tables into an inherit class with nested classes

Maybe this works for you?

public class MainTable
   {
       public int MainTableID;
       public int TableChildOneID;
       public int TableChildTwoID;
       //... many properties
   }

public class TableChildOne
   {
       public int TableChildOneID;
       //... many properties
   }

public class TableChildTwo
   {
       public int TableChildTwoID;
       //... many properties
   }

public class MainTable_Extended
   {
       public TableChildOne ChildOne { get; set; }
       public TableChildTwo ChildTwo { get; set; }
       public MainTable Main { get; set; }
   }

public class DB : DbContext
   {
       public DB()
           : base("name=YourName")
       {}

       public virtual DbSet<MainTable> MainTables { get; set; }
       public virtual DbSet<TableChildOne> TableChildOnes { get; set; }
       public virtual DbSet<TableChildTwo> TableChildTwos { get; set; }
   }

public class Test
   {
      public void TestTest()
        {
           var db = new DB();

           IQueryable<MainTable_Extended> listMainTable =
           from main in db.MainTables
           from childone in db.TableChildOnes.Where(childone => childone.TableChildOneID == main.TableChildOneID).DefaultIfEmpty()
           from childtwo in db.TableChildTwos.Where(childtwo => childtwo.TableChildTwoID == main.TableChildTwoID).DefaultIfEmpty()
           select new MainTable_Extended
            {
               ChildOne = childone,
               ChildTwo = childtwo,
               Main = main
            };
        }
    }

I let the MainTable_Extended class not inhier from the MainTable class. Instead i make a property with a MainTable object. So in the query i give the MainTable_Extended a value for the property.

So no you can query the propertys of MainTable by call the property Main of MainTable_Extended.