title: Linq - SelectMany Confusion
body: <p>From what I understand from the documentation of SelectMany, one could use it to produce a (flattened) sequence of a 1-many relationship.</p>
  
  <p>I have following classes</p>
  
  <pre><code>  public class Customer
    {
      public int Id { get; set; }
      public string Name { get; set; }
    }
  
    class Order
    {
      public int Id { get; set; }
      public int CustomerId { get; set; }
      public string Description { get; set; }
    }
  </code></pre>
  
  <p>I then try to use them using the query expression syntax like so</p>
  
  <pre><code>  var customers = new Customer[]
    {
      new Customer() { Id=1, Name ="A"},
      new Customer() { Id=2, Name ="B"},
      new Customer() { Id=3, Name ="C"}
    };
  
    var orders = new Order[]
    {
      new Order { Id=1, CustomerId=1, Description="Order 1"},
      new Order { Id=2, CustomerId=1, Description="Order 2"},
      new Order { Id=3, CustomerId=1, Description="Order 3"},
      new Order { Id=4, CustomerId=1, Description="Order 4"},
      new Order { Id=5, CustomerId=2, Description="Order 5"},
      new Order { Id=6, CustomerId=2, Description="Order 6"},
      new Order { Id=7, CustomerId=3, Description="Order 7"},
      new Order { Id=8, CustomerId=3, Description="Order 8"},
      new Order { Id=9, CustomerId=3, Description="Order 9"}
    };
  
    var customerOrders = from c in customers
                         from o in orders
                         where o.CustomerId == c.Id
                         select new 
                                { 
                                   CustomerId = c.Id
                                   , OrderDescription = o.Description 
                                };
  
    foreach (var item in customerOrders)
      Console.WriteLine(item.CustomerId + ": " + item.OrderDescription);
  </code></pre>
  
  <p>This gives to what I need.</p>
  
  <pre><code>1: Order 1
  1: Order 2
  1: Order 3
  1: Order 4
  2: Order 5
  2: Order 6
  3: Order 7
  3: Order 8
  3: Order 9
  </code></pre>
  
  <p>I assume this translates to using the SelectMany method when not using the query expression syntax?</p>
  
  <p>Either ways, I'm trying to wrap my head around using SelectMany. So even if my above query does not translate to SelectMany, given the two classes and mock data, could someone provide me with a linq query that uses SelectMany?</p>