Using LINQ's ToLookup

Often times you have a set of data and want to process the data in sets based on some property of the data. For example, you may want to process all employee records based on what state the employee lives in (presumably for tax purposes).

To organize the employees, we need something like a .NET dictionary class, but be able to store multiple employees per key (i.e. state). A dictionary only stores a single object per key so we're out of luck using that class.

This is where the LINQ ToLookup and GroupBy operators come to the rescue. ToLookup is a non-deferred operator and GroupBy is a deferred operator. The example below illustrates how ToLookup can be used to process employee objects, together, based on what state the employee lives in. The GroupBy is basically the same thing, but the data is grouped during enumeration because of its deferred nature.

private class Employee
{
   public int ID { get; set; }
   public string LastName { get; set; }
   public string FirstName { get; set; }
   public string StateAbbr { get; set; }
}

static void Main(string[] args)
{
   new Program().Run(args);
}

private void Run(string[] args)
{
   List<Employee> employees = new List<Employee>
   {
      new Employee { ID = 1, LastName = "Scott", FirstName = "Michael", StateAbbr = "CO" },
      new Employee { ID = 1, LastName = "Smoe", FirstName = "Joe", StateAbbr = "CA" },
      new Employee { ID = 1, LastName = "Lampton", FirstName = "Todd", StateAbbr = "CO" },
      new Employee { ID = 1, LastName = "Morgan", FirstName = "Jared", StateAbbr = "WA" },
      new Employee { ID = 1, LastName = "Smart", FirstName = "Laura", StateAbbr = "ID" },
      new Employee { ID = 1, LastName = "Seashell", FirstName = "Sally", StateAbbr = "CO" },
   };

   ILookup<string, Employee> folksByState = employees
      .ToLookup(e => e.StateAbbr);

   foreach (IGrouping<string, Employee> stateGroup in folksByState)
   {
      Console.WriteLine("\nState: {0}", stateGroup.Key);
      foreach (Employee e in stateGroup)
      {
         Console.WriteLine("{0}, {1}, {2}", e.LastName, e.FirstName, e.ID);
      }
   }
}