How to sort a list of objects by a specific member?

Question

Say I have a list of Person objects:

class person
{
  int id;
  string FirstName;
  string LastName;
}

How would I sort this list by the LastName member?

List<Person> myPeople = GetMyPeople();
myPeople.Sort(/* what goes here? */);

Answer 1

Under C# 3.0 you can use the following linq statement:

var sortedEnumerable = myPeople.OrderBy( p => p.LastName );

Answer 2

A custom IComparer<person>. If you are using c#3.0 and .net3.5, have a look into the LINQ Methods, espescially IEnumerable<T>.OrderBy().

Answer 3

One option is to write a comparer:

class LastNameComparer : IComparer<Person>
{
   public int Compare(Person x, Person y)
   {
       return String.Compare(x.LastName, y.LastName);
   }
}

An then

myPeople.Sort(new LastNameComparer());

Person may also implement IComparable<Person>, in which case myPeople.Sort() will suffice. However, you may want to sort by other properties on other places, so this isn't a general method; if you want to sort by ID on another report you can write another IComparer, but you can only have one IComparable<Person>.CompareTo(Person other) method.

If you're feeling lazy, or sure you won't use it again, you can also use a lambda:

myPeople.Sort((p1, p2) => String.Compare(p1.LastName, p2.LastName));

Answer 4

As others have suggested, you can use a custom IComparer method to sort your list. If you're only doing it once, the easiest way is using an anonymous method like such:

List<Person> myPeople = GetMyPeople();
myPeople.Sort(delegate(Person one, Person two)
{
    return one.LastName.CompareTo(two.LastName);
});

Alternatively, you can implement IComparable on your Person class, so that the class knows how to sort itself:

class Person : IComparable<Person>
{
    int id;
    string FirstName;
    string LastName;

    public int CompareTo(Person other)
    {
        return LastName.CompareTo(other.LastName);
    }
}

Answer 5

List<T>.Sort will sort the list in-place. If that's what you want, sweet: use the overload that takes a Comparison<T> delegate:

List<Person> myPeople = GetMyPeople();
myPeople.Sort((x, y) => x.LastName.CompareTo(y.LastName));

If you don't want to modify the actual list but want to enumerate over its contents in a sorted order, then, as others have suggested, use the LINQ extension method OrderBy (.NET 3.5 and up) instead. This method can take a simple Func<T, TKey> selector and order by the keys which this selector... you know, selects.

List<Person> myPeople = GetMyPeople();
var orderedByLastName = myPeople.OrderBy(p => p.LastName);

Answer 6

Nothing needs to go inside your "Sort" method call if your class implements IComparable.

Check this link for detail.

Code:

class person : IComparable<person>
{ 
  int id; 
  string FirstName; 
  string LastName; 

public int CompareTo(person other)
    {
        //If you want to sort on FirstName
        return FirstName.CompareTo(other.FirstName);
    }


} 

Answer 7

example of ICompare

   /// <summary>
   /// Sort class on LastName Descending Order
   /// </summary>
   private class PersonLastNameDescending : IComparer<Person>
   {
      public int Compare(Person aX, Person aY)
      {
         return aY.LastName.CompareTo(aX.LastName);
      } // end Compare(...)
   } // end inner class PersonLastNameDescending 

   MyPeople.Sort(new PersonLastNameDescending());