C# Snippet Tutorial - Custom List Sorting [Beginner]

The List is one of my favorite additions to the .NET framework. It has built-in mechanisms to perform efficient sorting and searching. In this snippet tutorial, I'm going to demonstrate how to customize the sorting routines used by the List class.

The default Sort function of the List uses the CompareTo method of the object if it implements IComparable. Fortunately, the .NET primitives already implement this interface, which means to sort a List of primitives, you don't have to do anything special.

List<string> myList = new List<string>();

myList.Add("one");
myList.Add("two");
myList.Add("three");
myList.Add("four");

myList.Sort();

foreach (string s in myList)
  Console.WriteLine(s); 

Output:

four
one
three
two

With that being said, to get the List to sort your own custom objects, all you'd have to do is have your object implement the IComparable interface.

public class MyObject : IComparable<MyObject>
{
  private int _myInt;

  public int MyInt
  {
    get { return _myInt; }
    set { _myInt = value; }
  }

  public MyObject(int value)
  {
    _myInt = value;
  }

  public int CompareTo(MyObject obj)
  {
    return _myInt.CompareTo(obj.MyInt);
  }
}

And here's what happens when you populate and sort a list of these objects.

List<MyObject> myObjectList = new List<MyObject>();

myObjectList.Add(new MyObject(3));
myObjectList.Add(new MyObject(1));
myObjectList.Add(new MyObject(4));
myObjectList.Add(new MyObject(2));

myObjectList.Sort();

foreach (MyObject obj in myObjectList)
  Console.WriteLine(obj.MyInt);

Output:

1
2
3
4

That works great when you have access to the object, but what happens if you want to sort an object that doesn't implement IComparable? Good thing for us, the Sort method can take an IComparer, which will take its place. If the object that is being sorted also implements IComparable, passing an IComparer will override the CompareTo function located inside the object.

Let's specify an IComparer that will sort the previous List in the opposite direction.

public class MyListSorter : IComparer<MyObject>
{
  public int Compare(MyObject obj1, MyObject obj2)
  {
    return obj2.CompareTo(obj1);
  }
}

Just like with the CompareTo function, the Compare function returns less than zero if the first object precedes the second one, 0 if they are identical, and greater than zero if the second object precedes the first. Since I'm comparing primitive types, I simply used their CompareTo function. Here's how we use this object.

myObjectList.Sort(new MyListSorter());

foreach (MyObject obj in myObjectList)
  Console.WriteLine(obj.MyInt);

Output:

4
3
2
1

If you don't want to go through all the work of creating a new object to do your comparisons, the last way to sort a List is to use a function. Here's the same sorting using a Comparison delegate.

public int MySortFunction(MyObject obj1, MyObject obj2)
{
  return obj2.CompareTo(obj1);
}

myObjectList.Sort(new Comparison<MyObject>(MySortFunction));

foreach (MyObject obj in myObjectList)
  Console.WriteLine(obj.MyInt);

Output:

4
3
2
1

That's it for sorting a List. The sort function has another override not shown here that specifies the range to sort, but I'm sure you can figure out how to use that by yourself.