The IComparable interface and its generic counterpart define a single CompareTo method. This method compares the current instance to another instance of the same type and returns an Int32 value indicating their relative order. A result less than zero indicates that the current instance comes before the other value, while a result greater than zero indicates that the other value comes first. A zero result indicates that the two values are equivalent.
All the primitive .NET data types, including String, implement the IComparable interface, allowing arrays and collections of such values to be sorted automatically, e.g.
C#
List<int> numbers = new List<int>();
numbers.Add(7);
numbers.Add(2);
numbers.Add(5);
numbers.Add(3);
Console.WriteLine("Before sorting:");
foreach (int number in numbers)
{ Console.WriteLine(number);}
numbers.Sort();
Console.WriteLine("After sorting:");
foreach (int number in numbers)
{ Console.WriteLine(number);}
VB
Dim numbers As New List(Of Integer)
numbers.Add(7)
numbers.Add(2)
numbers.Add(5)
numbers.Add(3)
Console.WriteLine("Before sorting:")For Each number As Integer In numbers
Console.WriteLine(number)
Nextnumbers.Sort()
Console.WriteLine("After sorting:")For Each number As Integer In numbers
Console.WriteLine(number)
NextSorting arrays is basically the same except that the Array.Sort method is a static/Shared method:
C#
int[] numbers = { 7, 2, 5, 3 };Console.WriteLine("Before sorting:");
foreach (int number in numbers)
{ Console.WriteLine(number);}
Array.Sort(numbers);Console.WriteLine("After sorting:");
foreach (int number in numbers)
{ Console.WriteLine(number);}
VB
Dim numbers As Integer() = {7, 2, 5, 3}
Console.WriteLine("Before sorting:")For Each number As Integer In numbers
Console.WriteLine(number)
NextArray.Sort(numbers)
Console.WriteLine("After sorting:")For Each number As Integer In numbers
Console.WriteLine(number)
NextThe same principle holds for String, DateTime and any other type that implements the IComparable interface. As such, we can define our own types that implement IComparable and then they will be able to be automatically sorted too. For instance, let’s say that we have a Person class and we want them to be inherently sortable by LastName and FirstName. Such a class could look like this:
C#
public class Person : IComparable, IComparable<Person>
{private string _lastName;
private string _firstName;
public string LastName
{ get {return this._lastName;
}
set {this._lastName = value;
}
}
public string FirstName
{ get {return this._firstName;
}
set {this._firstName = value;
}
}
public Person(string firstName, string lastName)
{ this._firstName = firstName; this._lastName = lastName;}
public int CompareTo(object obj)
{return this.CompareTo((Person)obj);
}
public int CompareTo(Person other)
{ // Compare by LastName by default.int result = this.LastName.CompareTo(other.LastName);
// If LastNames are the same... if (result == 0) { // ...compare by FirstName.result = this.FirstName.CompareTo(this.FirstName);
}
return result;}
}
VB
Public Class Person
Implements IComparable, IComparable(Of Person)
Private _lastName As String
Private _firstName As String
Public Property FirstName() As String
GetReturn Me._firstName
End Get
Set(ByVal value As String)
Me._firstName = valueEnd Set
End Property
Public Property LastName() As String
GetReturn Me._lastName
End Get
Set(ByVal value As String)
Me._lastName = valueEnd Set
End Property
Public Sub New(ByVal firstName As String, _
ByVal lastName As String)
Me._firstName = firstName Me._lastName = lastNameEnd Sub
Public Function CompareTo(ByVal obj As Object) As Integer _
Implements IComparable.CompareToReturn Me.CompareTo(DirectCast(obj, Person))
End Function
Public Function CompareTo(ByVal other As Person) As Integer _
Implements IComparable(Of Person).CompareTo
'Compare by LastName by default.Dim result As Integer = Me.LastName.CompareTo(other.LastName)
'If LastNames are the same...If result = 0 Then
'...compare by FirstName. result = Me.FirstName.CompareTo(other.FirstName)End If
Return resultEnd Function
End Class
Note that the Person class implements both the standard and generic forms of the IComparable interface, which is a good convention to follow. The Person.CompareTo method makes use of the String.CompareTo method. Using the CompareTo methods of primitive types in your own CompareTo methods is also a good convention to follow, where it’s feasible.
As you can see, two Person objects will first be compared by their LastName properties and then, if those are equivalent, they will be compared by their FirstName properties. Note that I say “equivalent” rather than “equal”. We might reimplement our CompareTo method to ignore case, in which case strings that are not equal may still be equivalent.
We can now sort an array or collection of Person objects automatically like so:
C#
List<Person> people = new List<Person>();
people.Add(new Person("Mary", "Smith"));
people.Add(new Person("John", "Williams"));
people.Add(new Person("John", "Smith"));
people.Add(new Person("Andrew", "Baxter"));
Console.WriteLine("Before sorting:");
foreach (Person person in people)
{Console.WriteLine("{0}, {1}",
person.LastName,
person.FirstName);
}
people.Sort();
Console.WriteLine("After sorting:");
foreach (Person person in people)
{Console.WriteLine("{0}, {1}",
person.LastName,
person.FirstName);
}
VB
Dim people As New List(Of Person)
people.Add(New Person("Mary", "Smith"))
people.Add(New Person("John", "Williams"))
people.Add(New Person("John", "Smith"))
people.Add(New Person("Andrew", "Baxter"))
Console.WriteLine("Before sorting:")For Each person As Person In people
Console.WriteLine("{0}, {1}", _person.LastName, _
person.FirstName)
Nextpeople.Sort()
Console.WriteLine("After sorting:")For Each person As Person In people
Console.WriteLine("{0}, {1}", _person.LastName, _
person.FirstName)
NextIn the next instalment we’ll look at how to perform custom sorting.
Part 2 here
Part 3 here
