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Using Variance in Interfaces for Generic Collections (C# and Visual Basic)

A covariant interface allows its methods to return more derived types than those specified in the interface. A contravariant interface allows its methods to accept parameters of less derived types than those specified in the interface.

In .NET Framework 4, several existing interfaces become covariant and contravariant. These include IEnumerable<T> and IComparable<T>. This enables you to reuse methods that operate with generic collections of base types for collections of derived types.

For a list of variant interfaces in the .NET Framework, see Variance in Generic Interfaces (C# and Visual Basic).

The following example illustrates the benefits of covariance support in the IEnumerable<T> interface. The PrintFullName method accepts a collection of the IEnumerable<Person> type (IEnumerable(Of Person) in Visual Basic) as a parameter. However, you can reuse it for a collection of the IEnumerable<Employee> type (IEnumerable(Of Person) in Visual Basic) because Employee inherits Person.

// Simple hierarchy of classes. 
public class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

public class Employee : Person { }

class Program
{
    // The method has a parameter of the IEnumerable<Person> type. 
    public static void PrintFullName(IEnumerable<Person> persons)
    {
        foreach (Person person in persons)
        {
            Console.WriteLine("Name: {0} {1}",
            person.FirstName, person.LastName);
        }
    }

    public static void Test()
    {
        IEnumerable<Employee> employees = new List<Employee>();

        // You can pass IEnumerable<Employee>,  
        // although the method expects IEnumerable<Person>.

        PrintFullName(employees);

    }
}

The following example illustrates the benefits of contravariance support in the IComparer<T> interface. The PersonComparer class implements the IComparer<Person> interface (IComparer(Of Person) in Visual Basic). However, you can reuse this class to compare a sequence of objects of the Employee type because Employee inherits Person.

// Simple hierarchy of classes. 
public class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

public class Employee : Person { }

// The custom comparer for the Person type 
// with standard implementations of Equals() 
// and GetHashCode() methods. 
class PersonComparer : IEqualityComparer<Person>
{
    public bool Equals(Person x, Person y)
    {            
        if (Object.ReferenceEquals(x, y)) return true;
        if (Object.ReferenceEquals(x, null) ||
            Object.ReferenceEquals(y, null))
            return false;            
        return x.FirstName == y.FirstName && x.LastName == y.LastName;
    }
    public int GetHashCode(Person person)
    {
        if (Object.ReferenceEquals(person, null)) return 0;
        int hashFirstName = person.FirstName == null
            ? 0 : person.FirstName.GetHashCode();
        int hashLastName = person.LastName.GetHashCode();
        return hashFirstName ^ hashLastName;
    }
}

class Program
{

    public static void Test()
    {
        List<Employee> employees = new List<Employee> {
               new Employee() {FirstName = "Michael", LastName = "Alexander"},
               new Employee() {FirstName = "Jeff", LastName = "Price"}
            };

        // You can pass PersonComparer,  
        // which implements IEqualityComparer<Person>, 
        // although the method expects IEqualityComparer<Employee>.

        IEnumerable<Employee> noduplicates =
            employees.Distinct<Employee>(new PersonComparer());

        foreach (var employee in noduplicates)
            Console.WriteLine(employee.FirstName + " " + employee.LastName);
    }
}
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