Covariance and contravariance real world example

Question

I\'m having a little trouble understanding how I would use covariance and contravariance in the real world.

So far, the only examples I\'ve seen have been the same o

Answer 1

class A {}
class B : A {}

public void SomeFunction()
{
    var someListOfB = new List();
    someListOfB.Add(new B());
    someListOfB.Add(new B());
    someListOfB.Add(new B());
    SomeFunctionThatTakesA(someListOfB);
}

public void SomeFunctionThatTakesA(IEnumerable input)
{
    // Before C# 4, you couldn't pass in List:
    // cannot convert from
    // 'System.Collections.Generic.List' to
    // 'System.Collections.Generic.IEnumerable'
}

Basically whenever you had a function that takes an Enumerable of one type, you couldn't pass in an Enumerable of a derived type without explicitly casting it.

Just to warn you about a trap though:

var ListOfB = new List();
if(ListOfB is IEnumerable)
{
    // In C# 4, this branch will
    // execute...
    Console.Write("It is A");
}
else if (ListOfB is IEnumerable)
{
    // ...but in C# 3 and earlier,
    // this one will execute instead.
    Console.Write("It is B");
}

That is horrible code anyway, but it does exist and the changing behavior in C# 4 might introduce subtle and hard to find bugs if you use a construct like this.