C# 4.0 Feature - CoVariance and ContraVariance

Lets see the new feature in C# 4.0 “CoVariance and ContraVariance". This is a very tricky concept and there are many illustrations in the web.

If we had to do the following prior to C# 4.0

class Drivers { }
class LaptopDriver : Drivers { }
class DesktopDriver : Drivers { }
class LanDriver : LaptopDriver { }

delegate T GetDriver();

static void Main(string[] args)
{
GetDriver getLaptopDriver = () => new LaptopDriver();
GetDriver getDrivers = getLaptopDriver;

Console.ReadLine();

}
We would have got a error as follows

Cannot implicitly convert type 'ConsoleApplication2.Program.GetDriver' to 'ConsoleApplication2.Program.GetDriver'

To overcome this in C# 4.0 we have CoVariance which is mainly used for assignment compatability. We need to modify the code by adding a “out” keyoword in the delagate declaration as seen below

class Drivers { }
class LaptopDriver : Drivers { }
class DesktopDriver : Drivers { }
class LanDriver : LaptopDriver { }

delegate T GetDriver();

static void Main(string[] args)
{
GetDriver getLaptopDriver = () => new LaptopDriver();
GetDriver getDrivers = getLaptopDriver;

Console.ReadLine();

}

Now lets see the what is Contravariance ? Lets see the below example if tried in C# 3.0

class Drivers { }
class LaptopDriver : Drivers { }
class DesktopDriver : Drivers { }
class LanDriver : LaptopDriver { }

delegate void GetDriver(T driver);

static void Main(string[] args)
{
GetDriver getDrivers = (drivers) => Console.WriteLine("Drivers Initialised");
GetDriver getDesktop = getDrivers;
}
We would have got the error as follows

Cannot implicitly convert type 'ConsoleApplication2
.Program.GetDriver' to 'ConsoleApplication2.Program.GetDriver'

To Overcome this in C# 4.0, we have Contravariance where we use “in” keyword in the delagate declariation which does the opposite direction of assignment.

class Drivers { }
class LaptopDriver : Drivers { }
class DesktopDriver : Drivers { }
class LanDriver : LaptopDriver { }

delegate void GetDriver(T driver);

static void Main(string[] args)
{
GetDriver getDrivers = (drivers) => Console.WriteLine("Drivers Initialised");
GetDriver getDesktop = getDrivers;
}

Hope this was cool feature. Please add your comments or suggestions.

Cheers!!!