CSharp - Covariance and Contravariance in Delegates

Introduction

Covariance allows that we can assign it a method that has a "more derived return type" than the "originally specified return type."

contravariance allows a method with parameter types that are less derived than in the delegate type.

In the following code we can do the following assignment

ShowVehicleTypeDelegate del2 = bus1.ShowBus;

Although our delegate return type was Vehicle, its del2 object received a derived type "Bus" object.

Demo

using System;
delegate Vehicle ShowVehicleTypeDelegate();
class Vehicle//w  w w.j  a va2s. c  om
{
    public Vehicle ShowVehicle()
    {
        Vehicle myVehicle = new Vehicle();
        Console.WriteLine(" A Vehicle created");
        return myVehicle;
    }
}
class Bus : Vehicle
{
    public Bus ShowBus()
    {
        Bus myBus = new Bus();
        Console.WriteLine(" A Bus created");
        return myBus;
    }
}

class Program
{

    static void Main(string[] args)
    {
        Vehicle vehicle1 = new Vehicle();
        Bus bus1 = new Bus();
        ShowVehicleTypeDelegate del1 = vehicle1.ShowVehicle;
        del1();
        //Note that it is expecting a Vehicle(i.e. a basetype) but received a Bus(subtype)
        //Still this is allowed through Covariance
        ShowVehicleTypeDelegate del2 = bus1.ShowBus;
        del2();
    }
}

Result

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