How Can I Safely Cast a List to List in C#?

Convert List<Derived Class> to List<Base Class>

When dealing with inheritance, it is sometimes necessary to convert a list of derived class objects into a list of base class objects. However, this can lead to errors if not handled properly.

Error in list conversion using writable data

Consider the following code:

using System;
using System.Collections.Generic;

class Animal
{
    public virtual void Play(List<Animal> animal) { }
}

class Cat : Animal
{
    public override void Play(List<Animal> animal) { }
}

class Program
{
    static void Main(string[] args)
    {
        Cat cat = new Cat();
        cat.Play(new List<Cat>());
    }
}

This code will generate a compilation error due to a mismatch in the list object types. Animal expects a list of its own type (Animal), but this method is called with a list of Cat objects.

Error reason

This error occurs because the list is writable. If the conversion is allowed, it may cause data manipulation problems. For example, a dog object could be added to a list of cats, which would violate type safety.

Generic covariance

In C# 4, a feature called generic covariance was introduced. This allows a list of derived classes to be converted to a list of base classes, provided these types ensure type safety. An example of a safe type for generic covariance is IEnumerable<T>, which represents a sequence of objects but cannot be written to.

Implementing generic covariance

To implement generic covariance, you can change the method signature in the base class to use IEnumerable<T> as the parameter type, like this:

class Animal
{
    public virtual void Play(IEnumerable<Animal> animals) { }
}

class Cat : Animal
{
    public override void Play(IEnumerable<Animal> animals) { }
}

class Program
{
    static void Main()
    {
        Cat cat = new Cat();
    cat.Play(new List<Cat>());
    }
}

In this example, the Play method in Animal takes IEnumerable<Animal> as a parameter, which allows it to accept a sequence of Animal and Cat objects.

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