Can I Cast a `List` to a `List` in C#?

Convert derived class list to base class list

Covariance means that a derived type can replace its base type without losing any functionality. In this case, we have a base class (Animal) and a derived class (Cat), where the base class contains a virtual method Play that accepts a List as input parameter.

The following code demonstrates the conversion problem:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication9
{
    class Animal
    {
        public virtual void Play(List<Animal> animals) { }
    }
    class Cat : Animal
    {
        public override void Play(List<Animal> animals)
        {
        }
    }

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

Compiling the code will result in the error: Argument 1: cannot convert from 'System.Collections.Generic.List' to 'System.Collections.Generic.List'.

This error occurs because List is a writable data structure. If we allow conversion from List to List, dogs can be added to the list of cats, which may lead to unexpected behavior.

To solve this problem, C# 4 introduced a concept called "generic covariance" for known-safe interfaces. IEnumerable is such an interface. Since IEnumerable does not allow mutation, it is considered covariantly safe.

We can solve the conversion problem by modifying the Play method to accept an IEnumerable instead of a List:

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>());
    }
}

By using the IEnumerable<t></t> interface instead of List<t></t>, we take advantage of the covariance feature of C#, avoid type conversion errors, and ensure the safety of the code. IEnumerable<t></t> only allows data to be read, not modified, so it is safe to convert List<cat></cat> to IEnumerable<animal></animal>.

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