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

Converting a List of Derived Classes to a List of Base Classes in C#

This example demonstrates a common problem: passing a list of derived class objects (e.g., List<cat></cat>) to a method expecting a list of base class objects (List<animal></animal>). Direct assignment isn't allowed because a base class reference cannot directly point to a derived class object.

The Issue:

Attempting to pass a List<cat></cat> to a method with a List<animal></animal> parameter results in a compiler error.

The Solution: Using IEnumerable and Covariance

The solution leverages C#'s generic covariance. Instead of using List<animal></animal>, change the method parameter type to IEnumerable<animal></animal>. IEnumerable is a read-only interface; this prevents the method from modifying the original collection, addressing potential type safety concerns.

Revised Code:

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

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

class Program
{
    static void Main()
    {
        Cat myCat = new Cat();
        myCat.Play(new List<Cat>()); // This now compiles successfully
    }
}

This works because List<cat></cat> implicitly converts to IEnumerable<cat></cat>, and IEnumerable<cat></cat> is covariant with IEnumerable<animal></animal>. The Play method can now safely iterate through the collection without attempting to modify it. This approach maintains type safety while allowing for flexible method parameterization.

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