Casting List<> of Derived Class to List<> of Base Class

Convert List DerivedClass to List BaseClass

The way to make this work is to iterate over the list and cast the elements. This can be done using ConvertAll:

List<A> listOfA = new List<C>().ConvertAll(x => (A)x);

You could also use Linq:

List<A> listOfA = new List<C>().Cast<A>().ToList();

Casting List of Derived class to List of base class

The reason you cannot do this is because a list is writable. Suppose it were legal, and see what goes wrong:

List<Cat> cats = new List<Cat>();
List<Animal> animals = cats; // Trouble brewing...
animals.Add(new Dog()); // hey, we just added a dog to a list of cats...
cats[0].Speak(); // Woof!

Well dog my cats, that is badness.

The feature you want is called "generic covariance" and it is supported in C# 4 for interfaces that are known to be safe. IEnumerable<T> does not have any way to write to the sequence, so it is safe.

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>());    
    }    
}  

That will work in C# 4 because List<Cat> is convertible to IEnumerable<Cat>, which is convertible to IEnumerable<Animal>. There is no way that Play can use IEnumerable<Animal> to add a dog to something that is actually a list of cats.

Convert List of Base Class to a List of Derived Class - possible?

Cast<T> method applies cast operation to all elements of the input sequence. It works only if you can do the following to each element of the sequence without causing an exception:

ParentClass p = ...
ChildClass c = (ChildClass)p;

This will not work unless p is assigned an instance of ChildClass or one of its subclasses. It appears that in your case the data returned from the server API contains objects of ParentClass or one of its subclasses other than ChildClass.

You can fix this problem by constructing ChildClass instances, assuming that you have enough information from the server:

List<ChildClass> childList = parentList
    .Select(parent => new ChildClass(parent.Name, ... /* the remaining fields */))
    .ToList();

Cannot convert from List DerivedClass to List BaseClass

It is because List<T> is in-variant, not co-variant, so you should change to IEnumerable<T> which supports co-variant, it should work:

IEnumerable<BaseClass> bcl = new List<DerivedClass>();
public void doSomething(IEnumerable<BaseClass> bc)
{
    // do something with bc
}

Information about co-variant in generic

Most efficient way to cast List SubClass to List BaseClass

The syntax for this sort of assignment uses a wildcard:

List<SubClass> subs = ...;
List<? extends BaseClass> bases = subs;

It's important to realize that a List<SubClass> is not interchangeable with a List<BaseClass>. Code that retains a reference to the List<SubClass> will expect every item in the list to be a SubClass. If another part of code referred to the list as a List<BaseClass>, the compiler will not complain when a BaseClass or AnotherSubClass is inserted. But this will cause a ClassCastException for the first piece of code, which assumes that everything in the list is a SubClass.

Generic collections do not behave the same as arrays in Java. Arrays are covariant; that is, it is allowed to do this:

SubClass[] subs = ...;
BaseClass[] bases = subs;

This is allowed, because the array "knows" the type of its elements. If someone attempts to store something that isn't an instance of SubClass in the array (via the bases reference), a runtime exception will be thrown.

Generic collections do not "know" their component type; this information is "erased" at compile time. Therefore, they can't raise a runtime exception when an invalid store occurs. Instead, a ClassCastException will be raised at some far distant, hard-to-associate point in code when a value is read from the collection. If you heed compiler warnings about type safety, you will avoid these type errors at runtime.

Converting a List of Base type to a List of Inherited Type

If you have linq available you can do

var ListOfA = ListOfB.Cast<A>().ToList();

.OfType () Extract Derived types from a Base Type List

This is asking the framework to figure out a little bit too much:

List<Melee> melee = chars.Where(c => c is Melee).ToList(); // doesnt work

Even though you're using Where to filter out items that are only of type Melee, the list you're filtering is of type Character. So no matter how you filter the list of Character, the result is an IEnumerable<Character>, or List<Character> after calling ToList().

You can't assign a List<Character> to a variable declared as List<Melee>.

You could do

List<Melee> melee = chars.Where(c => c is Melee).Cast<Melee>().ToList();

but the previous line where you use OfType is simpler and clearer. OfType is doing the same thing - selecting elements of the given type and casting them as that type.

Copying a List BaseClass to List DerivedClass

List<DerivedClass> result = 
    listBaseClass.ConvertAll(instance => (DerivedClass)instance);

Actually ConvertAll is good when you need to create new objects based on the original, when you just need to cast you can use the following

List<DerivedClass> result = 
    listBaseClass.Cast<DerivedClass>().ToList();

If not all of the items in your list can be cast to DerivedClass then use OfType instead

List<DerivedClass> result =
    listBaseClass.OfType<DerivedClass>().ToList();

Downcasting a list of objects in C#

Using LINQ:

    var baseList = new List<BaseClass>();
    var derivedList = baseList.Cast<DerivedClass>();

Note: Having to downcast usually is a 'smell' and indicates that the inheritance hierarchy is wrong, or wrongly implemented. The idea of having a base class is that you can treat all subclasses as superclass without having to downcast to individual subclass types.

Instead of Cast you might want to use OfType to 'fish out' certain derived classes from a collection of superclasses. But again, there should be no need to do that.

Ask yourself, why you need to have a subclass - maybe you need to move some functionality to base class?

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