Question

I’m attempting to convert Java code to C# but I’m running into a problem when it comes to generic casts.

I have an interface type called Copyable which exposes the following method…

interface Copyable { void copyTo(Copyable target); }

And an AtomicObject type which takes as a parameter a generic type which is restricted to implement the Copyable interface…

class AtomicObject<T> where T : class, Copyable {
    public T openRead();

I store AtomicObjects in a Dictionary of object types, but I need to cast the objects back to AtomicObjects to call the “openRead” method that is specific to the AtomicObject class…

foreach (KeyValuePair<object, object> entry in dict)
{
    AtomicObject<Copyable> obj = (AtomicObject<Copyable>)entry.Key;
    Copyable dest = (Copyable)obj.openRead();//get the destination
}

I have implemented a ListNode type which plays the role of the parameter for the AtomicObject…

class ListNode<T> : Copyable
AtomicObject<ListNode<string>> atomic = new AtomicObject<ListNode<string>>();

I can compile this code, but when I run this program it produces an “InvalidCastException” because the runtime system is unable to convert an object of type…

AtomicObject<ListNode<string>> 

to…

AtomicObject<Copyable>. 

Yet the ListNode is of type Copyable. Can anyone explain why this cannot be cast correctly? Most importantly, can anyone suggest a fix which does not rely on the Dictionary or the foreach loop having to know what type of parameter a ListNode contains? For example, in Java I could have done…

AtomicObject<?> obj = (AtomicObject<?>)entry.Key;

But this is not possible in C-Sharp.

Answer 1

You won’t be able to do that, because you need covariance, and classes can’t be covariant.

So you’ll have to extract interface from your AtomicObject:

interface IAtomicObject<out T> where T : class, Copyable
{
    public T openRead();
}

(please note out in generic declaration), and make your AtomicObject class implement it.

After that you should be able to run following code

foreach (KeyValuePair<object, object> entry in dict)
{
    IAtomicObject<Copyable> obj = (IAtomicObject<Copyable>)entry.Key;
    Copyable dest = obj.openRead();
}

Also please note that .net standards recommend prefixing interface names with I.