Why can't I cast this interface to a concrete class?

Question

I have an interface IApiDataWithProperties. A class called Event implements this interface.

Normally I would be able to cast an object of IApiDataWithProperties to Event (assuming it is one) and for the compiler to let me do that no problem.

In this case, the type is actually a generic TApiData which has a where restriction to interface IApiDataWithProperties. However, I am unable to cast TApiData to Event even with the type restriction. I get Cannot convert type 'TApiData' to 'Event'

Why is this? Am I missing something?

public class Event : IApiDataWithProperties, IXmlSerializable
{
    // ...
}


public abstract class AbstractBatchPropertyProcessor<TApiData> : AbstractBatchProcessor<TApiData>, IBatchProcessor
    where TApiData : IApiDataWithProperties
{
    protected virtual string Build(ConcurrentBag<TApiData> batch)
    {
        foreach (var newItem in batch)
        {   
           if(newItem is Event)
           {
              // This cast fails: Cannot convert type 'TApiData' to 'Event'
              ((Event)newItem).Log();
           }
        }

        // ...  
    }
}

Edit:

I'm just looking to know why this is a compilation error.

I'm aware this is a strange design, and you wouldn't normally cast like that inside a generic method anyway. This was something I came across when I wanted to add some quick logging info there during a test, and this was the path of least effort.

Answer 1

The reason is that newItem can be any type that implements IApiDataWithProperties, so compiler can't guarantee that it's type is convertible to Event. Even if you check it using is operator it means nothing to the compiler. As a workaround you can use double cast:

((Event)(object)newItem).Log();

Even though this works, it doesn't mean you should use it. You shouldn't check the type in a generic method. Instead try using polymorphism, add the Log method to IApiDataWithProperties or some other interface and implement it in your types. Then have another constraint for that interface, then you can call the method without needing a cast.

Answer 2

Short version - you can use:

(newItem as Event).Log();

instead of your cast.

Long version - your TApiData objects implements IApiDataWithProperties so it can be Event, but it can be everything else implementing it. This is called downcasting and must be done at runtime with help of as/is operators. Compiler does not know during compilation if newItem of generic type is really an Event so it cannot assure such cast.

Answer 3

Because the where restriction promises that TApiData is an IApiDataWithProperties; it does not guarantee about IApiDataWithProperties implemented types.

MORE:

Every Child would be an instance of Parent, but not vice versa. Think about this model:

interface I { void InterfaceNethod(); }

class A : I {
    void InterfaceMethod() { }
    void AMethod() { }
}

class B : I {
    void InterfaceMethod() { }
    void BMethod() { }
}

Now, lets get some instances, and call their methods:

I i = new A();
i.InterfaceMethod(); // it works
i.AMethod(); // it doesn't work, cause I has not a method named AMethod

And now, castings:

A a = new A();
a.InterfaceMethod(); // exists
a.AMethod(); // exists

I i = (I)a; // correct
i.InterfaceMethod(); // exists

B b = (B)i;
// if this be correct, then we should be able to call B's methods on b, right?
// While b hasn't any of B's members.
// I mean calling this:
b.BMethod();
// is logically incorrect. right? because, following the object's reference in memory
// would tell us that b is pointing to an A instance actually. Am I right?
// so the cast will fail. Because compiler knows about logic :) a little bit at least. cheers

Answer 4

You could declare the Log method inside the interface IApiDataWithProperties. Implement the Log method in your Event class and then where you are getting the exception cast the newItem to IApiDataWithProperties rather than Event.

((IApiDataWithProperties)newItem).Log

Or actually thinking about it you might not need a cast and can do

 newItem.Log

Answer 5

Your design is not the best to start with as it assumes an implementation of a specific class. But in case you want it done this way....

using as will work but you probably want it restricted also to reference types.

 where TApiData : IApiDataWithProperties, class

...

 (newItem as Event).Log();

Though this is not entirely safe as it can result in null you probably should check for that also

 var item = newItem as Event;
 if(item != null)
     item.Log();

You also can eliminate the line if(newItem is Event) as it is not required.