Question

interface IExecutor
{
   void Execute();
}

class Executor2<T> where T : IExecutor
{
    public void Execute()
    {
        var ex = (T)Activator.CreateInstance(typeof(T));
        ex.Execute();
    }
}

It was a question at an interview. They told me that sometimes this code falls (causes exceptions) and there are at least 3 reasons that could cause the problems. It’s unknown what exceptions were there. But the method Execute was created good, its implementation has no fault.

Does anybody have a suggestion about that?

Edit: There are at least 3 reasons that could cause the problems. What are these reasons?

Answer 1

On the face of it I can see a few issues.

1. The code doesn’t compile, but I can ignore that and make it compile.
2. The code doesn’t do what you think it does.

To explain 2: you specify a type T at the class definition with a constraint on IExecutor, but you then define another type T at the method level without a constraint. This doesn’t compile.

If I fix this and remove the <T> definition from the method I can see a number of reasons for it failing without much warning:

1. ex is null.
2. Type T doesn’t have a public parameterless constructor defined.
3. Perhaps it cannot load the DLL containing T.

As spotted by Jakub:

1. T could be an interface (no constructor).
2. T could be an abstract class, these don’t allow instances to be created directly.

The first can be guarded against using a null check if (ex != null) and the second can be guarded against using another generic constraint new():

class Executor2<T> where T : IExecutor, new()
{
}

Obviously you could also amend your code to including exception logging. This might be useful in figuring out what the actual problems are instead of just stabbing in the dark:

public void Execute<T>()
{
    try
    {
        var ex = (T)Activator.CreateInstance(typeof(T));
        ex.Execute();
    }
    catch (Exception ex)
    {
        Log(ex); // Mystical logging framework.
        throw;
    }
}

This is the only answer I can cobble together considering I didn’t understand the question.

If I were asked this in an interview I’d likely say I couldn’t name all 3, but I’d know how to change the code to be more maintainable and tell me what was wrong. Then I’d likely walk for asking pointless interview questions.