How can I overcome the restriction of Windows components for Windows environment types?

I have a Windows Store app that uses a background task. The background task is stored in a Windows Runtime Component project. (This structure seems to be the only way to get background tasks to work.)

In a background task project, I have external visible public methods that have return / parameter types that are my own classes and not Windows Runtime classes.

For instance:

public MyClass DoSomething()
{
    return null;
}

When I build, I get errors such as the following regarding these methods:

The "X" method returns "Y", which is not a valid Windows runtime type. Windows Runtime-related methods should only return Windows run-time types.

and

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