Question

I’m using a third party library class that has the following (abbreviated) method:

public void DoSomethingAsync(Action<ResultInfo> callback)

Rather than have a reference to this third party library in my application code project, I’ve created a wrapper class that abstracts the third party dependencies. So, what I’m trying to do is have a wrapper method that looks like the following:

public void MyDoSomethingAsync(Action<MyResultInfo> callback)
{
    this.wrappedClass.DoSomethingAsync(callback);
}

The problem is that I need to convert the callback parameter from Action<MyResultInfo> to Action<ResultInfo>. Is this possible with a custom implicit cast operator or is there an alternative approach that someone can recommend?

Answer 1

No, you wouldn’t be able to do so directly, even if MyResultInfo derived from ResultInfo; delegate contravariance wouldn’t apply here as the Action<MyResultInfo> wouldn’t be able to handle any instance derived from ResultInfo (so that disqualifies an Action<MyResultInfo> from being used in place of an Action<ResultInfo>)

However, that doesn’t mean you can’t use an adapter, like so:

public void MyDoSomethingAsync(Action<MyResultInfo> callback)
{
    // Create the action which will transform ResultInfo
    // into MyResultInfo and then pass to the wrapped
    // class.
    // Create the action first.
    Action<ResultInfo> a = ri => {
        // Translate ri into MyResultInfo.
        MyResultInfo mri = (translate here);

        // Call the callback with the translated result.
        callback(mri);
    };

    // Pass the action to the wrapped class.
    this.wrappedClass.DoSomethingAsync(a);
}