Question

I’ve recently started using MEF in my application, and have had no issues using it or testing classes that it acts upon inside of my main application; however, I’m just now starting to create a licensing library for my application, have run into some issues, since I’m unsure of how to set it up.

In my licensing library, I’ve got LicenseManager, LicenseValidator, and LicenseSigner classes:

public class LicenseManager
{
    [Import]
    private ILicenseValidator _validator;

    [Import]
    private ILicenseSigner _signer;

    public LicenseManager()
    {
        var catalog = new AssemblyCatalog(System.Reflection.Assembly.GetExecutingAssembly());
        var container = new CompsitionContainer();
        container.ComposeParts();
    }
}

[Export(typeof(ILicenseSigner))]
public class LicenseSigner : ILicenseSigner
{
    ...
}

[Export(typeof(ILicenseValidator))]
public class LicenseValidator : ILicenseValidator
{
    ...
}

I find that both the validater and the signer are null after accessing them.

Which I assume is due to the fact that I’m instantiating LicenseManager before the constructor for LicenseManager can create the container; however, I’m not entirely sure how to solve the problem, since LicenseManager is intended as the only entry point into the class library. I could possibly create some sort of factory to initialize the container and return a LicenseManager instance, but that would result in a less nice library API.

I’m sort of new to the whole MEF thing. Is there something I could be doing better? Is there a way for me to take care of all the MEF loading in my main application? Any solution is better than the lack of one that I have now.

Answer 1

The ComposeParts call needs at least one attributed object to compose. You should pass in the LicenseManager as the argument to ComposeParts. So, your full set of code would look like:

public class LicenseManager
{
    [Import]
    private ILicenseValidator _validator;

    [Import]
    private ILicenseSigner _signer;

    public LicenseManager()
    {
        var catalog = new AssemblyCatalog(System.Reflection.Assembly.GetExecutingAssembly());
        var container = new CompsitionContainer(catalog);
        container.ComposeParts(this);
    }
}

[Export(typeof(ILicenseSigner))]
public class LicenseSigner : ILicenseSigner
{
    ...
}

[Export(typeof(ILicenseValidator))]
public class LicenseValidator : ILicenseValidator
{
    ...
}

In my testing, the above code worked.

Another idea would be to delegate the choosing of the catalogs to the code implementing the library. Your constructor would become:

public LicenseManager(AggregateCatalog catalog)
{
    var container = new CompsitionContainer(catalog);
    container.ComposeParts(this);
}

This allows the program implementing your licensing library to specify where the exports will come from.