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.netc#-4.0mef

MEF - Does every Import need an Export to be present?

I have tried searching for this from other posts, but was not able to find an answer, so creating a new question.

I am building an ETL system.

Now I have a class which has the following Imports:

        [Import]
        public IDataExtractor DataExtractor { get; set; }
        [Import]
        public IDataTransformer DataTransformer { get; set; }
        [Import]
        public  IDataValidator DataValidator { get; set; }
        [Import]
        public IDataPublisher DataPublisher { get; set; }
        [Import]
        public  IDataUpdater DataUpdater { get; set; }

My program.cs has the following code for composition:

            var catalog = new AggregateCatalog();
            if (!string.IsNullOrEmpty(ConfigKeys["Extractor"]))
                catalog.Catalogs.Add(new DirectoryCatalog(ConfigKeys["Extractor"]));
            if (!string.IsNullOrEmpty(ConfigKeys["Transformer"]))
                catalog.Catalogs.Add(new DirectoryCatalog(ConfigKeys["Transformer"]));
            if (!string.IsNullOrEmpty(ConfigKeys["Validator"]))
                catalog.Catalogs.Add(new DirectoryCatalog(ConfigKeys["Validator"]));
            if (!string.IsNullOrEmpty(ConfigKeys["Publisher"]))
                catalog.Catalogs.Add(new DirectoryCatalog(ConfigKeys["Publisher"]));
            if (!string.IsNullOrEmpty(ConfigKeys["Updater"]))
                catalog.Catalogs.Add(new DirectoryCatalog(ConfigKeys["Updater"]));
            var container = new CompositionContainer(catalog);
            container.ComposeParts(this);

Where ConfigKeys[] is my config management system, where I can specify, the path of the dll for each extractor, transformer, Validator, etc.

The full process is: Extraction, Transformation, Validation, Publish, Updating.

Certain times, all that is require are 3 functions to be executed: Extraction, Transformation,Publish. So if the Config system, does not have a path for the Validator dll, I dont want an instance of the IDataValidator interface. But if I specify a blank, I get an error during composition.

Error: The composition remains unchanged. The changes were rejected because of the following error(s): The composition produced multiple composition errors, with 4 root causes. The root causes are provided below. Review the CompositionException.Errors property for more detailed information. "The composition produced a single composition error. The root cause is provided below. Review the CompositionException.Errors property for more detailed information.\r\n\r\n1) No exports were found that match the constraint: \n\tContractName\tCTP.DCU.Models.Interfaces.IDataTransformer\n\tRequiredTypeIdentity\tCTP.DCU.Models.Interfaces.IDataTransformer\r\n"

My question is: For every IMPORT, I may not have an EXPORT and I want to handle that gracefully. Is there a way to do that in MEF? How can I handle this scenario? I thought of creating empty dummy stubs for substituting, but really is that what everyone does?

Thank you!

Solution

  • If you want to make an [Import] optional, you can use AllowDefault:

    [Import(AllowDefault = true)]
public IDataTransformer DataTransformer { get; set; }

    If there is no matching [Export] then the DataTransformer property will be left null.