ComposeParts method not working

Question

I've got an ITagger and an IWpfTextViewMargin, both are exported as MEF components. I want to import the ITagger in my Margin code, and then use some members in that Tagger.

Now I tried to use ComponentContainer in the Margin class, then import the IViewTaggerProvider. I used the following code, which can be found in many MEF tutorials

[Import(typeof(IViewTaggerProvider))]
public IViewTaggerProvider vt_provider { get; set; }

var catalog = new AggregateCatalog();
catalog.Catalogs.Add(new AssemblyCatalog(typeof(TestMargin).Assembly));
_container = new CompositionContainer(catalog);
//Fill the imports of this object
try
{
    this._container.ComposeParts(this);
}
catch (CompositionException compositionException)
{
    System.Diagnostics.Trace.WriteLine(compositionException.Message);
}

and the export code.

[Export(typeof(IViewTaggerProvider))]
[ContentType...

The exported class is defined in another namespace but same assembly.

Here I got problem that ComposeParts(this) throws ImportCardinalityMismatchException. I don't know why the parameter is this. I tried to pass the catalog to it, there is no exception but the import is also null. I also referred to debug mef failures and believe that the exported class has the right contract name and export type identity.

---

After checking the assembly with Visual MEFx and debugging, I found that probably it's because the IViewTaggerProvider imports a Visual Studio IClassificationTypeRegistryService, which is also an MEF part and results in a rejection of the IViewTaggerProvider.

[Primary Rejection]
 [Exception] System.ComponentModel.Composition.ImportCardinalityMismatchException: No valid exports were found that match the constraint '((exportDefinition.ContractName == "Microsoft.VisualStudio.Text.Classification.IClassificationTypeRegistryService") AndAlso (exportDefinition.Metadata.ContainsKey("ExportTypeIdentity") AndAlso "Microsoft.VisualStudio.Text.Classification.IClassificationTypeRegistryService".Equals(exportDefinition.Metadata.get_Item("ExportTypeIdentity"))))', invalid exports may have been rejected.

So one solution is to add the assembly that exports IClassificationTypeRegistryService. It's a Visual Studio core editor service but I cannot find which assembly exports it. Anyone knows this?

Or any better possible solutions?

Answer 1

Try VisualMEFx. Here is a short blog entry about getting started https://ihadthisideaonce.com/2012/02/22/getting-started-with-visual-mefx/. Once you have it up and running, use VisualMEFx to load the TestMargin assembly and see if any IViewTaggerProvider is exported from that assembly.

Also remember that ImportCardinalityMistmatch doesn't only mean that an export is missing. It can also mean that there are too many exports available that can satisfy the import and MEF has no way of choosing which one to use. So when you are examining your composition in VisualMEFx, check to see if there are too many.

This parameter:

void Bootstrap()
{
  var catalog = new AggregateCatalog();
  catalog.Catalogs.Add(new AssemblyCatalog(typeof(TestMargin).Assembly));
  _container = new CompositionContainer(catalog);

 //Fill the imports of this object
 try
 {
    var objectToSatisfy = this;
    // var objectToSatifsy = new SomeOtherObjectWithImports();

    this._container.ComposeParts(objectToSatisfy);
 }
 catch (CompositionException compositionException)
 {
    System.Diagnostics.Trace.WriteLine(compositionException.Message);
 }
}

When you call ComposeParts you pass an object to the method. MEF will take the object that you pass and see if there are any imports on it that need to be satisfied. If it finds any imports, it will look in the catalog and try to satisfy them. You can pass any object you want to the ComposeParts method. So I've modified your sample code a little to show two different options. One option is to create some object that needs to be satisfied, and then give it to the container for composition. This is what I have done in the commented out line var objectToSatisfy = new SomeOtherObjectWithImports(). But it is often the case that the object we want to compose is the same object that's calling ComposeParts. So we don't need to create a new object to pass to the container, we already have the object, we just need a reference to it. In C# we can get a reference to the current object instance using the keyword this (in VB.NET the keyword is Me). So, when we want to satisfy imports on the same object that is calling ComposeParts, we can do so by using the this reference as the argument to ComposeParts.

The argument to the ComposeParts method is a parameter array. Informally, this just means that when you write container.ComposeParts(this) it is interpreted as if you had written container.ComposeParts(new object[] { this }). In practice this means you can pass multiple objects to MEF at once, like this:

container.ComposeParts(this, objectToSatifsy, thirdObjectToCompose);

If the object calling ComposeParts has no imports on it, then you should not be using this as the argument. Instead, create an object of the type that you want to compose and pass that to the method. Also, unless all the parts that you want to compose are available in the TestMargin assembly, you need to create more AssemlbyCatalogs for the assemblies that do provide the parts and add them to your AggregateCatalog.