c# - MEF Constructor Injection

Question

I'm trying to figure out MEF's Constructor Injection attribute. I have no idea how I tell it to load the constructor's parameters.

This is the property I'm trying to load

[ImportMany(typeof(BUsers))]
public IEnumerable<BUsers> LoadBUsers { get; set; }

Here is the code I'm using to import the assemblies.

try
{
    var catalog = new AggregateCatalog();
    catalog.Catalogs.Add(new AssemblyCatalog(System.Reflection.Assembly.GetExecutingAssembly()));
    catalog.Catalogs.Add(new DirectoryCatalog("DI")); 
    var container = new CompositionContainer(catalog);
    container.ComposeParts(this);
}

Here is the class I'm trying to load

[Serializable]
[Export(typeof(BUsers))]
public class EditProfile : BUsers
{
    [ImportingConstructor]
    public EditProfile(string Method, string Version)
    {            
        Version = "2";
        Action = "Edit";
        TypeName = "EditProfile";
    }

question from:https://stackoverflow.com/questions/2008133/mef-constructor-injection

Answer 1

When you use the ImportingConstructor attribute, the parameters to the constructor become imports. By default, what you are importing (the contract name) is based on the type of the parameter or property that your are importing into. So in this case the contract type for both your imports is string, and there's no real difference between the first and second parameter.

It looks like you are trying to use imports to supply configuration values, which isn't necessarily what it was designed for. To get it to do what you want, you should override the contract name for each of the parameters, like this:

[ImportingConstructor]
public EditProfile([Import("Method")] string Method, [Import("Version")] string Version)
{ }

Then you need exports for Method and Version in your container. One way to do this is just to add them directly:

var container = new CompositionContainer(catalog);
container.ComposeExportedValue("Method", "MethodValue");
container.ComposeExportedValue("Version", "2.0");
container.ComposeParts(this);

(Note that ComposeExportedValue is actually an extension method defined on the static AttributedModelServices class.)

If you want to read these values from a configuration file of some sort, you could create your own export provider which reads the configuration and provides the values in it as exports to the container.

An alternative way to handle this would be to just import an interface that provides access to the configuration values by name, and get the values you need from the body of the constructor.