Question

I got a ViewModel which I export with MEF. I’d like this ViewModel to be initialized differently each time it’s being imported, according to an enum/specific object parameter that will be provided to it.

I’ve been reading a little on the subject and I found that maybe this –

http://msdn.microsoft.com/en-us/library/ee155691.aspx#metadata_and_metadata_views

would be able to fit my needs, but I’m not sure that this would be the best way to do it.

Another method I’ve been thinking about is importing the class normally, and then once I’ve an instance, to call a special initialization method that would receive my parameter. However this doesn’t seem like a classic MEF implementation, and maybe losses some of its “magic”.

I’m hoping someone would be able to point out for me what would be the recommended method to achieve this.

Thanks!

Answer 1

A workaround is exporting a factory that creates instances of your type. While this means you cannot directly import thos instances, it does have the benefit that the logic to create them is the responsability of the factory so users of the class do not have to know about it:

public class ServiceWithParameter
{
  public ServiceWithParameter( int a )
  {
    this.a = a;
  }

  private readonly int a;
}

[Export]
public class ServiceWithParameterFactory
{
  public ServiceWithParameterFactory()
  {
    instance = 0;
  }

  public ServiceWithParameter Instance()
  {
    return new ServiceWithParameter( instance++ );
  }

  private int instance;
}

  //now everywhere you need ServiceWithParameter:
[Import]
ServiceWithParameterFactory serviceFactory;

var instanceA = serviceFactory.Instance(); //instanceA.a = 0
var instanceB = serviceFactory.Instance(); //instanceB.a = 1

A more extensible way is telling the container you have a factory and an example is presented here: http://pwlodek.blogspot.com/2010/10/mef-object-factories-using-export.html