Inject many classes in view model class

Question

I develop wpf app with MVVM design. As MVVM framework I use Caliburn Micro. Service I have in external assembly, and it is injected in view models classes with MEF.

Problem in service I have many small class, I try respect SOLID principe.

[Export(typeof(IClassA)]
public class ClassA : IClassA
{}

[Export(typeof(IClassB)]
public class ClassB : IClassB
{}

[Export(typeof(IClassC)]
public class ClassC : IClassC
{}

//...
[Export(typeof(IClassK)]
public class ClassK : IClassK
{}

Classes count is about 12-15.

I need use this classes in view model class. So I use this:

public class MyViewModelClass
{
 private interface IClassA _a;
 private interface IClassB _b;
 private interface IClassC _c;


//...
 private interface IClassK _k;


[ImportingConstructor]
public MyViewModelClass(IClassA a, IClassB b, IClass c, ..., IClassK k)
{
_a=a; _b=b; _c=c; ...  _k=k

}

}

I don’t that this way is correct. Or it exist something elegant, simple. Thank for your opinion and advices.

score 1 · Answer 1 · 2011-01-14T22:06:43.057

1

To: Abe Heidbrecht.

I am little confuse. If I use this:

public class MyViewModelClass : IPartImportsSatisfiedNotification
{
    [Import]
    private IClassA _a;  // I need init _a?
    [Import]
    private IClassB _b; 
    [Import]
    private IClassC _c;

    ...

    public void OnImportsSatisfied()
    {
        // what code must be here ?
        //how can I check if all import was success
    }
}

Because.

IPartImportsSatisfiedNotification has only a single method: OnImportsSatisfied, which is called when all imports that could be satisfied have been satisfied.

I think it is better for unit testing init interfaces of services class in contstructor of view model class.

edited Jan 14 '11 at 22:06

answered Jan 14 '11 at 22:01

OnImportsSatisfied won't be called unless all imports were successful. Basically the code that goes in that method would be the code you put in your constructor to initialize your object normally in your constructor. Since the imports won't be available in your constructor, you can't use them until that method is called. If any imports can't be satisfied, the normal MEF exceptions will be thrown, alerting you to missing exports. – Abe Heidebrecht Jan 20 '11 at 15:46

score 0 · Accepted Answer · answered Jan 14 '11 at 21:27

MEF can import to fields (even private ones). If you want to make your life a little easier, you can just decorate the fields with the ImportAttribute. If you want to know when all the imports have finished, just implement the IPartImportsSatisfiedNotification interface:

public class MyViewModelClass : IPartImportsSatisfiedNotification
{
    [Import]
    private IClassA _a; 
    [Import]
    private IClassB _b; 
    [Import]
    private IClassC _c;

    ...

    public void OnImportsSatisfied()
    {
        // add initialization code here
    }
}

This pretty much requires you to use MEF to ensure that your ViewModel is instantiated correctly, but depending on your scenario that may not be a big deal.

Inject many classes in view model class

2 Answers2