CODEWITHSUNDEEP
c#wpfmvvmservicecatel
Kilazur
Kilazur

Reputation: 3188

NullReferenceException when MyViewModel is instanciated with service dependency injection

I try using the best approach to use services in Catel, which is to inject services in the viewmodel's constructor:

public MyViewModel(IMessageService msgService)
{
    Argument.IsNotNull(() => msgService);
    this.messageService = msgService;
    //stuff
}

Yet when I put arguments in this constructor, my program throws an NullReferenceException, supposedly at the Argument.IsNoNull line (debugging through Catel's code seems to be a pain). When I remove the argument or the IsNotNull validation, it launches fine.

So I do not receive a IMessageService, msgService is always null. What am I doing wrong? Missing assembly?

EDIT: It seems like the IMessageService type is registered. I manage to get a reference by using ResolveType:

messageService = ServiceLocator.Default.ResolveType<IMessageService>();

So as far as my project is concerned, the problem is solved. But it doesn't answer the original question: why isn't dependency injection working?

Upvotes: 0

Views: 169

Answers (3)

MrWombat
MrWombat

Reputation: 649

The code for Argument can be found on Git, The usage is correct, i fear you have to check the injection. Try to set a breakpoint in the constructor and go through the code until it reaches the breakpoint.

Are you using a special DI-Framework? Yes - then check the registration/setup otherwise find the code that composes the ViewModel

I've just seen in the documentation that the types are set up automatically. Is it possible to create the object-graph for your service (are there any circular dependencies, or dependencies that cannot be created due to own dependencies or not matching constructors?)

Upvotes: 0

Geert van Horrik
Geert van Horrik

Reputation: 5724

If you get a null, it means the dependency cannot be resolved. If you are using the default ServiceLocator of Catel, enable logging to see what's going on under the hood in your application startup:

#if DEBUG
    LogManager.AddDebugListener();
#endif

You can also check if the type is registered by using this code:

var serviceLocator = ServiceLocator.Default;
serviceLocator.IsTypeRegistered<IMessageService>();

Upvotes: 3

sanguaire
sanguaire

Reputation: 76

Have you tried this?

Argument.IsNotNull("msgService", msgService);

In the documentation video they do similar with Argument.ArgumentIsNullOrWhitespace

Upvotes: 0

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