ResolutionFailedException with Unity

Question

I'm using Patterns and Practices' Unity to inject dependencies into my objects and have hit a weird (to me, anyway) issue. Here's my class definitions:

public class ImageManager : IImageManager 
{
    IImageFileManager fileManager;

    public ImageManager(IImageFileManager fileMgr)
    {
        this.fileManager = fileMgr;

    }
}

public class ImageFileManager : IImageFileManager
{
    public ImageFileManager(string folder)
    {
        FileFolder = folder;
    }
 }

And here's the code to register my classes

container.RegisterInstance<MainWindowViewModel>(new MainWindowViewModel())
         .RegisterType<IPieceImageManager, PieceImageManager>(
              new InjectionConstructor(typeof(string)))
         .RegisterType<IImageFileManager, ImageFileManager>()
         .RegisterType<IImageManager, ImageManager>(
              new InjectionConstructor(typeof(IImageFileManager)));

I originally resolved this in the code behind (I know, it defeats the purpose. Bear with me.) of the XAML file like this

IImageManager imageManager = MvvmViewModelLocator.Container.Resolve<IImageManager>(
    new ParameterOverride("folder", "/images"));

And it worked. But I created a view model for my main view and when I copied the same line into it, I get an exception. Here are the two most inner exceptions:

InnerException: Microsoft.Practices.Unity.ResolutionFailedException
  HResult=-2146233088
  Message=Resolution of the dependency failed, type = "SwapPuzzleApp.Model.IImageManager", name = "(none)".
Exception occurred while: while resolving.
Exception is: InvalidOperationException - The type IImageManager does not have an accessible constructor.

At the time of the exception, the container was:

Resolving SwapPuzzleApp.Model.IImageManager,(none)

        Source=Microsoft.Practices.Unity
        TypeRequested=IImageManager
        StackTrace:
             at Microsoft.Practices.Unity.UnityContainer.DoBuildUp(Type t, Object existing, String name, IEnumerable`1 resolverOverrides)
             at Microsoft.Practices.Unity.UnityContainer.DoBuildUp(Type t, String name, IEnumerable`1 resolverOverrides)
             at Microsoft.Practices.Unity.UnityContainer.Resolve(Type t, String name, ResolverOverride[] resolverOverrides)
             at Microsoft.Practices.Unity.UnityContainerExtensions.Resolve[T](IUnityContainer container, ResolverOverride[] overrides)
             at SwapPuzzleApp.ViewModel.MainWindowViewModel..ctor() in c:\Users\Carole\Documents\Visual Studio 2012\Projects\SwapPuzzle\SwapPuzzle\ViewModel\MainWindowViewModel.cs:line 17
             at SwapPuzzleApp.ViewModel.MvvmViewModelLocator..cctor() in c:\Users\Carole\Documents\Visual Studio 2012\Projects\SwapPuzzle\SwapPuzzle\ViewModel\MvvmViewModelLocator.cs:line 51
        InnerException: System.InvalidOperationException
             HResult=-2146233079
             Message=The type IImageManager does not have an accessible constructor.
             Source=Microsoft.Practices.Unity
             StackTrace:

             StackTrace:
                  at Microsoft.Practices.ObjectBuilder2.DynamicMethodConstructorStrategy.ThrowForNullExistingObject(IBuilderContext context)
                  at lambda_method(Closure , IBuilderContext )
                  at Microsoft.Practices.ObjectBuilder2.DynamicBuildPlanGenerationContext.<>c__DisplayClass1.<GetBuildMethod>b__0(IBuilderContext context)
                  at Microsoft.Practices.ObjectBuilder2.DynamicMethodBuildPlan.BuildUp(IBuilderContext context)
                  at Microsoft.Practices.ObjectBuilder2.BuildPlanStrategy.PreBuildUp(IBuilderContext context)
                  at Microsoft.Practices.ObjectBuilder2.StrategyChain.ExecuteBuildUp(IBuilderContext context)
                  at Microsoft.Practices.Unity.UnityContainer.DoBuildUp(Type t, Object existing, String name, IEnumerable`1 resolverOverrides)
             InnerException:

I'm not sure what the problem is, as ImageManager clearly has a public constructor. I thought it might be due to an invalid path, but if I concretely instantiate the object, everything works.

// this line has no problems
IImageManager imageManager = new ImageManager(new ImageFileManager("/images"));

I also wondered if I needed to pass in new InjectionConstructor(typeof(string)) when I register IImageManager, but it doesn't seem to help and why would it be needed now and not before? So I'm stumped. This is my first attempt at using Dependency Injection, so it's probably something basic. I'm just not seeing what, though.

You are resolving IImageManager and passing in parameter to the contructor called "folder" of type string. But ImageManager doesn't have a constructor that matches that definition; ImageManager's contructor accepts IImageFileManager. — Randy supports Monica
Hm. That did look a bit odd, but it worked when I put it in MainWindow.xaml.cs, so I assumed the line is correct. Any guesses about why it would work in one file and not another? — VanillaBean
I left this alone for several months and finally returned to it. I see now that I was doing things very, very wrong. — VanillaBean

Chris Tavares Chris Tavares · Accepted Answer · 2013-07-12T06:57:41

Look very closely at the error message. Notice this part:

 Message=The type IImageManager does not have an accessible constructor.

Notice the type name is IImageManager, not ImageManager. Somewhere along the line you lost your type mapping.

Your registration of FileImageManager has a problem as well, since you don't specify the folder parameter in the registration, so Unity has no idea what string to pass.

ResolutionFailedException with Unity

2 Answers