Understanding Default Type Mapping with Codon FX

25 Mar 2018

Codon FX is a cross-platform framework for building maintainable applications. I use it for all of my .NET based applications. Codon is built on .NET Standard and uses platform specific assemblies to support various platforms such as Xamarin Android, iOS, WPF, and UWP. It’s a zero-configuration framework. By that I mean that it doesn’t require bootstrapping; services used internally and in user code are resolved automatically, despite being potentially located in platform specific assemblies. The way that Codon resolves platform specific services deserves some explanation. That’s the purpose of this post.

There are various abstractions within Codon, such as the ISynchronizationService, IDialogService, IClipboardService, INavigationMonitor, ISettingsService, IMemoryUsage, IPowerService, and IStateManager to name just a few. If you take a look at one of these services you’ll notice that it is decorated with a Codon.InversionOfControl.DefaultTypeAttribute and/or a Codon.InversionOfControl.DefaultTypeName. These attributes give Codon’s IoC container FrameworkContainer a hint on how to resolve service implementations at run-time. For example, the IDialogService is a decidedly platform specific service. It is used to display dialogs, ask the user questions, and popup toasts. Implementations exist for it in each of the platform specific assemblies. The IDialogService is decorated with a DefaultTypeName attribute, as shown in the following excerpt:

[DefaultTypeName(AssemblyConstants.Namespace + "." 
        + nameof(DialogModel) + ".DialogService, " 
        + AssemblyConstants.PlatformAssembly, Singleton = true)]
public interface IDialogService

Consumers of the IDialogService retrieve the service implementation either via dependency injection (constructor or property), or by leveraging the IoC container directly. This is ordinarily done by calling the static Dependency.Resolve<IDialogService>() method.

When a request to resolve the IDialogService arrives, the FrameworkContainer looks for a registered type implementing IDialogService. If it doesn’t find one, it then turns to the DefaultTypeName and DefaultType attributes to locate the implementation. In the case of the IDialogService it probes for the implementation using the fully qualified type name Codon.DialogModel.DialogService, Codon.Platform. Notice that the assembly name is Codon.Platform. Codon uses this assembly name for all of its platform specific core assemblies. Thus making it easy to reference types in, for example, XAML regardless of what platform the application is running on.

NOTE: The same convention used by the Codon.Platform assemblies is also used by the platform specific Extras assemblies; platform specific Extras assemblies are all named Codon.Extras.Platform.

NOTE: When set to true the Singleton property of the DefaultType and DefaultTypeName attributes indicate to the IoC container that only one instance of the specified mapped type should be created, which is reused for all subsequent requests. When the Singleton property is set to false, the IoC container creates a new instance of the default type upon each request. The default value of the Singleton property is true.

The IDialogService only makes use of the DefaultTypeName attribute. That’s because there isn’t a non-platform-specific implementation for this service. There are, however, other services that do have default implementations, such as the IExceptionHandler interface. The IExceptionHandler allows your application to process exceptions raised within other services and types. The commanding infrastructure uses the IExceptionHandler to hand-off exceptions that are thrown during command excecution, potentially not from the UI thread. This saves you from having to wrap all your command logic in try/catch blocks. But, I digress. The default implementation for the IExceptionHandler is the LoggingExceptionHandler and it is specified by decorating the IExceptionHandler with a DefaultType attribute, as shown in the following excerpt:

public interface IExceptionHandler

The default implementation of the IExceptionHandler simply logs the exception message using the registered ILog instance and returns true; indicating to the caller that the exception should be re-thrown. See Listing 1.

NOTE: The IExceptionHandler interface is one service for which you should really consider providing your own implementation.

Listing 1. LoggingExceptionHandler Class

class LoggingExceptionHandler : IExceptionHandler
    public bool ShouldRethrowException(
        Exception exception, 
        object owner, 
        string memberName = null, 
        string filePath = null,
        int lineNumber = 0)
        var log = Dependency.Resolve<ILog>();
        if (log.ErrorEnabled)
            log.Error("LoggingExceptionHandler: Unhandled exception occurred. " + owner,
                    exception, null, memberName, filePath, lineNumber);

        return true;

We’ve seen how Codon’s DefaultType and DefaultTypeName attributes are used to resolve default implementations. But, what happens if you want to specify a default implementation from an assembly that does not reference the Codon core .NET Standard library.

To achieve that, you can use the System.ComponentModel.DefaultValueAttribute. This attribute allows you to specify a default implementation as demonstrated in the following example:

public interface IUndoService

NOTE: When using System.ComponentModel.DefaultValueAttribute to specify a default implementation, the implementation is considered to be a singleton. Unlike Codon’s DefaultType and DefaultTypeName attributes, the DefaultValueAttribute does not allow you to provide other parameters.

In this post we’ve seen how Codon is a zero-configuration framework; it doesn’t require bootstrapping. Services used internally and in user code are resolved automatically, despite being potentially located in platform specific assemblies. Codon’s DefaultType and DefaultTypeName attributes are used to specify default type mappings. Types can also being specified using the System.ComponentModel.DefaultValueAttribute, allowing you to specify default type mappings within class libraries that do not reference Codon’s core assembly.

I hope you find this post useful. Have a great day!