Dependency Injection

Using dependency injection in a Revit add-in can help you organize and manage dependencies, improve code maintainability, and make your code more testable. However, Revit add-ins have specific constraints due to Revit's architecture and the way add-ins are loaded. Here's how you can implement dependency injection in a Revit add-in:

Step 1: Define Your Dependencies

First, define the dependencies you want to inject into your add-in classes. These dependencies can be services, utility classes, or other components that your add-in relies on.

public interface IMyService
{
    void DoSomething();
}
 
public class MyService : IMyService
{
    public void DoSomething()
    {
        // Implementation
    }
}

Step 2: Set Up a Dependency Injection Container

To perform dependency injection, you need a container that can manage and provide instances of your dependencies. Popular .NET dependency injection containers include:

Here, we'll use Autofac as an example. Install the Autofac NuGet package into your project.

Step 3: Register Dependencies

In your add-in's entry point, which is typically the OnStartup method of your IExternalApplication class, configure the dependency injection container and register your dependencies.

using Autodesk.Revit.UI;
 
public class MainClass : IExternalApplication
{
    private static IContainer _container;
 
    public Result OnStartup(UIControlledApplication application)
    {
        // Set up the container and register dependencies
        var builder = new ContainerBuilder();
        builder.RegisterType<MyService>().As<IMyService>();
 
        // Build the container
        _container = builder.Build();
 
        // Continue with add-in initialization
        // ...
        return Result.Succeeded;
    }
 
    // ...
}

Step 4: Resolve Dependencies

To use your dependencies within your add-in classes, you can resolve them from the container using constructor injection.

public class MyCommand : IExternalCommand
{
    private readonly IMyService _myService;
 
    public MyCommand(IMyService myService)
    {
        _myService = myService;
    }
 
    public Result Execute(
        ExternalCommandData commandData,
        ref string message,
        ElementSet elements)
    {
        // Use the resolved dependency
        _myService.DoSomething();
 
        return Result.Succeeded;
    }
}

Step 5: Run Your Add-In

Now, when you run your Revit add-in, the dependency injection container will automatically resolve and inject the dependencies into your add-in classes.

Important Considerations:

Lifetime Scope: Be mindful of the lifetime scope of your dependencies. In Revit add-ins, you typically have a single application instance, so using a singleton scope for dependencies is common.
Revit's Single-Threaded Model: Remember that Revit add-ins operate in a single-threaded context. Ensure that your dependencies are thread-safe when accessed from multiple threads.
Add-In Loading: Dependency injection frameworks might not work seamlessly with Revit's add-in loading process, especially when add-ins are loaded and unloaded dynamically. You may need to manage the container's lifetime explicitly to prevent memory leaks.

By incorporating dependency injection into your Revit add-ins, you can improve code modularity, maintainability, and testability while adhering to Revit's unique architectural constraints. Be sure to choose a dependency injection framework that suits your needs and accommodates the specific requirements of Revit add-in development.

Interface Unit Tests