IoC/DI Containers

Dependency Injection is used widely in many large applications. DotVVM allows you to have your services injected in the viewmodel constructor or properties.

public class CustomersViewModel 
{
    // the parameters will be injected automatically by the DI container
    public CustomersViewModel(CustomerService customerService, ILogging log) 
    {
        ...        
    }

    // this service can be injected too
    [Bind(Direction.None)]
    public IAdditionalService AdditionalService { get; set; }
}

Basically, if you need any services in your viewmodel, you can request them in the constructor as parameters, or you can declare a public property in the viewmodel. In that case, don't forget to use the [Bind(Direction.None)] attribute to tell the serializer that it should not care about this property.

Enabling the Dependency Injection

The way of handling dependency injection has been changed in DotVVM 1.1 and the dependency injection is based on the Microsoft.Extensions.DependencyInjection library.

Because there is no dependency injection mechanism built in .NET Framework and an external libraries have to be used for this purpose, you have to perform additional configuration steps to enable the dependency injection.

The DotvvmConfiguration object contains a property called ServiceLocator. This property contains a class that manages internal services of DotVVM, e.g. view compiler, viewmodel serializer etc.

One of the internal services is the viewmodel loader represented by the IViewModelLoader interface. This class is responsible for locating of the class specified by the @viewModel directive in the page and creating an instance of the viewmodel.

DotVVM uses the DefaultViewModelLoader class which locates the class and calls its default constructor. If you need to plug a dependency injection container in, you can create a class that inherits DefaultViewModelLoader and override the CreateViewModelInstance.

Custom ViewModelLoader for Castle Windsor

Castle Windsor is one of the favorite IoC/DI containers in .NET. Here is how to create the viewmodel loader using this container. Notice that we call container.Resolve in the CreateViewModelInstance and container.Release in the DisposeViewModel.

using System;
using Castle.Windsor;
using DotVVM.Framework.ViewModel.Serialization;

namespace DotvvmDemo.Web
{
    public class WindsorViewModelLoader : DefaultViewModelLoader
    {
        private readonly WindsorContainer container;

        public WindsorViewModelLoader(WindsorContainer container)
        {
            this.container = container;
        }

        protected override object CreateViewModelInstance(Type viewModelType)
        {
            return container.Resolve(viewModelType);
        }

        public override void DisposeViewModel(object instance)
        {
            container.Release(instance);
            base.DisposeViewModel(instance);
        }
    }
}

If you use another container, the implementation will be very similar. Don't forget to tell the container to release the instances in the DisposeViewModel method. This method is called when the HTTP request ends and DotVVM no longer needs the viewmodel object.

Some containers do this by initiating a "scope" in the CreateViewModelInstance method and disposing the scope in the DisposeViewModel method.

Registering Custom ViewModelLoader

The last thing is to replace the default viewmodel loader with the one you have just created. We should do this in the Startup.cs class:

dotvvmConfiguration.ServiceLocator.RegisterSingleton<IViewModelLoader>(() => new WindsorViewModelLoader(container));

Please note that the ServiceLocator property is no longer used in DotVVM 1.1.