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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.

The way of handling dependency injection has been changed in DotVVM 1.1.

DotVVM 1.1 uses the Microsoft.Extensions.DependencyInjection library to configure and resolve services.

OWIN

In the Startup.cs file, every DotVVM application calls the following method:

app.UseDotVVM<DotvvmStartup>(applicationPhysicalPath, options: options =>
{
    options.Services.AddSingleton...
});

The lambda method options => ... can be used to configure additional services. The options parameter exposes the property called Services of type IServiceCollection which can be used to register services.

You can register custom services using options.Services.AddTransient, options.Services.AddSingleton or options.Services.AddScoped.

One of the services that is already present in the collection, 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, IDotvvmRequestContext context)
        {
            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:

app.UseDotVVM<DotvvmStartup>(applicationPhysicalPath, options: options =>
{
    options.Services.AddSingleton<IViewModelLoader>(serviceProvider => new WindsorViewModelLoader(container));
});

ASP.NET Core

ASP.NET Core contains a built-in dependency injection mechanism. In the Startup.cs file, there is a method called ConfigureServices which registers all application services in the IServiceCollection parameter. The collection is managed by the Microsoft.Extensions.DependencyInjection library.

When you call app.UseDotVVM<DotvvmStartup>(...), it registers several internal services which DotVVM uses the IServiceCollection, for example the view compiler, viewmodel serializer and so on.

Registering Services

To register services, you can just call one of the following methods:

public void ConfigureServices(IServiceCollection services)
{
    ...
    services.AddSingleton<ICustomService, CustomService>();
    services.AddTransient<ICustomService2, CustomService2>();
    services.AddScoped<ICustomService3, CustomService3>();
}

DotVVM will be able to inject these services if they are specified as parameters of the viewmodel constructor.

Using Alternative Container

Optionally, the ConfigureServices method can return its own IServiceProvider which will be used instead of the default one. This is useful if you want to use an alternative IoC/DI container.

For example, the default Microsoft.Extensions.DependencyInjection library doesn't support advanced scenarios like registering services by conventions or working with open generic types. This can be a reason to use an alternative dependency injection container.

Autofac is one of the popular DI containers which works with ASP.NET Core. You can use this code to use Autofac:

public IServiceProvider ConfigureServices(IServiceCollection services)
{
    ...

    // create the Autofac container builder
    var builder = new ContainerBuilder();

    // find all modules with container configuration in current assembly
    builder.RegisterAssemblyModules(typeof(Startup).GetTypeInfo().Assembly);

    // combine the rules with the services already registered in the IServiceCollection
    builder.Populate(services);

    // create and return the container
    var applicationContainer = builder.Build();
    return new AutofacServiceProvider(applicationContainer);
}
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