How to use Dependency Injection with ASP.NET Web Forms

Question

I am trying to work out a way to use dependency injection with ASP.NET Web Forms controls.

I have got lots of controls that create repositories directly, and use tho

Answer 1

UPDATE 2019: With the introduction of Web Forms 4.7.2, there is now better support for DI. This invalidates the below. See: Wiring up Simple Injector in WebForms in .NET 4.7.2

You can use automatic constructor injection by replacing the default PageHandlerFactory with a custom one. This way you can use an overloaded constructor to load the dependencies. Your page might look like this:

public partial class HomePage : System.Web.UI.Page
{
    private readonly IDependency dependency;

    public HomePage(IDependency dependency)
    {
        this.dependency = dependency;
    }

    // Do note this protected ctor. You need it for this to work.
    protected HomePage () { }
}

Configuring that custom PageHandlerFactory can be done in the web.config as follows:

<?xml version="1.0"?>
<configuration>
  <system.web>
    <httpHandlers>
      <add verb="*" path="*.aspx"
        type="YourApp.CustomPageHandlerFactory, YourApp"/>
    </httpHandlers>
  </system.web>
</configuration>

Your CustomPageHandlerFactory can look like this:

public class CustomPageHandlerFactory : PageHandlerFactory
{
    private static object GetInstance(Type type)
    {
        // TODO: Get instance using your favorite DI library.
        // for instance using the Common Service Locator:
        return Microsoft.Practices.ServiceLocation
            .ServiceLocator.Current.GetInstance(type);
    }

    public override IHttpHandler GetHandler(HttpContext cxt, 
        string type, string vPath, string path)
    {
        var page = base.GetHandler(cxt, type, vPath, path);

        if (page != null)
        {
            // Magic happens here ;-)
            InjectDependencies(page);
        }

        return page;
    }

    private static void InjectDependencies(object page)
    {
        Type pageType = page.GetType().BaseType;

        var ctor = GetInjectableCtor(pageType);

        if (ctor != null)
        {
            object[] arguments = (
                from parameter in ctor.GetParameters()
                select GetInstance(parameter.ParameterType)
                .ToArray();

            ctor.Invoke(page, arguments);
        }
    }

    private static ConstructorInfo GetInjectableCtor(
        Type type)
    {
        var overloadedPublicConstructors = (
            from constructor in type.GetConstructors()
            where constructor.GetParameters().Length > 0
            select constructor).ToArray();

        if (overloadedPublicConstructors.Length == 0)
        {
            return null;
        }

        if (overloadedPublicConstructors.Length == 1)
        {
            return overloadedPublicConstructors[0];
        }

        throw new Exception(string.Format(
            "The type {0} has multiple public " +
            "ctors and can't be initialized.", type));
    }
}

Downside is that this only works when running your side in Full Trust. You can read more about it here. But do note that developing ASP.NET applications in partial trust seems a lost cause.

Answer 2

Autofac supports fairly unobtrusive dependency injection in ASP.NET WebForms. My understanding is it just hooks into the ASP.NET page lifecycle using an http module and does property injection. The only catch is that for controls I don't think this happens until after the Init event.

Answer 3

The best way is to have a base class for the controls like:

public class PartialView : UserControl
{
    protected override void OnInit(System.EventArgs e)
    {
        ObjectFactory.BuildUp(this);
        base.OnInit(e);
    }
}

That will inject any control that inherits from that base class (uses structuremap). Combining that with a property based config, you will be able to have controls like:

public partial class AdminHeader : PartialView
{
   IMyRepository Repository{get;set;}
}

Update 1: If you can't have the controls inherit, perhaps the CMS has a hook right after creating the controls, in there you can call the BuildUp. Also if the CMS allows you to hook something to fetch the instance you could use constructor based injection, but I prefer BuildUp on this specific scenario as asp.net doesn't have a hook for this.

Answer 4

Starting from .NET 4.7.2 (what's new), it is now easy for developers to use Dependency Injection in WebForms applications. With the UnityAdapter, you can add it to your existing WebForms application in 4 simple steps. See this blog.

Answer 5

You could also create some singleton instances in the Application_Start global.asax event and have them available as public static readonly properties.

Answer 6

This is a solution I recently used to avoid hooking into the pipeline (I find that confuses everyone that looks at my code in the future, but yes, I see its benefits as well):

public static class TemplateControlExtensions
{
    static readonly PerRequestObjectManager perRequestObjectManager = new PerRequestObjectManager();

    private static WIIIPDataContext GetDataContext(this TemplateControl templateControl)
    {
        var dataContext = (WIIIPDataContext) perRequestObjectManager.GetValue("DataContext");

        if (dataContext == null) 
        {
           dataContext = new WIIIPDataContext();
           perRequestObjectManager.SetValue("DataContext", dataContext);   
        }

        return dataContext;
    }

    public static IMailer GetMailer(this TemplateControl templateControl)
    {
        return (IMailer)IoC.Container.Resolve(typeof(IMailer));
    }

    public static T Query<T>(this TemplateControl templateControl, Query<T> query)
    {
        query.DataContext = GetDataContext(templateControl);
        return query.GetQuery();
    }

    public static void ExecuteCommand(this TemplateControl templateControl, Command command)
    {
        command.DataContext = GetDataContext(templateControl);
        command.Execute();
    }

    private class PerRequestObjectManager
    {
        public object GetValue(string key)
        {
            if (HttpContext.Current != null && HttpContext.Current.Items.Contains(key))
                return HttpContext.Current.Items[key];
            else
                return null;
        }

        public void SetValue(string key, object newValue)
        {
            if (HttpContext.Current != null)
                HttpContext.Current.Items[key] = newValue;
        }
    }
}

This shows how you can create your own life time manager pretty easily as well as hook into an IoC container if you so desire. Oh, and I am also using a query/command structure which is sort of unrelated, but more on the reasoning behind that can be found here:

Limit your abstractions: Refactoring toward reduced abstractions