In the previous blog post, I wrote about replacing the authentication with OWIN Security libraries. In this part, I would like to introduce DotVVM – an open source framework which allows building web applications using the MVVM pattern and is very easy to use. I will also show how you can modernize old Web Forms applications using DotVVM.

Parts of the series


I have started the DotVVM project when I noticed that Web Forms are not going to be supported on ASP.NET Core. I realized that there are thousands of companies still heavily relying on legacy Web Forms applications.

In many cases, it is just not possible to rewrite the applications from scratch. They have often been developed for more than 10 years and the idea of rewriting them in less than half of that time is ridiculous. Especially in the current world of over-complicated JavaScript frameworks which add tremendous amount of complexity and make almost impossible to be a full-stack web developer.


How does DotVVM work?

The idea behind DotVVM was quite simple – to build a successor of ASP.NET Web Forms, with support of .NET Core, and get rid of the biggest pain points Web Forms had.

No ViewState

Web Forms store state of all controls in the page in a cryptic hidden field which is called viewstate. Although it is possible to specify which controls can use it and which cannot, still, it is not possible to get rid of it completely, and some controls stop working without it.

In DotVVM, the controls in the page are stateless. The page is backed by a viewmodel, which is a C# class you need to write, and all the state is stored in the viewmodel instance. The purpose of viewmodel is to store page state and handle user actions (clicking a button for example).

You can make postbacks in DotVVM, same as in Web Forms. But you can control what will be transferred from the client to the server – instead of hundreds of kilobites of encrypted state, it is a plain JSON object which corresponds with the C# viewmodel. You can specify bind direction to particular properties, so they won’t be transferred in both ways.

And what’s best – for most actions, you don’t need to do a full postback. DotVVM 2.0 brought REST API bindings, which can significantly reduce the amount of data in the viewmodel, and Static Command Services, which can also avoid transferring the entire viewmodel to the server and back. And we are working on even more interesting things right now.

Clean HTML

Web Forms controls contain many properties to change how the controls look like – FontSize, BackColor and so on. These properties emit inline styles, which is considered as a bad practise, and the HTML rendered by some controls is difficult to style using CSS.

DotVVM relies on CSS. All controls produce as simple HTML as possible, and everything is easily styleable. You can specify which HTML elements should be produced by some controls (Repeater, for example), so you can use the control to render unordered lists, or just a bunch of divs. Also, we have documented what each control produces.

Testability & Better Architecture

The viewmodels in DotVVM don’t reference controls from the UI – actually, they even don’t know which control will use a particular property. Thanks to the data-binding, any change made to the properties will be reflected by the controls, and vice-versa. This makes the viewmodels testable – you can just create an instance of the viewmodel, set some properties, call a method and make sure it did what it should do.

Also, DotVVM natively supports dependency injection, action filters and many other concepts known from MVC or ASP.NET Core. It supports both old ASP.NET and new ASP.NET Core, and can be used together with any other framework in one application. This is super-interesting for the rest of this article.



How can DotVVM help with existing Web Form applications?

If you need to extend Web Forms applications with new pages or areas, you should definitely consider writing these new parts in DotVVM. The syntax is cleaner, you can access your business layer the same way as in Web Forms, and finally, many things are easier to do in DotVVM.

If you know that you will need to run this application next five years or more, you may use DotVVM to modernize it, and possibly upgrade to .NET Core. You can take Web Forms pages and rewrite them one by one while keeping the entire application still working. After a while, when you get rid of all Web Forms pages, you might be able to upgrade the project to .NET Core (DotVVM supports both .NET Framework and .NET Core). DotVVM is a live project and is constantly adding new features, so your platform will be in sync with the world of web development which changes every year.


How to start?

I have started building a NuGet package called DotVVM.Adapters.WebForms. It will contain several controls that allow closer integration of the two technologies.

  • There are dotvvm:RouteLink and webforms:RouteLink controls, which can render a link to DotVVM or ASP.NET Web Forms route, which is helpful when making a transition between the two parts.
  • Currently, I am working on a control which can host DotVVM user control and its viewmodel in a Web Forms page. This is more difficult to do as it requires some changes in the framework itself, but still, it would be a nice step forward.

The project is hosted on GitHub and I am looking for a feedback from community – it will help us to choose the next steps and prioritize the ideas we have.

I have also created a DotVVM cheat sheet for Web Forms developers so you can see the differences between these two technologies.


Installing DotVVM package

To integrate DotVVM with Web Forms, you need to perform the following steps:

  1. Install DotVVM.Owin and Microsoft.Owin.Host.SystemWeb packages in the project.

  2. Open the csproj file in a text editor and add change the ProjectTypeGuids element to this:
    <ProjectTypeGuids>{94EE71E2-EE2A-480B-8704-AF46D2E58D94};{349c5851-65df-11da-9384-00065b846f21};{fae04ec0-301f-11d3-bf4b-00c04f79efbc}</ProjectTypeGuids>

  3. Add DotvvmStartup.cs file in the project. This file contains the configuration of DotVVM:
    public class DotvvmStartup : IDotvvmStartup, IDotvvmServiceConfigurator
    {
        public void Configure(DotvvmConfiguration config, string applicationPath)
        {
            config.AddWebFormsAdapterControls();
    
            RegisterRoutes(config, applicationPath);
            RegisterControls(config, applicationPath);
            RegisterResources(config, applicationPath);
        }
    
        private void RegisterRoutes(DotvvmConfiguration config, string applicationPath)
        {
            // ...
        }
    
        private void RegisterControls(DotvvmConfiguration config, string applicationPath)
        {
            
        }
    
        private void RegisterResources(DotvvmConfiguration config, string applicationPath)
        {
            
        }
    
        public void ConfigureServices(IDotvvmServiceCollection services)
        {
        }
    }

  4. Add Startup.cs in the project. This file contains the OWIN pipeline configuration. You may already have this file in the project, if you are using SignalR or another OWIN-based library.
    [assembly: OwinStartup(typeof(YourNamespace.Startup))]
    
    namespace YourNamespace
    {
        public class Startup
        {
            public void Configuration(IAppBuilder app)
            {
                app.UseDotVVM<DotvvmStartup>(HostingEnvironment.ApplicationPhysicalPath);
            }
        }
    }

  5. Create Views and ViewModels folders somewhere in the project.

  6. Create a DotVVM Master Page and copy all contents of ASP.NET Web Forms Master Page. The goal is to make DotVVM page look exactly the same as the Web Forms one. You need to have two – one for Web Forms, one for DotVVM. It is not possible to reuse it.

  7. Replace all links (e.g. in the menu) with <webforms:RouteLink> controls.

  8. Replace <asp:ContentPlaceHolder> with <dot:ContentPlaceHolder>.

  9. Look for runat=”server” in all controls and replace all Web Forms controls with their DotVVM equivalents:
    1. There are no HyperLink, Label, Image, Panel controls in DotVVM – use their HTML equivalents.
    2. DotVVM doesn’t have controls like Menu or Calendar – you need to use HTML and CSS to make any menu you may need. We have the Calendar control in DotVVM Business Pack.
    3. Form controls (TextBox, CheckBox…), Repeater, FileUpload, GridView and many other controls have the same names.
    4. Replace all appearance properties (FontSize etc.) with proper CSS. I strongly recommend using LESS or SCSS.
    5. Replace all properties which are set from the code-behind, with data-binding, and place the corresponding properties in the master page viewmodel. If there are any values which cannot change while the page is loaded, you can use resource binding to render them directly in HTML.

    The migration shouldn't bee difficult - most of the master pages I have seen, are quite simple and use only basic controls.

  10. Start creating new pages in DotVVM using the master page. You can always make a link to a Web Forms page using <webforms:RouteLink> control, and you can always link to a DotVVM page using <dotvvm:RouteLink runat=”server”> control in a Web Forms page.

  11. Make sure the single-sign on works as expected. DotVVM should see the user identity even if you are using Forms Authentication. I strongly recommend to move to OWIN Security, which was described in the previous part.


Of course, there are so many Web Forms specific stuff that I couldn’t cover everything. However, I have tried to apply all these steps in 10 various Web Forms applications I made years ago, and I was able to integrate DotVVM very quickly.

If you try this approach, definitely let us know on our Gitter – we are very interested in any issues you might have run into. And if you have any ideas, what controls you’d like to have ported, we are listening.

In the previous blog post, I was writing about the problems of legacy ASP.NET Web Forms applications. It is often not possible to rewrite them from scratch, as new features should be introduced every month, and I’d like to dig into one of the ways of modernizing these applications.

Parts of the series

In this part, I will write about how to replace Forms Authentication with OWIN Security middlewares. I needed to do this in one of my older Web Forms apps recently, because it was using Microsoft.WebPages.OAuth library to authenticate users using Facebook, Twitter and Google. This library was part of ASP.NET Web Pages and is very old, so it doesn’t support the current version of OAuth protocol.

OWIN, which was a new infrastructure for ASP.NET applications on .NET Framework, a predecessor of ASP.NET Core, supports various authentication middlewares, and besides Cookie Security Middleware which can replace Forms Authentication, there are also middlewares for all commonly used social networks and other authentication scenarios.


Forms Authentication

I have created a sample ASP.NET Web Forms application (it’s in ModernizingWebFormsSample.Auth folder) with a few pages and Forms Authentication.

It is using the built-in membership provider to store information about users. The database should be created automatically and it should reside in App_Data folder.

In Global.asax, there is a code which creates a default user account (e-mail: [email protected], password: Pa$$w0rd) when the application is launched. 

The Login.aspx page contains a login form made by the <asp:Login> control. This control uses the membership provider to validate the user credentials (by calling Membership.ValidateUser) and then it calls FormsAuthentication.SetAuthCookie to issue a cookie with an authentication token.

If you don’t want to use <asp:Login> control, you can always build the form by yourself and issue the authentication cookie manually by calling the methods mentioned above.


The concepts

If you are not familiar with how membership providers and identity in ASP.NET works, here is a brief explanation.

Almost every web application uses some concept of user accounts, often with different permissions and access levels.

The information about the users needs to be stored somewhere. Sometimes, the users are stored in a SQL database, sometimes, they are retrieved from Active Directory, and there are many other options.

Membership provider is an abstract mechanism that provides a unified API for the application no matter how and where exactly the user info is stored. The membership provider exposes methods for creating, editing and deleting users, validating, changing and resetting their passwords, and so on.

ASP.NET comes with a built-in membership provider, which stores user information in a SQL database which is located in App_Data folder (it can be changed in web.config).

You can implement your own membership provider to be able to store users in your own SQL database, or any other place. You just need to implement several methods like CreateUser, ValidateUser etc.

All ASP.NET controls that work with users (<asp:Login>, <asp:ChangePassword> etc.) work thanks to the concept of membership providers. If you haven’t heard about membership providers before, you are probably using the default one.

You should consider replacing membership providers by ASP.NET Identity, which provides much more options and features, but that’s another story - I will address it in some of the next parts of this series.


The authentication in ASP.NET can work in multiple modes. Most Web Forms applications are using Forms Authentication, which uses an authentication cookie. You can set the authentication mode in web.config, using the system.web/authentication element.


The authentication and membership providers are decoupled, so you should be able to use any authentication mode with any membership provider (or other solution, like ASP.NET Identity I have already mentioned).


OWIN

ASP.NET was always tightly integrated with IIS. It was quite difficult to run .NET web apps outside IIS. The reason for OWIN was to provide a modular and extensible HTTP server pipeline that would be able to run without IIS dependencies. This allows e.g. self-hosting ASP.NET Web API or SignalR applications in Windows Service and other interesting scenarios.

OWIN works with a concept of middlewares. You can register any number of middlewares in the pipeline. Basically, middlewares are just functions with two parameters – request context and next middleware. The middleware can look at the request context and process the HTTP request, or pass the request to another middleware (and optionally do something with the response produced by the next middleware).

This mechanism is very flexible and allows to combine multiple frameworks in one application. The authentication also benefits from this concept and can be totally independent on the application itself.

For example, you can register Web API in the OWIN pipeline. When the request URL matches some API controller, Web API will process the request and produce the response. If Web API doesn’t recognize the URL, it will pass the request to the next middleware in the pipeline, which can be e. g. static files middleware. It will look in the filesystem and return the appropriate file, or again, pass the control to the next middleware in the pipeline. If no middleware wants to process the request, a HTTP error will be produced by OWIN.


OWIN Security is a set of middlewares which handle the authentication. These middlewares are typically registered at the beginning of the pipeline.

When a HTTP request is received, the authentication middleware will make sure the request is authenticated (by looking at cookies, the Authorization header or something like that), set the user identity in the request context and pass the request to the next middleware.

The application will be able to read the information about the user and doesn’t need to know whether the user was logged through an authentication cookie or through Windows Authentication.

If the application notices that the user doesn’t have permissions to perform some operation, it may throw an exception, or set the HTTP status code to 401. The authentication middleware will notice that and can take an appropriate action, for example redirect the user to the login page.

OWIN can be integrated with ASP.NET quite easily thanks to the Microsoft.Owin.Host.SystemWeb package. If you install it in the application, the OWIN pipeline will be added before the ASP.NET stack, so you can use authentication middlewares in combination with Web Forms pages (or MVC controllers). And that’s exactly what I needed in my app recently to be able to use OWIN Security libraries, like Cookies, Facebook, Twitter and so on.


In the demo app, I am using Microsoft.Owin.Security.Cookies package as a replacement of Forms Authentication, because it works in a very similar way – it generates and stores the authentication ticket in a cookie.


1. Install Microsoft.Owin.Host.SystemWeb package in the application.

2. Then, install Microsoft.Owin.Security.Cookies package.

3. Add new OWIN Startup class in the project. This class is used to configure OWIN pipeline.

image


4. Add the following code snippet in the Configuration method in Startup.cs file. It will register the cookie authentication middleware in the OWIN pipeline.

If you need to adjust the default values like the expiration timeout or cookie domain, you can do it right here in the CookieAuthenticationOptions initializer.

app.UseCookieAuthentication(new CookieAuthenticationOptions()
{
    LoginPath = new PathString("/Login.aspx")
});


5. To issue an authentication cookie, call Context.GetOwinContext().Authentication.SignIn instead of FormsAuthentication.SetAuthCookie.

You will need to build a ClaimsIdentity which represents the current user. This identity contains a collection of claims – e.g. username, user ID, list of roles, e-mail and other information about the user. You can add any custom claims in the identity – they will be encrypted and stored in the authentication cookie.

If you are already using ASP.NET Identity, there is a method called CreateIdentity, which will build the ClaimsIdentity for you.

protected void LoginButton_OnClick(object sender, EventArgs e)
{
    if (Membership.ValidateUser(UserName.Text, Password.Text))
    {
        // get user info
        var user = Membership.GetUser(UserName.Text);

        // build a list of claims
        var claims = new List<Claim>();
        claims.Add(new Claim(ClaimTypes.Name, user.UserName));
        claims.Add(new Claim(ClaimTypes.NameIdentifier, user.ProviderUserKey.ToString()));
        if (Roles.Enabled)
        {
            foreach (var role in Roles.GetRolesForUser(user.UserName))
            {
                claims.Add(new Claim(ClaimTypes.Role, role));
            }
        }

        // create the identity
        var identity = new ClaimsIdentity(claims, CookieAuthenticationDefaults.AuthenticationType);

        Context.GetOwinContext().Authentication.SignIn(new AuthenticationProperties()
        {
            IsPersistent = RememberMe.Checked
        }, 
        identity);

        Response.Redirect("~/");
    }
    else
    {
        FailureText.Text = "The credentials are not valid!";
    }
}


To sign out, you need to call SignOut method:

protected void LogoutButton_OnClick(object sender, EventArgs e)
{
    Context.GetOwinContext().Authentication.SignOut();
    Response.Redirect("~/");
}


6. If you are using ASP.NET login controls, you may need to replace them with your own mechanisms as the controls are hard-coded to use the Forms Authentication. Unfortunately, there are no extensibility points to replace static method calls to FormsAuthentication.SetAuthCookie.

This applies to the following controls:


If you are using these controls without specifying your own templates for them, you may use the following quick action to generate their code.

image

You can now get rid of the Login control and use only the code inside the template. Just add OnClick to the button and use the code from step 5 to perform the authentication.


7. The last thing you need to do is to disable Forms authentication in web.config. Change the authentication mode to None. Don’t worry, there is still the OWIN authentication.

<authentication mode="None" />


Note that the URL authorization mechanism (system.web/authorization element) will still work. OWIN still sets the ClaimIdentity in the HttpContext.Current.User property, so the URL authorization module will be able to see whether the user is authenticated and in which roles it is.


You can see the working demo on GitHub in the ModernizingWebFormsSample.Auth.New project.

In my last live coding session, I have a web application built with DotVVM which doesn’t connect to a database directly. It consumes a REST API instead, which is quite common approach in microservices applications.

Application architecture

 

 

The validation logic should be definitely implemented in the backend service. The REST API can be invoked from other microservices and we certainly don’t want to duplicate the logic on multiple places. In addition to that, the validation rules might require some data which would not be available in the admin portal (uniqueness of the user’s e-mail address for example).

Basically, the backend service needs to report the validation errors to the admin portal and the admin portal needs to report the error in the browser so the text field can be highlighted.

 

The solution is quite easy as both DotVVM and ASP.NET MVC Core use the same model validation mechanisms.

 

Implementing the Validation Logic

In my case, the API controller accepts and returns the following object (some properties were stripped out). I have used the standard validation attributes (Required) for the simple validation rules and implemented the IValidatableObject interface to provide enhanced validation logic for dependencies between individual fields.

public class EventDTO : IValidatableObject
{
    public string Id { get; set; }

    [Required]
    public string Title { get; set; }
    public string Description { get; set; }
    public List<EventDateDTO> Dates { get; set; } = new List<EventDateDTO>();
    ... 
    public DateTime RegistrationBeginDate { get; set; }
    public DateTime RegistrationEndDate { get; set; }

    [Range(1, int.MaxValue)]
    public int MaxAttendeeCount { get; set; }

    public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
    {
        if (!Dates.Any())
        {
            yield return new ValidationResult("The event must specify at least one date!");
        }

        if (RegistrationBeginDate >= RegistrationEndDate)
        {
            yield return new ValidationResult("The date must be greater than registration begin date!", new [] { nameof(RegistrationEndDate) });
        }

        for (var i = 0; i < Dates.Count; i++)
        {
            if (Dates[i].BeginDate < RegistrationEndDate)
            {
                yield return new ValidationResult("The date must be greater than registration end date!", new[] { nameof(Dates) + "[" + i + "]." + nameof(EventDateDTO.BeginDate) });
            }
            if (Dates[i].BeginDate >= Dates[i].EndDate)
            {
                yield return new ValidationResult("The date must be greater than begin date!", new[] { nameof(Dates) + "[" + i + "]." + nameof(EventDateDTO.EndDate) });
            }
        }
        ...
    }
}

Notice that when I validate the child objects, the property path is composed like this: Dates[0].BeginDate.

 

Reporting the validation errors on the API side

When the client posts an invalid object, I want the API to respond with HTTP 400 Bad Request and provide the list of errors. In ASP.NET MVC Core, there is the ModelState object which can be returned from the API. It is serialized as a map of property paths and lists of errors for the particular property.

I have implemented a simple action filter which verifies that the model is valid. If not, it produces the HTTP 400 response.

public class ModelValidationFilterAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext context)
    {
        if (!context.ModelState.IsValid)
        {
            context.Result = new BadRequestObjectResult(context.ModelState);
        }
        else
        {
            base.OnActionExecuting(context);
        }
    }
}

You can apply this attribute on a specific action, or as a global filter. If something is wrong, I am getting something like this from the API:

{
    "Title": [
        "The event title is required!"
    ],
    "RegistrationEndDate": [
        "The date must be greater than registration begin date!"
    ],
    "Dates[0].BeginDate": [
        "The date must be greater than registration end date!"
    ]
}

 

Consuming the API from the DotVVM app

I am using Swashbuckle.AspNetCore library to expose the Swagger metadata of my REST API In my DotVVM application. I am also using NSwag to generate the client proxy classes.

This tooling can save quite a lot of time and is very helpful in larger projects. If you make any change in your API and regenerate the Swagger client, you will get a compile errors on all places that needs to be adjusted, which is great.

On the other hand, I am still missing a lot of features in Swagger pipeline, for example support for generic types. Also, even that NSwag provides a lot of extensibility points, sometimes I had to use nasty hacks or regex matching to make it generate the things I really needed.

Swagger generates the ApiClient class with async methods corresponding to the individual controller actions. I can call these methods from my DotVVM viewmodels.

public async Task Save()
{
    await client.ApiEventsPostAsync(Item);
    Context.RedirectToRoute("EventList");
}

Now, whenever I get HTTP 400 from the API, I need to read the list of validation errors, fill the DotVVM ModelState with them and report them to the client.

 

Reporting the errors from DotVVM

I have created a class called ValidationHelper with a Call method. It accepts a delegate which is called. If it returns a SwaggerException with the status code of 400, we will handle the exception. Actually, there are two overloads - one for void methods, one for methods returning some result.

public static class ValidationHelper
{

    public static async Task Call(IDotvvmRequestContext context, Func<Task> apiCall)
    {
        try
        {
            await apiCall();
        }
        catch (SwaggerException ex) when (ex.StatusCode == "400")
        {
            HandleValidation(context, ex);
        }
    }

    public static async Task<T> Call<T>(IDotvvmRequestContext context, Func<Task<T>> apiCall)
    {
        try
        {
            return await apiCall();
        }
        catch (SwaggerException ex) when (ex.StatusCode == "400")
        {
            HandleValidation(context, ex);
            return default(T);
        }
    }

    ...
}

The most interesting is of course the HandleValidation method. It parses the response and puts the errors in the DotVVM model state:

private static void HandleValidation(IDotvvmRequestContext context, SwaggerException ex)
{
    var invalidProperties = JsonConvert.DeserializeObject<Dictionary<string, string[]>>(ex.Response);

    foreach (var property in invalidProperties)
    {
        foreach (var error in property.Value)
        {
            context.ModelState.Errors.Add(new ViewModelValidationError() { PropertyPath = ConvertPropertyName(property.Key), ErrorMessage = error });
        }
    }

    context.FailOnInvalidModelState();
}

And finally, there is one little caveat - DotVVM has a different format of property paths than ASP.NET MVC uses, because the viewmodel on the client side is wrapped in Knockout observables. Instead of using Dates[0].BeginDate, we need to use Dates()[0]().BeginDate, because all getters in Knockout are just function calls.

This hack won’t be necessary in the future versions of DotVVM as we plan to implement a mechanism that will detect and fix this automatically.

private static string ConvertPropertyName(string property)
{
    var sb = new StringBuilder();
    for (int i = 0; i < property.Length; i++)
    {
        if (property[i] == '.')
        {
            sb.Append("().");
        }
        else if (property[i] == '[')
        {
            sb.Append("()[");
        }
        else
        {
            sb.Append(property[i]);
        }
    }
    return sb.ToString();
}

 

Using the validation helper

There are two ways how to use the API we have just created. Either you can wrap your API call in the ValidationHelper.Call method. Alternatively, you can implement a DotVVM exception filter that will call the HandleException method directly.

public async Task Save()
{
    await ValidationHelper.Call(Context, async () =>
    {
        await client.ApiEventsPostAsync(Item);
    });

    Context.RedirectToRoute("EventList");
}

 

In the user interface, the validation looks the same as always - you mark an element with Validator.Value="{value: property}" and configure the behavior of validation on the element itself, or on any of its ancestors, for example Validation.InvalidCssClass="has-error" to decorate the elements with the has-error class when the property is not valid.

The important thing is that you need to set the save button's Validation.Target to the object you are passing to the API, so the property paths would lead to corresponding properties on the validation target.

 

You can look at the ErrorDetail page and its viewmodel in the OpenEvents project repository to see how the things work.

 

Next Steps

Currently, this solution validates only on the backend part, not in the DotVVM application and of course not in the browser. To make the client-side validation work, the DotVVM application would have to see the validation attributes on the EventDetailDTO object. Right now, the object is generated using Swagger, so it doesn’t have these attributes.

The NSwag generator could be configured not to generate the DTOs, so we would be able to have the DTOs in a class library project that would be referenced by both admin portal and backend app. This would allow to do the same validation in the admin portal and because DotVVM would see the validation attributes, it could enforce some validation rules in the browser.

 

My third live coding session is scheduled to Tuesday, January 2nd on 6:30 PM (Central Europe Time).

Last time, I have built a simple user interface for editing the events in my registration portal. This time, I will do a little bit of CSSing, implement the validation and show how Bootstrap for DotVVM can make your life easier.

 

Watch the live stream on Twitch.

The recording from the second live stream is here:


Few days ago, I have done a live coding session - in the first part, I was building the core of event registration system.

I really enjoyed it, so here is the date for the next session: Tuesday, December 19 at 6:30 PM (Central Europe Time). 

This time, I will be building the core of the admin portal in DotVVM that will allow to manage individual events and reservations.