Things I’ve Learnt This Week (5th February)

I’m keeping up my plan (week 2 yay!) to record and share things I’ve learned during the last week. Less from me this week as it’s been fairly hectic and I’ve been feeling a bit unwell.

Things I’ve Learned

Identity Server 4

As part of my work for Humanitarian Toolbox I’ve been actively investigating Identity Server 4 as an option to handle our authentication. The allReady application currently uses ASP.NET Core Identity within the application to support login and user management. As the product nears v1 release discussions have begun around the use cases for the application, including potential for multi-tenancy and considerations around storage of user accounts. As a result this led Richard Campbell to suggest we look into Identity Server 4 to help with this identity flow.

I was lucky enough to have an audience with Brock Allen and Dominick Baier this week to chat through some of the basics about Identity Server. It was really useful to chat with them both and I want to thank them for offering their time and support to the project. One of the key take-aways for me from the call was getting a better understanding of where Identity server fits into the puzzle. It’s about authentication and helping with the protocol of OAuth2 / OpenId communications. It’s not a user management / user store product, although it does sit nicely on top of ASP.NET Core Identity 3 as an option.

As I continue investigating and testing Identity Server 4, I hope to put together some more details posts about how we’re using it and what I learn along the way.

Things I’ve Read

In no particular order here’s some of the blogs and posts that I’ve read this week.

Things I’ve Listened To

Things I’ve Watched

Things I’ve Learnt This Week (29th January)

Whether this will become a regular thing, I’m not sure. But starting this week I’ve been keeping notes on the things I’ve learned, problems I’ve faced and resources that I’ve read, watched or listened to.

I try to consume as much information as possible about ASP.NET and development in a continual drive to learn more and get better at what I do. This includes listening to a regular set of podcasts on my daily commute, reading any blog posts that I can find that relate to things I do or may be doing in the future, and watching videos online. My current focus is around ASP.NET Core so a bulk of the materials I am reading tend to be focused in that area.

I won’t go into explicit details in these posts, as realistically I won’t have time. But I hope to highlight key points of information I have found useful and to share links to things I’ve learned from, hopefully so that others sharing my passion can save some time. It’s also a shameless way to help me remember things as my brain will only hold information for so long!

Things I’ve Learned

Not an exhaustive list (as we’re always learning and that’s one of the things I love about development) but here are a few key things which came to mind after the week has ended. I’ll contain this section to small snippets of information that do not generally warrant a longer, dedicated post.

ASP.NET Core RTM SDK Tooling

I picked up on a point that Damian Edwards mentioned on the weekly ASP.NET community standup this week around the final SDK tooling where I thought I heard him say that for RTM tooling we had to be using VS 2017 when it’s released. I must admit I hadn’t realised this or considered the implications of the move to a refined csproj (from project.json) for ASP.NET Core.

I tweeted Damian to clarify this and he was kind enough to answer my questions. The outcome, as I’ve interpreted it, is that indeed there will be no supported RTM tooling for ASP.NET Core on Visual Studio 2015. The tooling we have now which is preview tooling, will remain available, but unsupported. To get a supported ASP.NET Core tooling experience, developers will need to move to VS 2017 or use VS Code.

The nature of the all new csproj format is that it cannot/will not be implemented in VS 2015. Any projects which are opened on VS 2017 will auto migrate to the newer csproj format and after that, cannot be developed on VS 2015 any longer. It also seems that when ASP.NET Core 2.x lands, that will be csproj and VS 2017 supported only.

I was a bit shocked to learn (and perhaps I was just slow on the uptake) that the above was the case. I had assumed we might get a RTM tools for VS 2015 ASP.NET Core since people have adopted this new platform and will be left with an upgrade if they do want to continue with the full IDE and proper support. Working on an open source ASP.NET Core project as I do, this means we have to think carefully about when / if we bite the bullet and force a VS 2017 / VS Code only experience by upgrading the project.

Setting cache expiry for static files using OWIN

This week I needed to ensure that javascript and css files served via our site had a proper long cache expiry to help optimise page load times. I’ve used middleware in ASP.NET Core a fair bit, but not touched the ASP.NET 4.x OWIN code much. Our project was using the Microsoft.Owin.StaticFiles.StaticFileMiddleware for serving the static files and it turned out that the change to add the Expiry header was very similar to code I’d used in ASP.NET Core for a different, but similar requirement.

In our case all I had to do was ensure that the StaticFileOptions passed into the StaticFileMiddleware included an OnPrepareResponse action to handle setting the expires header like so.

return new StaticFileOptions
{
	OnPrepareResponse = (ctx) =>
	{
		ctx.OwinContext.Response.Expires = DateTimeOffset.UtcNow.AddYears(1);
	}
};

Misc

  • In VS 2017 we will be able to remote debug ASP.NET Core over SSH.

Things I’ve Read

In no particular order here’s some of the blogs and posts I read this week.

Things I’ve Listened To

 Things I’ve Watched

  • Domain-Driven Design: The Good Parts – Jimmy Bogard – I’m keen to understand DDD better and this was a nice talk on some of the concepts. I’m still keen to find some more code based examples but I felt this provided some good foundations.
  • ASP.NET Community Standup – I listen to this weekly to catch up on what’s happening with ASP.NET Core.
  • Public speaking with Scott Hanselman, Kendra Havens, Maria Naggaga Nakanwagi, Kasey Uhlenhuth, and Donovan Brown – This really came at a great time as I start to work on my public speaking. I want to be able to share my passion for ASP.NET Core with others and while I’ve done a few smaller talks at work I want to build up the level, quality and quantity of speaking I do. I will be continuing to build my confidence in smaller groups at work but I would like to get to the point where I can do wider audiences of strangers. I have my next talk on ASP.NET Core nearly finalised for a group of developers at work.

Updating an ASP.NET Core Site to the December 2016 Release How to upgrade a site on the LTS 1.0.3 version of ASP.NET Core

I run into an issue this week during what should have been a simple ASP.NET Core application update. I wanted to share my experience in case others run into similar problems. Also, I’m sure to be back here myself to remember this in the future!

On December 13th Microsoft released their second minor patch release for the LTS (Long Term Support) track of .NET Core. ASP.NET Core releases on two tracks depending on how cutting edge you want to be. LTS is the “safer” track, which will be supported and bug fixed during the support lifespan. The other track is FTS (Fast Track Support) which will be where new features appear. You can read more about this on the Microsoft Blog.

As you may be aware from reading my other posts, I’m contributing to an opensource charity project called allReady. We’re currently using the LTS track packages and at the time of writing still targeting the full .NET framework (as opposed to .NET Core). We had applied the last patch release 1.0.1 packages in September without any major problems so I was hoping for the same experience with this patch release.

The details for the release were made available in this Microsoft blog post. If you follow the links to the release notes you will see that the ASP.NET Core updates are considered version 1.0.3. This is where the versioning starts to get a little murky in my opinion. ASP.NET Core itself has a version number (now 1.0.3) which tracks general “releases” of the framework. However, the individual packages that actually make up .NET Core and ASP.NET Core also have version numbers and revisions. Those numbers don’t track with the main release version, so it starts to get a bit confusing. You won’t for example find a package for Microsoft.AspNetCore.Mvc at version 1.0.3. The latest for that package is 1.0.2.

I’ll now step through how I upgraded our project and then discuss the issue I experienced with the EF commands for entity framework. Before starting to update the project I made sure to install the latest version of the 1.0.3 SDK from the Microsoft website.

Update Package.json

This is where the first pain point came for me. It wasn’t listed specifically in the blogs posts or release notes all of the package which had updated and what the latest package versions were. So my initial solution was to turn to the VS Nuget Package Manager where I was hoping I could simply update all of the Microsoft packages to the latest versions. However, since the package manager lists the latest (non pre-release) versions, it was offering me the FTS 1.1.x versions. So a simple, upgrade all option was out of the question.

Next I went into the project.json manually planning to update each package by hand, allowing autocomplete to give me the latest versions. However autocomplete didn’t always seem to pick up the latest version number for me automatically and I was worried about missing something. So I reverted back to the Nuget Package Manager and went one by one through the Microsoft packages. I used the install dropdown to select the newest LTS version 1.0.x for each one. This was slow and manual but at least meant I knew what options I had and could be explicit in choosing the latest version i wanted.

Here’s a rundown the packages from our project.json that I needed to update and the versions number they are are now on (which should be the latest LTS release). Note that our project.json may well differ for newly generated projects so you may not have all of these packages and you may even have dependencies listed that we do not.

"Microsoft.EntityFrameworkCore.SqlServer": "1.0.2",
"Microsoft.EntityFrameworkCore": "1.0.2",
"Microsoft.ApplicationInsights.AspNetCore": "1.0.2",
"Microsoft.AspNetCore.Mvc": "1.0.2",
"Microsoft.AspNetCore.Mvc.TagHelpers": "1.0.2",
"Microsoft.AspNetCore.Authentication.Cookies": "1.0.1",
"Microsoft.AspNetCore.Authentication.Facebook": "1.0.1",
"Microsoft.AspNetCore.Authentication.Google": "1.0.1",
"Microsoft.AspNetCore.Authentication.MicrosoftAccount": "1.0.1",
"Microsoft.AspNetCore.Authentication.Twitter": "1.0.1",
"Microsoft.AspNetCore.Diagnostics": "1.0.1",
"Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore": "1.0.1",
"Microsoft.AspNetCore.Identity.EntityFrameworkCore": "1.0.1",
"Microsoft.AspNetCore.Server.IISIntegration": "1.0.1",
"Microsoft.AspNetCore.Server.Kestrel": "1.0.2",
"Microsoft.AspNetCore.StaticFiles": "1.0.1",
"Microsoft.AspNetCore.Cors": "1.0.1",
"Microsoft.Extensions.Configuration.Abstractions": "1.0.1",
"Microsoft.Extensions.Configuration.Json": "1.0.1",
"Microsoft.Extensions.Configuration.UserSecrets": "1.0.1",
"Microsoft.Extensions.Logging": "1.0.1",
"Microsoft.Extensions.Logging.Console": "1.0.1",
"Microsoft.Extensions.DependencyInjection.Abstractions": "1.0.1",
"Microsoft.Extensions.Options.ConfigurationExtensions": "1.0.1",
"Microsoft.VisualStudio.Web.BrowserLink.Loader": "14.0.1",
"Microsoft.AspNetCore.Mvc.WebApiCompatShim": "1.0.2",
"Microsoft.Extensions.Logging.Debug": "1.0.1",

I also had to update the dependencies for some of these in our test library, so remember to check there too.

To complete the upgrade I also adjusted the SDK version in our solution’s global.json as follows…

"sdk": {
  "version": "1.0.0-preview2-003156",
  "runtime": "clr",
  "architecture": "x86"
},

With the above changes made I was able to build and run our site via Visual Studio. Great!

It wasn’t until a couple of days later when I hit an issue. During an issue I was working on I’d updated our model classes Entity Framework and needed to build my next entity framework migration. I did this by running the usual command…

dotnet ef migrations add AddNotifications

After building the project I was faced with the following error:

Could not load file or assembly 'Microsoft.EntityFrameworkCore, Version=1.0.1.0, Culture=neutral, PublicKeyToken=adb9793829ddae60' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)

At this stage I tried a few things, none of which fixed the problem outright, although they may have contributed to the overall solution. Firstly I cleaned my solution and rebuilt, no joy. Then I wondered if Nuget had cached any incorrect versions of the package, so I cleared my local Nuget cache and tried restoring my project dependencies again. Still no joy! Finally I hopped onto the ASP.NET Core Slack channel and sought help there. It was with huge thanks to Chad Tolkien then he suggested a manual deletion of my bin and obj folders within the project. I did that and rebuilt the solution. Success! Finally I was able to generate a migration using the EF CLI tooling. So it seems the clean and restore steps previously hadn’t cleaned everything they needed to.

I’d love to know if there’s a better way to manage these updates currently? I’m hoping that with the final tooling release and VS 2017 things will get easier. It would be useful for example, to be able to choose which track you want to use within Nuget Package Manager. I’m not sure how that would be achieved exactly but it would distinguish the packages you really want to get the latest within your chosen support track. It would also be handy if Microsoft blog posts about each release include specific details of each updated package and it’s latest version number. Having a quick reference when updating dependencies would have made my life a little easier. There are some release notes which hint at the main packages and their new version number, but it didn’t include all components that I ended up changing.

Developers working hard during the code-a-thon

Running a Humanitarian Toolbox Code-A-Thon

I’ve been involved with the Humanitarian Toolbox allReady project for about one year now. I’ve really enjoyed being able to contribute to the cause, while learning plenty along the way. Contributing has made me a better programmer and exposed me to new libraries and techniques which I am already benefiting from in my day job and in other projects.

During my time as a contributor I’ve be aware of the code-a-thons which have taken place in the US, Canada and more recently Europe. They’ve always seemed like great events and I’m excited to be signed up for the 2-day code-a-thon in London next month. A few months ago I discussed my experience with allReady and my wiliness to attend the London event with the management at my employer, Madgex Ltd. They were very receptive and were keen to support the cause. In fact, they are now funding 4 developers (including myself) to attend the full two days in London. Madgex are covering the transportation costs and hotel accommodation as well as loosing 4 developers for 2 days so this is a generous contribution.

In addition to the NDC event, we had discussions about arranging a code-a-thon at our office in Brighton, one evening after work. I was really keen to get something planned in and allow other developers to be able to contribute and experience allReady. With the concept green lighted by senior management, we set about making the idea a reality.

Planning

The starting point was to arrange a 30 minute presentation about Humanitarian Toolbox and allReady to gauge interest and demonstrate the application. I prepared a set of slides and a short demo which I presented in our board room one lunchtime. We had a good turnout of about 16 people in the room and it planted the seed with a few of the attendees, who talked to me afterwards about getting involved.

With the awareness raised, I followed up with some emails to the Madgex staff to further determine if people would be willing to join an event locally. I got some positive indications from enough people, so we picked a date and the invites went out to the staff. We planned our initial event for one evening after work. We agreed that 3 hours would be most practical given that people had already done a full day of work. Over the next few weeks I continued to promote the idea and started to confirm attendees for the evening. We had 7 or 8 people showing good interest as we closed in on the planned date. This was all supported by our developer lead at Madgex, Steve (great name!)

1 week to go

With a week to go we started to ramp up our planning activities. Steve (developer lead), helped with the physical space we needed and secured budget for some pizzas on the night (always a great lure for developers!).

I started combing through issues on GitHub that would be suitable for new contributors. We have a lot of work on-going with the project, pushing towards our v1 release but many of the issues that are left are quite complex and require a reasonable knowledge of the codebase. The balance was finding interesting and diverse issues, whilst ensuring that they wouldn’t require too much time up-front having to learn about the entire application. I had decided to try the new project feature inside GitHub to plan and organize the event. I created a few statuses and dropped issues into the “Not Started” category. The project feature is a basic Kanban board which allows cards to be dragged between statuses as the work progresses.

I also put together a pre-requisites list for the confirmed and tentative attendees, guiding them to ensure they had the latest tooling for .NET core installed, had forked and cloned the repository and were able to get it building. This is an important activity since it’s a little time consuming and we didn’t want to spend most of the time at the code-a-thon performing the setup work. As the big day approached we started to firm up numbers so we knew what cabling infrastructure we would need.

On the day

On the morning of the event, I did a final round up of the staff to confirm final attendees. After completing this we had about 6 locally able to attend as well as one of our developers from our North American office in Toronto, Canada. I started collecting GitHub usernames so that we could add people as contributors on the repository. This isn’t absolutely required, but aids in assigning issues to specific people. We also arranged to get people added to the Humanitarian Toolbox Slack channel so they could ask questions from our project experts and the Humanitarian Toolbox founders during the event.

At 4:30pm (with 30 mins to go) I moved into the meeting room we would be using for the evening. I wanted to get the webcam setup and ensure we had the cables for power and networking. Our fantastic systems technician Ricky had beaten me to it and already sorted the cabling requirements. We tested out the remote Skype link to Luke in Canada and made sure we had everything ready. A big thanks to Ricky for volunteering his time to provide some tech support and make sure we got up an running so smoothly.

Humanitarian Toolbox Code-A-Thon Sign
You must have a nice sign when running a code-a-thon!

At 5pm our team of developers migrated like birds in the winter, to the meeting room. We had 5 developers joining me locally, plus Luke video conferencing with us from across the pond. We’d decided to relocate our desktop PCs into this common area so that it would be easier to support each other during the event. While this required a little time up-front to disconnect and reconnect the PCs, it proved a good move as I was able to answer questions, share information and demo things very easily for everyone.

Preparing for the event
Developers getting setup for the event

By about 5:20pm we were in good shape, the computers were moved, developers were settled, pizza orders had been taken (priorities people). We started with a brief Google Hangouts standup with the Humanitarian Toolbox team. James and Tony introduced the project, it’s goals and thanked everyone for taking the time at the end of their working day to stay on and code for the greater good. We also had one of the most regular project contributors Mike on the call, showing his support for the event. Being able to speak to the founders and project team made for a great start and really set the tone for a fun evening, supporting code to save lives.

It’s worth pointing out here a couple of things that James and Tony highlighted during the standup. Firstly, the initial use case for the application will be to aid the American Red Cross with the effort to install free smoke alarms in people’s homes. Already this initiative has helped save lives when disaster struck and a family’s home caught fire. Thanks to a smoke alarm installed by the Red Cross, the family were alerted to the fire and all able to evacuate safely. Also important to note is that for each hour of coding time spent on the application we can expect about 40 hours of volunteer time saved. That’s a huge return and really shows that even sparing a few hours of personal time can have a massive impact.

With the project introduced and the devs eager to get going we started the team off by finding issues people were keen to work on. Sarah, Roberto and Luke took on some unit tests, Patrick picked up a new feature requirement, while Chris dove in with some EF and migrations code for the first time. Mark one of our front end developers started looking at the homepage UI improvements. I floated around the room to answer questions, demonstrate the site functionality and to help with the GitHub flow. I really enjoyed watching new contributors getting up to speed with the code and being able to assist their learning as they progressed. It was good to be able to witness some of the common questions new contributors have so we can focus on lowering the barrier to entry in the future.

Developers working hard during the code-a-thon
Developers working hard during the code-a-thon

There were a lot of new things for everyone to learn and they did a fantastic job of absorbing the information and becoming productive quickly. All of the developers were new to GitHub and OpenSource, so there was learning to be done around the processes to ensure an up-to-date master branch, to manage rebasing and prepare pull requests. Most of the team were also experiencing ASP.NET Core for the first time, which in itself has a lot of new concepts to learn. It was also the first exposure to Entity Framework for everyone, so that had it’s own learning curve too. This really highlights another benefit of contributing to the project for developers. It’s a great learning experience that puts a real-world, production-ready code base in the hands of developers.

During the 3 hours everyone knuckled down and other than a brief break to load up our plates with some pizza, we worked solidly. I had hoped to use the GitHub project feature to keep track of things, but we hit some issues with people not being able to move the cards themselves. While I was able to do this, it proved more of a hindrance as my time was better spent helping people around the room. In the end we abandoned that feature and in hindsight a post-it-note board might have been easier to manage. I still like the concept of using GitHub so that others not physically at the event can monitor progress, but we need to find a way to allow contributors to manage the cards as they work on issues. I suspect it might just be a permissions thing, so I’ll investigate it soon.

By the end of the evening we had submitted two pull requests which were reviewed and merged into the project before we left. We also had three other issues very close to being completed which will be finished off in the coming days and hopefully submitted soon. Given the setup time, huge learning curve and relatively short coding period of three hours, I’m very pleased with this achievement. Everyone was amazed when we realised that we had hit the 8pm finish already. Time flew, which is a great sign and from feedback so far, people would have liked to have even longer with the code. Perhaps an all day event ison the cards for the future.

I really hope that everyone left feeling as positive and happy as I did. Certainly the sense I got was that everyone enjoyed learning some new things, getting to grips with the code and contributing to a good cause. I feel proud to be part of such a generous team of people who were able to join this code-a-thon after a full day of development at work first. Everyone should be very proud of what they managed to contribute. I’d love to run another session to continue the great start we’ve made and if people are willing, perhaps we can make it a regular thing or even look at a longer full day event.

From a personal perspective I enjoyed sharing what I’d learned during my time with the project and seeing other developers pick up the concepts for themselves. Prior to this, I’ve never been too keen on the idea of being a “teacher” and even presenting is not in my normal comfort zone, but I found a bit of a passion for instructing people. I’m a firm believer that by sharing information, it helps our own understanding as you are challenged to know enough to be able to articulate the concepts.

Code-a-Thon Team Photo
Code-a-Thon Team Photo (Sarah had escaped just prior to this being taken!)

Feedback

The feedback the day after has been extremely positive. I’m very happy to hear that people enjoyed themselves and had a positive and fun experience. It’s nice to spend time coding for fun, outside of the normal day-to-day work. Being able to put your skills to use towards such a positive concept is also very rewarding. From a quick survey afterwards, the team are keen to continue to contribute to the project and would like to take part in another code-a-thon in the future.

Thanks again to everyone who took part:

Our developers: Sarah, Patrick, Chris, Mark, Roberto and Luke
Tech support: Ricky
Planning and management: Steve K
Humanitarian Toolbox Support: James, Tony and Mike

Custom ModelBinding in ASP.NET MVC Core How to HTML Encode Strings

In a previous post I showed how we could automatically HTML encode data when deserialising it from a JSON request body. In my case this was to meet some specific security requirements we had for an ASP.NET Core API we were building. This time around I will discuss a similar requirement which stated that we should also ensure we HTML encode any strings bound from the route, querystring and form data. We’ll do this by creating a custom ModelBinder.

Much like our earlier requirement, we needed to ensure that we are not storing un-encoded data containing HTML or script tags in our database. While our application escapes the data on the way out, we want to prevent anyone accidentally rendering un-escaped HTML in future applications. We have no requirement to accept HTML code on our API so we decided to ensure each value was encoded by default during binding. As before we wanted to enable this globally so that no developer would have to take specific steps to enable this per controller or action.

Model Binding Flow

Before I jump into the solution, I’ll firstly explain at a high level how the model binding flow works in ASP.NET Core MVC. To understand this better I ended up following steps from this blog post where I discuss how we can add the MVC source solution to our code, allowing us to debug into it. With this in place I was able to step through the model binding code to watch and understand what was happening.

Model binding in principle is quite straight forward. It attempts to match up values coming in on the request to any properties expected by the parameters of the controller and action. Each value is run through the model binding flow which looks to find a suitable binder to handle the value. To find a suitable binder, ASP.NET MVC Core uses binder providers. These providers are registered when MVC is initialised and by default includes 14 different providers. Each of these implements the IModelBinderProvider interface.

There is a provider to handle key value pairs for example, another for complex types and another for simple types. These binders are registered in a specific order and MVC checks each provider in that order during the binding process until it find the first provider which can provide a suitable binder for the object being bound. Each binding provider will have some conditions that check the binding provider context. Once these conditions are met, the provider will return a binder that can handle the binding.

Once we have a model binder available it’s BindModelAsync method is called. This method expects to handle the value of the object being bound and returns a ModelBindingResult. If the binding succeeds as expected then a Success is returned, including the final value to be bound to a property. I hope to spend more time exploring the binding process in a future post. It’s a little beyond the scope here to explain the deeper details of how everything is hooked up. For now, let’s look at how we meet the requirement above to url encode values during the binding process.

Let’s assume we have the following action.

We want to ensure that the newTitle property is HTML encoded by the time we can access it within the action.

Creating a ModelBinder

The starting point for our solution was to create a model binder based on the IModelBinder interface.

We initialize the binder passing in an IModelBinder as a fallback. In my case I am specifically expect to handle string values but I don’t want to worry about the cases for handling null or empty strings. That is already covered in the default SimpleTypeModelBinder provided in MVC. Therefore I chose to pass in a binder which I will pass the responsibility onto in those cases.

Within the BindModelAsync method we first call the the ValueProvider.GetValue method on the value provider in the bindingContext. We pass in the model name which returns a valueProviderResult. As long as there is a value in the result we first check if it’s null or empty. If so, this is where we call the fallback binder’s BindModelAsync method. If we do have a suitable value then we proceed to HTMLEncode it before creating a success ModelBindingResult. Finally we return a completed task using the internal helper TaskCache.CompletedTask.

Creating a ModelBinderProvider

Now that we have a model binder defined we need to create a provider which will determine if the modelbinder is suited to the object being bound. Here is the code…

This provider is pretty simple. We must implement the GetBinder method on the IModelBinderProvider interface. We use the MetaData on the ModelBinderProviderContext to determine if it can provide a suitable binder. This meta data includes some helper properties such as the IsComplexType flag to help us determine if we can provide binding for the object. In this case we are looking only for strings. If the object being bound is a string, then we can return our custom HtmlEncodeModelBinder. Otherwise we return null. When we return null the next binding provider will be given the chance to provide a binder. This continues until a suitable binder is found. You’ll notice that we pass in a new SimpleTypeModelBinder which will act as our fallback for any string null or empty cases we encounter during binding.

Now that we have a binder and a provider, the final step is to add this provider to the list of binding providers. Since these are executed in a set order we also need to place our provider in the right place. I’ve achieved this using the following extension to the MVC options.

What we are doing here is using LINQ to find the SimpleTypeModelBinderProvider in the list of ModelBinderProviders. Our binder provider need to run just before the simple type binder to ensure that we can have the opportunity to handle the string types with our HTML encoding logic. If we placed it after the SimpleTypeModelBinderProvider we would find that the binding flow never reached our code as the binding would already have been handled. We then get the index of the SimpleTypeModelBinderProvider and using that index we insert our binder provider into the list at that position. Now, when MVC binding occurs, our binder provider will be part of the binding process. If we inspect the ModelBinderProviders during debug it should now look like this:

ASP.NET Core ModelBinders List

You can see our new custom HtmlEncodeModelBinderProvider listed before the SimpleTypeModelBinderProvider.

Finally, with everything complete we can do the final wiring up. We call our options extension when adding MVC within the ConfigureServices method in Startup.cs

Now when the newTitle parameter is bound on our example action it will ensure that any html tags are safely encoded. As with all of my posts, I’ve taken the best approach I could think of to implement this. I welcome any comments and suggestions to improve this code or correct any mistakes!