Precompiling MVC Views in ASP.NET Core with .csproj’s

Update 8/29/2017: 

When running ASP.NET Core 2:

  • Using .NET Core 2
    • You no longer have to do anything.  The app will precompile by default on publish.
  • Using full .NET Framework
    • The only thing you need is the package reference below.


Below is the original post for ASP.NET Core 1


I didn’t see this documented on yet for .csproj style projects, so I thought I’d blog about it, as it’s something I do on every project.


Add this to your .csproj file:


Alternatively, you can check out this GitHub commit that just adds pre-compiled views here.

Also – if you’re running on .NET Core and not Full Framework, then make sure your TargetFramework is netcoreapp1.1+ and not netcoreapp1.0.

What does precompiling views do?

Without precompiling your views, none of your views are compiled until they are requested for the first time.  This results in an extra few seconds of wait time for the end user.  Instead, most developers want to take a little bit of extra time in the publish step to precompile their views, so that users don’t have to eat that performance hit on the first request on every page.

Another benefit of precompiling your views is if there is a C# error on one of your .cshtml Razor pages (such as you changed the name of a property on your View Model and the rename didn’t update your .cshtml file), you will find out up front when you go to publish (via VS or a build server) rather than at run time when a user goes to request that page.

So now that we’ve seen what it does, let’s look at what it used to be in project.json.

What did it look like in project.json?

In project.json land, the pre-compiling of Views was a little verbose.  You had to do the following:

  1. Add a reference to “Microsoft.AspNetCore.Mvc.Razor.ViewCompilation.Design” under the “dependencies” section
  2. Add a reference to “Microsoft.AspNetCore.Mvc.Razor.ViewCompilation.Tools” under the tools section.
  3. Under buildOptions, set “preserveCompilationContext”to true
  4. And finally you had to add a fairly verbose post-publish script to actually use the pre-compile tool.All of that looked like this (removing the rest of the project.json for brevity):

This then generates a .PrecompiledViews.dll in the root of your publish directory.  You will have no refs folder as well.

How does it look in .csproj?

In project.json, it was pretty verbose.  So what does it look like in .csproj?  It’s much cleaner.

  1. Add <MvcRazorCompileOnPublish>true</MvcRazorCompileOnPublish>
  2. Add <PackageReference Include=Microsoft.AspNetCore.Mvc.Razor.ViewCompilation Version=1.1.0 />


So the final output of a .csproj might look a little something like this.  Note lines 5,6, and 32.

This will still generate a .PrecompiledViews.dll in the root of your publish directory and you will still have no refs folder.

This simplicity in order to get precompiled views is yet another win for the .csproj style ASP.NET Core projects over project.json.

Using the ASP.NET Core Script TagHelper to Polyfill The Latest JavaScript Features


The primary use case for fallbacks in the Script Tag Helper is checking if a CDN is down and loading a local script instead.  Another use case is you can use fallbacks to make it easy to polyfill missing JavaScript features in older browsers, while not having your users using the latest browsers take a network hit.

Background – What is a Polyfill?

A polyfill is a fallback, written in JavaScript, that fills in a new JavaScript feature that is missing in an older browser and replicates the same functionality that newer browsers provide natively.  This allows JavaScript code using the latest features only available in newer browsers to work on older browsers as well.

To polyfill the API, you simply just check if that API is available, and if not, load the polyfill that adds that API to your browser.  That way, if your user is using the latest browser, you don’t punish them by downloading something they don’t need and hurting their performance.

How a Polyfill works

This JavaScript code will polyfill the “new” fetch API for making AJAX calls, that is currently available in all of the latest major browsers (Chrome, Edge, Firefox, Safari, etc.), but not in any of the Internet Explorer family tree (not even IE11).

First, the code looks to see if the Promise API is available, because fetch relies on Promises in order to work.  If the API is not available, it loads a script from a CDN that polyfills the Promise API.  It does the same thing for the fetch API.

The built-in Script Tag Helper – CDN fallback

The built-in Script Tag Helper in ASP.NET Core has fallback support baked in that generates code very close to this.  The original intent of their fallback support was for attempting to load a script from a CDN.  After loading from that CDN, test to see if a JavaScript API is available, and if it’s not (due to the CDN being down, blocked, or whatever) fall back to loading a local JavaScript file.  That code looks like this:

The fallback works for polyfills too!

What I haven’t seen anyone mention is to use this to conditionally polyfill if a JavaScript feature doesn’t exist.  And it works right out of the box, simply omit the src attribute, and the rest will be taken care of:

The code above will generate nearly identical code to what’s in the first code sample.  As mentioned above, fetch needs Promises to work, so I’m also polyfilling that conditionally as well.  In the real world, I would likely bundle these two together (locally or using something like and make one network request, but using unpkg (an NPM CDN) was just for simplicity’s sake.

With the code above, I get the best of both worlds.  My code using fetch runs on older browsers, but my users who use the latest browsers don’t take a network hit for something they don’t need.  This same methodology will work for other ES2015 and above features and is nice for small apps that want to use some of the latest features and not have to deal with the build config of Babel or TypeScript transpiling down to ES5.

I’m not sure if this was an intended feature of the Script Tag Helper, but it’s a good use case for the fallback feature that I hadn’t seen anyone talk about, so I wanted to call attention to it.

And if you are lucky enough to only have to support Chrome at work.  I assume this is you right now reading the hoops us plebes have to go through to get our code to run on all browsers:

Custom Tag Helper: Toggling Visibility On Existing HTML elements

A lot of times we need a way to toggle the visibility of an element based on some server-side values.  The most common example is if someone is logged in, show them more options than someone who is not logged in.

Option without Tag Helpers

This solution always felt a little dirty to me

What I’d like to do is to add an attribute off of div to toggle visibility.  So instead I’d like something that looks like this:

So let’s create a custom Tag Helper to do this

  • Create a TagHelpers folder in the root of your MVC project
  • Create a new class called VisibilityTagHelper
  • Copy and paste the super simple code below
  • Go to /Views/_ViewImports.cshtml and add a line for all the TagHelpers in your project by doing @addTagHelper *,  .  Example below:
  • Build your project *this is important or you won’t get intellisense*
  • Start using it such as the example shown below (same as above)
  • That’s it!


Or view the GitHub Commit that makes this happen

I also have a repo, where the first two commits are just the initial commit with a File => New Project.  This 3rd commit outlines all the changes needed to make to get this to work if you’d rather learn via a Commit.


Breaking Down The Code

The code for the Visibility Tag Helper is pretty straight forward, but here’s a breakdown of it:

  • HtmlTargetElement
    • You first decorate your class with the HtmlTargetElement attribute to tell it that you’re targeting div’s.
    • You can target more than just divs by adding more attributes.
    • You could add the is-visible attribute to ALL elements by using *.  This could be quite handy
    • UPDATE: 10/4/2018 – Per Andrew’s comment below, you can have this target all elements like above, but skip going through this TagHelper rendering by saying only run it if they have the is-visible attribute.  This should save some perf as well as not give you purple tags in Visual Studio on all of your HTML elements.
    • [HtmlTargetElement("*", Attributes = "is-visible")]
  • Inherit from TagHelper.
    • Naming the TagHelper doesn’t matter, as we are choosing the HtmlTargetElement.  If we were making our own tag, then the tag would be based off of the name.
      public class VisibilityTagHelper : TagHelper
  • Add a property to store the visibility of the tag.
    • We need to default it to true, otherwise none of the HTML tags specified in the HtmlTargetElement will be visible, because booleans default to false.  THAT IS REALLY IMPORTANT.  The name will be kebab-cased which means each capital letter will be split by a dash.  Example IsVisible becomes is-visible when using the tag.
      public bool IsVisible { get; set; } = true;
  •   Override the Process method
    • If the tag is set to not IsVisible, then we need to supress the output which will not render the tag at all.  (Thanks to Damian Edwards for the tip about SuppressOutput())
      public override void Process(TagHelperContext context, TagHelperOutput output)
          if (!IsVisible)
          base.Process(context, output);
  • That’s about it!  Super simple.


Update 1/3/2017: As mentioned above – thanks to Damian Edwards for informing me of the output.SuppressOutput() method.  That made things simpler.

How to fix a failure in upgrading from ASP.NET Core 1.0.x to ASP.NET Core 1.1

Error I received

  1. The project has not been restored or restore failed – run dotnet restore

  2. The project does not list one of ‘win10-x64, win81-x64, win8-x64, win7-x64’ in the ‘runtimes’ section.

  3. You may be trying to publish a library, which is not supported. Use dotnet pack to distribute libraries.

Can not find runtime target for framework ‘.NETCoreApp,Version=v1.1’ compatible with one of the target runtimes: ‘win10-x64, win81-x64, win8-x64, win7-x64’.

How to fix it

Open your project.json and under “dependencies” change

"Microsoft.NETCore.App": "1.1.0",


"Microsoft.NETCore.App": { "version": "1.1.0", "type":  "platform" },

Then re-run your app.


How it happened

I’m still using Visual Studio 2015, but I have .NET Core 1.1 downloaded and installed and I wanted to upgrade to .NET Core 1.1 and update to all the latest ASP.NET Core 1.1 packages.

I went to File => New Project => ASP.NET Core Web Application, which proceeded to give me .NET Core 1.0.1, which was expected.  I followed the upgrade path, but I forgot to hit “Save” on my project.json in step 2, when I changed the values to .NET Core 1.1.0.  Therefore, it did not restore the .NET Core 1.1 NuGet package.

So when I went to the NuGet UI under Updates, .NET Core was an option to update, which I didn’t realize when I hit “Update All.”  Well that jacked with my project.json in this way…





NuGet removed my JSON nested object with just the version number.  What the “type”:”platform” does is it tells my project that it expects .NET Core will already be installed on the machine, therefore it will not bundle .NET Core with my app.

When “type” : “platform” is NOT specified, then it assumes I am bundling .NET Core WITH my app.  In that case, I need to specify the exact platforms that I’m targeting (i.e. Windows 10, Linux, Mac, etc.) in the “runtimes” section.  However, that “runtimes” section was not there previously, because I was targeting the platform.

This seems like it would be a bug with NuGet.  It should only update the Version, and not overwrite anything else.  Maybe this gets fixed during the switch to csproj coming soon.  I’m not going to file a bug at this point on GitHub, because I doubt they make the investment to fix this in project.json.

Feature Folder Structure in ASP.NET Core

A feature folder structure is organizing your app by feature as opposed to technology (Controllers, Views, etc.), which is the default structure in MVC.  A feature folder structure is easier to maintain than the default structure and even Areas (which is a poor man’s feature folder structure).  It looks something like this:

Why would you do this?

Anyone who has built any ASP.NET MVC app of any size often runs into the same problem.  You’re implementing a feature, and you end up spending a lot of time bouncing between folders while you’re implementing the feature.  You keep opening and closing the Controllers folder, the Views folder, the View Models (or Models) folder, the Scripts folder, the Content/CSS folder, the Shared folder under Views, etc.  As your app grows, there are more and more files in these various folders that you have to wade through to find the files you actually need.  All the while, you get the sense you’re fighting your folder structure and fighting the default framework conventions.

Wouldn’t it be nice for all these things that make up the feature to live in the same folder?  That way, you’re just living in one folder (on the front end anyways, business logic and data access logic is likely in another project), and everything is laid out nicely next to each other.  I’ll show you exactly how to do that in ASP.NET Core.  Instead of “feature” you could use the word component, module, etc., but I like the word feature best in this scenario.  I actually like the word component better as a more consistent descriptor across other web stacks (such as Angular, React, etc.), but because View Components are a new feature of ASP.NET Core, the name “component” might be a bit misleading in ASP.NET Core land.  Naming is hard.

I’m not alone in liking this structure

This is not a unique idea.  Not even close.  Jimmy Bogard does it.  Tim Thomas wrote a blog post on it.  Matthew Renze wrote a blog post about doing it in ASP.NET 4.x.  Even other stack experts like Angular guru John Papa structures his apps this way.  And there are many, many other people who like this structure better, even if it goes against the default conventions that Microsoft expects.

If a ton of people do this, why write this blog post?

Because I haven’t seen an implementation of this for ASP.NET Core.

Ok, so how do we do this?

To do this, we simply need to do move some files around and change some configuration about Views.  JavaScript, CSS, ViewModels, etc. are not things known by MVC’s conventions, so we don’t have to worry about those, and Controllers are automatically picked up.

1. Rename the Views folder to Features

2. Add a custom ViewLocationExpander to tell ASP.NET where to find the Views

a. Create a folder to house this config.  I usually call it Startup Customizations or Infrastructure and place it under the root of your csproj, but you can call it whatever you’d like.

b. Create a new file under this folder called FeatureLocationExpander.cs under the folder from the step above and copy the code at this gist (shown below):

3. Wire up the FeatureLocationExpander in Startup.cs

a. Add the code at this gist (shown below) to your ConfigureServices method in your Startup.cs after the .AddMvc()

4. Move any feature-specific View Models, JS, and CSS to their respective feature folders.

5. Move the Controllers into their respective feature folders and delete the Controllers folder

6. That’s it!

The only other thing you probably want to do is run your CSS and JS through some sort of task runner (to bundle, minify, and copy to wwwroot), whether that task runner’s Gulp, Grunt, or just plain ol’ NPM scripts which seems to be what the cool kids are doing these days.  There are plenty of other blog posts that explain how to do that, including on the official ASP.NET Core documentation.

Final Thoughts

This leaves us with a much easier to maintain folder structure that allows us to stay focused and productive while writing code, and not wasting time hunting for files in different folders.


UPDATE 8/24/2017: For Resharper users – click here to figure out how to fix Resharper’s intellisense.  Thanks again to Bill Sorensen in the comments below who first pointed out this solution to me.