Introduction to Blazor

/ 29 May, 2018

Blazor is a new experimental .NET web framework created by Steve Sanderson of Microsoft. It utilizes the WebAssembly technology together with Mono to render the User Interface in the client’s browser. The cool thing about Blazor is that it can be used to create a web front-end (single page application), rendering regular HTML, with all the coding in C# and Razor syntax instead of JavaScript! For the back-end, you can use whatever technology you like. Using ASP.NET Core with C# provides a nice development flow and creates less context switching between languages in the back- and front-end: they just use the same language! Even the places in which we would normally use JavaScript, can now be coded using C#. How cool is that! 

This article guides you through the steps needed to start developing with Blazor and get a feel for the new possibilities provided by this new framework. I am working with version 0.3.0 for this article. 

Note: this new framework is marked ‘Experimental’ for a reason: it still has some rough edges! Known issues at the time of writing this article:  

  • You can’t run and debug in Visual Studio in combination with IIS (both dotnet run and IIS Express work well).  
  • All modern web browsers (even mobile) accept the WebAssembly, but older browsers use a fallback with a JavaScript polyfill. These fallbacks aren’t available yet for IE11. 
  • Debugging with Visual Studio inside the WebAssembly itself isn’t supported yet. 
  • Do not use a hyphen or space in your new project name. Currently, there is a bug in Blazor that will break the tooling. 
  • The current update cadence is about two weeks from version 0.1.0 to 0.2.0 and 0.3.0, and the team is planning to stick to that speed. 

Prerequisites for installation:  

For a full installation guide, you can follow the steps in Microsoft’s Preview Announcement. In short, the prerequisite steps are:  

  1. Install the .NET Core 2.1 Preview 1 SDK. 

To also have Visual Studio tooling available, you also need the latest Visual Studio Preview version and the Blazor Extension: 

  1. Install the latest preview of Visual Studio 2017 (15.7) with the ASP.NET and web development workload.
    Note: You can install Visual Studio previews side-by-side with an existing Visual Studio installation without impacting your existing development environment. 
  2. Install the ASP.NET Core Blazor Language Services extension from the Visual Studio Marketplace. 

Note: you can find the links to download the prerequisites in the announcement link. 

Specifics for version 0.3.0 are mentioned here. 

.NET Core 

If you don’t want to wait for the full Visual Studio download and install, you can start by dropping into your command line and getting the Blazor Templates: 

dotnet new -I Microsoft.AspNetCore.Blazor.Templates

After that, you will see that there are three new Blazor Templates available: 

  • Blazor (hosted in ASP.NET server) 
  • Blazor (standalone) 
  • Blazor Library 

Navigate to a folder in which you want to save the new solution and trigger the create project from the template command: 

dotnet new blazor -o BlazorTestApp

Change into the new directory and run it. 

You can now test the application in your browser: 

This is a project, coded in Razor and C#, compiled to an assembly and running in a browser!  
If you open the developer tools and check the network calls, you can see the files that your browser downloads to render the page: 

  1. Localhost is the initial call to the hosting web application. 
  2. BlazorTestApp is our new front-end application, compiled into one assembly! 
  3. The rest is the Blazor framework and some .NET Core dependencies. 


You can see that our new BlazorTestApp is delivered to the browser as one file and that it doesn’t return HTML like a regular web application! The BlazorTestApp assembly hosts all the code for our web application: i.e. every page, every function, all our code now lives inside the browser on the client! You don’t need any more round trips to the server to load new pages. All you need is a set of REST calls for loading additional data. 

And because these are all static files, they can be cached or served from a CDN for even faster performance.  

Since WebAssembly is supported in all modern browsers, the same client application also works on a phone’s browser. This is Safari on iOS: 

Visual Studio 15.7 Preview 

After you have installed the Preview version of Visual Studio and the extension from the marketplace, you can use Visual Studio to create a new application.
Create a new ASP.NET Core Web Application: 

Now you can choose how the application will be hosted:  

The ASP.NET Core hosted option provides a good starting point that utilizes .NET Core to host your application, which makes it easy to run. If you don’t have the Blazor option available, check if the setting for the .NET Core version is set to ASP.NET Core 2.0.  

After this, you’ll have 3 projects in your solution:  

  1. BlazorApp.Client: The WebAssembly project holding all the front-end pages and logic. 
  2. BlazorApp.Server: The server hosting with a start for MVC controllers and to provide a WebAPI endpoint for loading data from a callback. 
  3. BlazorApp.Shared: A shared project for central objects that are used, e.g. model classes to move data between the front- and the back-end. 

Server hosting 

In the example solution, a server web application is provided to host the Blazor WebAssembly file that is compiled from the Client project. A starting point in the Server project to load the Client assembly can be found inside the Startup class in the Configure method: 

You can see there is a dependency on the Client project that loads the client assembly and passes it back to the browser with all the information the browser will need to load the WebAssembly.  

The MVC part is the entry point for hosting an example of the WebApi you can host to provide the Client with data. 

Blazor Client 

The wwwroot folder contains the entry point of the client application: index.html. 

This file only contains the basic HTML elements to show a ‘loading’ message to the user and trigger the loading of the WebAssembly. This happens inside the blazor-boot script. 


Things get really interesting inside a Blazor ‘page’. Remember, Blazor is based on Razor syntax. The first lines contain the standard Razor using statements to reference classes like the ones in the Shared library and a new @page directive. The page directive contains the route for the page, so it can be addressed inside the WebAssembly. Only pages with this directive can receive direct requests from other pages in the WebAssembly. This means that without it, it’s not possible to navigate directly to that page. 

For example, the index page:  

The @page directive indicates that this page is routable as the main index.  

The SurveyPrompt tag in the index page is a tag helper for a reference to a component. For more information about tag helpers, see here. I will show how this works in Blazor in the next paragraph. 


Blazor components are used just like tag helpers in ASP.NET Core. They can be found as a Razor page inside the ‘Shared’ folder. As mentioned earlier, they don’t have an @page directive so they cannot receive any direct request from the browser. They can be used inside other pages or other components. 

SurveyPrompt component 

The component file contains the logic required to display a survey prompt: a text message with a link to the survey you want to display: 

The @functions directive includes parameters that can be used to call a component, in this example ‘Title’. A member tagged with the Parameter directive is converted into a parameter for the tag helper. In the file ‘Index.cshtml’ we’ve seen the Title parameter being entered into the call to the component. That parameter is now used to show the title inside of the SurveyPrompt component. 

By running the application (by default on IISExpress), you can follow the loading process of this component: 

  • the browser loads the index.html page, 
  • displays the ‘Loading…’ message, 
  • triggers the blazor-boot script, 
  • which in turn triggers the downloading and loading of the actual WebAssembly, 
  • displays the HTML objects configured using C# in the index page, 
  • calls into the SurveyPrompt component, 
  • displays its message with the link. 

Very cool! 

Already provided in the preview version 

Even though Blazor has just been released in preview, it already contains a lot of stuff available out of the box:  

  • Redeploy on file save 
  • Client-side debugging 
  • Dependency injection into Razor pages 
  • Page lifecycle methods 
  • One-way and two-way databinding fields to inputs (right now only for strings and Boolean fields) 
  • Event binding for inputs (all events, even custom ones available as of version 0.2.0). 

Dependency injection 

The dependency injection system is used to inject objects into the Razor pages. By using the inject directive in a Razor page, you can request an instantiated class:  

@inject HttpClient Http

HttpClient is one of the two system services provided out of the box, the other system service is an IUrihelper for navigation options. The injection of the HttpClient is visible inside the FetchData page, which is then used to perform a callback for the weather data: 

Also notice the override on OnInitAsync, one of the page lifecycle methods available to start loading data from the back-end. The other lifecycle method you can currently use is OnParametersSetAsync for reacting as soon as the parameters are set. 

No JavaScript 

Below is an example of binding to an event and linking it to code that would normally use JavaScript. The variable currentCount is shown on the page and a button click event is linked to a C# function that increments that variable. Because of the binding between the variable and the displaying of the value, the new value will be updated client side, without any extra code to handle this. We can still use JavaScript if required. Blazor provides interops to call JavaScript from C# and vice versa. Should we need to include a library to perform some fancy animations, nothing will keep us from doing just that. 

Creating component libraries 

As of version 0.2.0, the team has invested in making it very easy to re-use a component (a Razor page used by Blazor) so that we are able to create libraries and NuGet packages of our default components to re-use them in other Blazor projects. Creating the library isn’t available in Visual Studio yet, but there is a template for .NET Core: 

dotnet new blazorlib -o BlazorTestAppBlazorComponentLibrary

You can now add it from Visual Studio to the solution. Then add a reference to it in your client project. You can now use the component (just a Razor page from the library) in any Razor page or component you like. For example in the index.cshtml: 

In the first line we added a reference to the library and in the second line, we imported all TagHelpers (remember: the page/component name works the same as a TagHelper) from the library so we can use the new component in the last line. If you check the network calls in the browser, you can find the new assembly is loaded on its own. We can build our own client-side components and re-use them wherever we need! 


We’ve now seen that Blazor already has some great features available out of the box. A lot of Razor concepts are already available, but it has some rough edges, which is logical for an experiment. It’s very cool that it has been open-sourced so soon after its creation. You can check out the GitHub repository and contribute to the project if you want. I am very excited to see what they can accomplish in the future! 

If you want to dive deeper into Blazor, there are a few awesome tutorials available. 


This article is part of our latest magazine; XPRT.#6   Download it here or get your free copy.