New in ASP.NET Core 3.0 - Blazor Client Side

In the last post, we had a quick look into Blazor Server-Side, which doesn't really differ on the hosting level. This is a regular ASP.NET Core application that will run on a web server. Blazor Client-Side, on the other hand, differs for sure, because it doesn't need a web server, it completely runs in the browser.

Microsoft compiled the Mono runtime into a WebAssembly. With this, it is possible to execute .NET Assemblies natively inside the WebAssembly in the browser. This doesn't need a web server. There is no HTTP traffic between the browser and a server part anymore. Except you are fetching data from a remote service.

Let's have a look at the HostBuilder

This time the Program.cs looks different compared to the default ASP.NET Core projects:

public class Program
    public static void Main(string[] args)

    public static IWebAssemblyHostBuilder CreateHostBuilder(string[] args) =>

Here we create IWebAssemblyHostBuilder instead of an IHostBuilder. Actually, it is a completely different Interface and doesn't derive from the IHostBuilder at the time I wrote this. But it looks pretty similar. In this case, also a default configuration of the IWebAssemblyHostBuilder is created and similar to the ASP.NET Core projects, a Startup class is used to configure the application.

The Startup class is pretty empty but has the same structure as all the other ones. You are able to configure services to the IoC container and to configure the application:

public class Startup
    public void ConfigureServices(IServiceCollection services)

    public void Configure(IComponentsApplicationBuilder app)

Usually, you won't configure a lot here, except the services. The only thing you can really do here is to execute code on startup, to maybe initialize a kind of a database or whatever you need to do on startup.

The important line of code here is the line where the root component is added to the application. Actually, it is the App.cshtml in the root of the project. In Blazor server side this is the host page that calls the root component and here it is configured in the Startup.

All the other UI stuff is pretty much equal in both versions of Blazor.

What you can do with Blazor Client Side

In general, you can do the same things in both versions of Blazor. You can also share the same UI logic. Both versions are made to create a single-page application with C# and Razor and without to learn a JavaScript framework like React or Angular. It will be pretty easy for you to build single-page applications if you know C# and Razor.

The Client-side version will live in the WebAssembly only and will work without a connection to a web server if no remote service is needed. Usually, every single page application needs a remote service to fetch data or to store data.

Blazor Client-Side will have a lot faster UI because it is all rendered natively on the client. All the C# and Razor code is running in the WebAssembly and Blazor Server-Side still needs to send UI from the server to the client.


In this part, you learned a different kind of Hosting in ASP.NET Core and this will lead us back to the generic hosting approach of ASP.NET Core 3.0.

In the next post, I will write about a different hosting model to run service workers and background services without the full web server stack.