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Choosing Between Blazor Server or WebAssembly

Building for the web using a language you're already comfortable with? Sounds like a great idea as long as it's easy to use and performs well for clients.

Building for the web using a language you’re already comfortable with? Sounds like a great idea as long as it’s easy to use and performs well for clients. That’s the promise of Blazor, and the team at Microsoft is working feverishly to deliver.

But one question that keeps popping up is “which flavor of Blazor should I use: Server or WebAssembly (WASM)?” Let’s dive in and learn the differences and use cases for both.

Before we move on: I’m stoked to be taking part in C# Advent again this year. Honestly, I don’t understand how I was allowed back. I guess mistakes were made.

Regardless, I’m grateful to all the folks that made C# Advent possible this year. If you haven’t read the other articles, be sure to check out the link and enjoy!

permalinkWhat the Blazor!?

C# developers have been dabbling or building with Blazor for a while now, but, for the uninitiated, what is it? In simplest terms, Blazor allows you to build web applications using C# instead of, or in tandem with, JavaScript (sorry for using the “J”-word.) With Blazor, you can build feature-complete apps with reusable web components that make it easy to isolate UI design, functionality, and testing.

Because you’re writing C#, you can use any .NET libraries that conform to the .NET Standard. That means most of the packages you would pull from Nuget can be included in your application. But the first choice you have to make is what flavor of Blazor you want to use.

Blazor can run your C# application in the browser, using WebAssembly, or on the Server. Both have pros/cons that should be considered depending on the use-case of your application.

permalinkBlazor Server

All Blazor Server code, including Razor pages, are compiled into libraries that run, as the name implies, on the server. When clients visit the application, they receive a page with a little JavaScript that initiates a connection to the server via SignalR.

The server then uses that websocket connection to send small payloads to the client that updates the page. This allows users to experience fast load & render times, but also means that every visitor has a persistent bi-directional connection to the server. You’ll definitely want to take that into consideration when thinking about scaling your application.

permalinkServer Pros

Render & Load Times

One of the biggest positives of Blazor Server is load & render times. Since users aren’t downloading the runtime or application libraries, they can start using the application faster.

Ease of On-Boarding

Getting started with Blazor Server is much faster because you’re writing the same C# code you would regardless. As long as your code conforms to .NET Standard 2.0, your existing classes, data-access, business logic, etc. will just work. Because all the code runs on the server, it also means you don’t have to build a separate front-end and back-end.

Browser Support

Unlike WASM, Blazor Server is supported in all major browsers because it doesn’t require the browser to support WebAssembly. This is a major consideration if you still need to support users with IE 11 or other older browsers.

permalinkServer Cons

No Offline Support

Blazor Server is not going to be the right fit for you if you’re looking to add offline support for your users. Since each page & component are served from the server, no code is maintained on the client.

Page Load/Change Latency

When changes need to be made to the UI, Blazor Server recognizes and sends the diff down to the client to update the presented UI. This is much faster than full page reloads, but can cause latency on the round-trip. You’ll want to be mindful of the datasets you’re passing around to minimize the size of the diff being sent.


Scalability isn’t necessarily a “con” for Server Blazor, but it is something to consider. Since every visitor has a separate websocket connection to the server, the amount of memory consumed by the application per user may require scaling hardware more quickly than Blazor WASM.

Servers Not Included

Because your site will be running server-side, you will need something to serve your application and handle requests. Serverless options are available, but processing is handled server-side and not client-side, so all logic, etc. must be handled there.

permalinkBlazor WebAssembly

Mozilla defines WebAssembly, or WASM, as a “low-level assembly-like language that provides near-native performance that allows other languages to run on the web.”

Blazor WebAssembly is a single-page app (SPA) framework for building client-side web applications with .NET. Developers can write C# and utilize code that already exists in their back-end applications like models, business logic, and more. When a Blazor WASM app is built, all your C# code and Razor files are compiled into .NET assemblies that are downloaded to and run in the client’s browser.

If you’ve had experience with the popular JavaScript frameworks, Blazor WASM will feel very familiar.

permalinkWASM Pros

Fast User Experience

Once loaded, the user experience is blazing fast (pun intended.) With your code running in the browser, the UI will feel nearly instantaneous for users other than calls to external APIs.

Offline Support

Blazor WASM allows you to build applications that users can still use when their internet connection isn’t available. Your apps can also take a hybrid approach to allow certain parts of your site to be accessible offline but not others.

Servers Not Needed

Because your Blazor WASM code can run offline, your app can be delivered via a CDN. This effectively removes the need for a server to host your application. If your application requires a back-end API, those would still need to live on a server.

.NET Standard 2.0 Is Ready To Go

Any .NET Standard 2.0 code can run in a Blazor WASM application. That means, in most cases, the internal & external libraries your app depends on will work as they do today.

permalinkWASM Cons

Initial Load Time

The most often mentioned downside is the initial payload size. All Blazor WASM apps bootstrap the .NET runtime. While Microsoft has made gains in trimming this down, it can still result in a large initial download for clients. One way the compiler tries to help is by tree-shaking unused code with the Intermediate Language Trimmer.


When dealing with data or things that need to be secured, you’ll need to make calls to an API. This is a paradigm that’s popular with exiting JavaScript frameworks and is likely familiar to C# developers who have built with ASP.NET MVC.

permalinkOther Observations

If you’re used to existing JavaScript frameworks like Vue.js, React, or Angular, then you’ll understand the paradigm that Blazor WASM uses. It just sprinkles in the magic of C# for .NET developers.

Blazor Server will be an easier entry point for WinForm & XAML developers. I hesitate to compare it to WebForms because it is NOT WebForms, but it seems like an evolution from that world. It’s all C# developers wanted WebForms to be.

The choice of which to use will be dependent on several factors, but an important one will be: “where will my code & user state live?”

If you choose Blazor Server, you will want to be mindful of what kind of state you’re maintaining for each user. Blazor Server handles state well, but the size and scope of that state are controlled by you. The larger state, the fewer users per server you’ll be able to support.

With Blazor WASM, that state remains on the client-side but requires you to use back-end APIs for data access and sensitive processes.

Luckily, both use components that work in both flavors. This means that switching between the two isn’t a daunting task and can be very simple with the right architecture.

Hopefully, you now have a better understanding of the features & limitations of Blazor Server and Blazor WebAssembly. I’d love to hear about what you’re building. Reach out on socials or join our Builders Club community Discord to share with an amazing group of developers who are focused on improving as developers and humans. We’d love to have you as part of our tribe.