What’s the difference between Blazor Server and Blazor Web Assembly (WASM)

What’s the difference between Blazor Server and Blazor Web Assembly (WASM)?

All the code references in this article can be cloned from github  under the Tasks/BlazorTasks Folder.

In this Visual Studio Solution, I have created 3 projects:

  • A Blazor Server Project
  • A Blazor WebAssembly Project
  • A Blazor Hosted Solution where the Client App and the Server API are in
    different projects

What I want to do in this demo is first investigate the difference in behaviour of 2 projects created from the default blazorserver template and the blazorwasm template respectively.

Next I want to look at what’s different between the build and configuration sequence of the 2 difference projects

Finally, I want to look at what my Solution would look like if I was to separate out the Client App from a typical API Controller REST endpoint and what components we can share in common.

This should be really interesting.


First Let’s run each and look at the behaviour.

Blazor Server

I’ll start with the blazor server project. I’ll set as default startup project and spin up.

I’m going to set the Default Browser to Edge and open up the inspector.

We get 12 requests, 336kb transferred. Top 5 files by size are:

1. bootstrap.min.cs163 kB
2. blazor.server.js132 kB
3. <fonts>15 kB
4. <css>9.6 kB
5. <home page>3.9 kB

We also have a couple of resource beginning with _blazor such as _blazor?id=-GphP43cN2yeT5hzs4XYRA
_blazor/negotiate?negotiateVersion=1 and

I think it’s safe to assume this is the SignalR wiring between the rendered client and the server.

Let’s navigate to the Counter page.

Click a couple of times, we can see the number incrementing.

Notice in the Network tab that no new files are loaded from the server.

What magic is this?

Blazor keeps the client DOM synced with the server by making tiny binary message calls to the server over SignalR which is a Websocket type infrastructure that has fall-back to polling and a few other features.

To see these binary messages, reload the page and then navigate to the _blazor?id and click across from the headers tab and the payload tab to the Messages tab. You will see binary messages flowing up and down between the server as you click.

Put a breakpoint inside the OnInitializedAsync() of the FetchData.razor page and you will see that the code is hit every time you navigate from the counter to the fetch data component.

What does this mean?

It means that the @code in the OnInitializedAsync() method to GetForecastAsync() is called by the client which sends the request over the wire to the Server than then runs the Methods and returns the data over SignalR.

Show the Elements of the page. Watch what happens as you navigate between counter and fetch-data.

You can see parts of the page flashing as they change but the page doesn’t reload or load new pages.

Blazor Apps are SPAs or Single Page Application. A single page is loaded and then the DOM is manipulated to load up new components and elements depending on the what routes you choose.

our new terminology for SPA is to use routes rather than paths because technically, the paths don’t change, only the route that changes the DOM is updated.

Notice also that when we navigate from Counter to Fetch Data that no new pages are loaded via the network tab. The only thing that changes in the network tab are these binary messages that flow to and from the server.

Every client has to sync it’s DOM state with the server, Microsoft have told us that we can easily support 20,000 simultaneous connections on a decent sized web server (D3_V2), after that we are going to get memory pressure.

Stop the App. Let’s now look at Blazor Wasm

Blazor Web Assembly

I’ve set the startup project to the Blazor Wasm App and let’s start it again.

The first thing to notice is that you get a blank “Loading…” screen for 10-20 seconds before anything starts.

This is because the .NET WebAssembly runtime has to be download alongside the client app

On my 2015 MacBook it takes about 15 seconds – your milage may vary.
Currently for this demo I’m using .net 6.0.400

The magic behind Blazor WASM is the blazor.webassembly.js script which downloads the .net wasm runtime and the app and it’s dependencies and start the whole show running.

Once up and running, Blazor WebAssembly apps are really fast, because of a very special architecture: In all SPAs that I am aware of, the elements and components are updated by traversing the DOM tree hierarchy and updating elements. Blazor works different because it maintains a flat array of references to every element and so traversal time of the array is linear. This should make client UI updates really fast especially for rich client apps like Data Grids.

Some other great advantages of WASM include the potential for offline work, using local storage in the browser (e.g. utilizing a local and native sqlite wasm instance for caching data), hosting the client app as a static web resources or CDN, utilizing client resources and Aheadof Time Compilation (AOT) performance increase.

The biggest disadvantage is a longer load time, although assembly trimming and optimisation in .NET 7 should see load performance get better. Currently an AOT-compiled WASM apps are about twice the size of their IL-compiled versions

Apart from that, the apps are almost identical. Later I’ll talk about the boot sequence for each and the Entry Points to the app, but for now, it’s only important to know that the code and technology that is run for both WASM and Blazor Server is almost identical.

Here are the difference from the perspective of your Application Code.

Difference between Blazor Server and Blazor WASM

The key different you have to think about when buildig for Blazor Server or Blazor WASM is in how you interract with services.

Fetch Data is a great example.

Notice the difference in the text heading for each example:

For Blazor Server we have :

<h1>Weather forecast</h1>

<p>This component demonstrates fetching data from a service.</p>

In this example, we are injecting a service directly into a Razor component, which magically sets up the wiring for the client page to communicate with the service running on the server.

This is a very subtle point and worth emphasising.

This page is rendered as client page, run on the client browser, but the GetForcaseAsync still runs on the server. Every time the component is activated, the OnInitializedAsync is run and the client page takes to the service running on the server using SignalR to do the wiring.

This is the _blazor?id Binary Messages you see going over the network.

@code {
    private WeatherForecast[]? forecasts;

    protected override async Task OnInitializedAsync()
                forecasts = await ForecastService.GetForecastAsync(DateTime.Now);

Set a breakpoint in visual studio in the OnIniitializedAsync method and you will see the code hit the breakpoint every time you navigate to the page/route.

For Blazor Web Assembly we have:

<h1>Weather forecast</h1>

<p>This component demonstrates fetching data from the server.</p>

Again, there is a subtle difference here. In this case, the client is running is WASM and it may not even have a .NET Core Server backend – it could be deployed serverless to a CDN or as a static resource.

In this case, we are not injecting the service, we are injecting the HttpClient and making a http call to a server somewhere else.

The Http.GetFromJsonAsync is literally downloading a static json file called sample-data/weather.json from the server, but this could just as easily hit a REST API endpoint and consume data this way too.

What is important to remember is that:

  • Blazor Server app can inject Services directly into the App and the wiring is taken care of by Blazor Server SignalR Hub.
  • Blazor Wasm apps have to manage to the server calls by injecting Http client into the client and making calls directly to the server – if you are accessing static json resources from a static endpoint or
    CDN, then this will also be blisteringly fast.

If you are coming from an existing SPA framework, then WASM may be a more intuitive transition for you.

If you are coming from a Razor Pages or MVC framework, then Blazor Server is probably right for you to get started.

There is a third model and that’s the Hosted Model.

In this case, we separate out the WASM Client and the API Controllers on the server – there is also a dotnet template available by passing the --hosted argument when creating a blazorwasm project/solution.

Let’s have a quick Look

Blazor Wasm Hosted Model

In this example, the common Domain Model is moved to a shared project because both the wasm client and api contoller on the server need a definition to the same model.

This is probably the closest of how most people will use Blazor App. You have a standalone Client Blazor App that uses well defined API endpoint and maps these json based endpoints directly to POCO’s or Model objects.

The Server only has a small change to a standard MVC or API Web App.


Is that it? Well – almost, we still need to reference the WASM client project and load the Web Assembly Server.

If you edit the Server project, you will see the Web Assembly server. Currently mine looks like this.

    <PackageReference Include="Microsoft.AspNetCore.Components.WebAssembly.Server" Version="6.0.8" />

This will setup everything to configure the SignalR wiring and setup the blazor server files.

Now let’s look at the Client App.

Program.cs reference the RootComponent as “#app” which is pretty standard for SPAs.

wwwroot/index.html contain the ‘#app’ div that contains the client app. the “Loading …” will get replaced by the running app once blazor is up and running.

 <div id="app">Loading...</div>

Notice that the index.html file contains a script that loads the blazor server.

    <script src="_framework/blazor.webassembly.js"></script>

Next the Razor App is loaded – App.razor into this div. This sets up the routing and fallback if routes are not found, we also setup the MainLayout here. The most important line is below:

    <RouteView RouteData="@routeData" DefaultLayout="@typeof(MainLayout)" />

Notice the MainLayout reference, this will give us our NavMenu and you will see the @Body Property of the LayoutComponentBase. I’ve simplified it below:

@inherits LayoutComponentBase

    <NavMenu />




As we navigate through our app, the routes are loaded into this @Body property.

**Tip: here is a useful tip. Blazor and Razor directives are lower case – like @code and @if and @bind. On the other hand – Properties are uppercase like @Body and @RenderBody

And Finally, we can see each of our Razor components in the Pages folder of the Client App.

  • Pages/Index.razor
  • Pages/Counter.razor
  • Pages/FetchData.razor

Notice that all the razor components are upper case, and it is this naming convention that allows us to embed an existing Razor Component in a different page. For example if we want to embed the Counter.razor component in the Index.razor page, we just add the component tag

<Counter />

And that’s really all there is. Everything else is just an extension of what you have learned here like adding new components.

I do really hope you found this useful.

Let me know.

P.S. For more Hosting and Deployment Advanced issues see