WebAssembly Fundamentals

Learning objectives#

• Explain how Blazor WebAssembly runs
• Use Interactive WebAssembly and Interactive Auto
• Know what belongs in the client project
• Keep secrets out of client code

Explanation#

WebAssembly is a fast binary format all modern browsers run. Blazor ships a .NET runtime compiled to WebAssembly, plus your app’s assemblies.

In a Blazor Web App, WebAssembly components live in the .Client project. Use @rendermode InteractiveWebAssembly, or InteractiveAuto: server first, then WebAssembly once it has downloaded.

After loading, interactions need no server round trips and can work offline. The first load is heavier, so trimming and caching matter.

Everything in the client is downloadable and inspectable — no connection strings, secrets or direct database access. Call an API instead.

Code example: A counter that runs in the browser#

1@page "/counter"
2@rendermode InteractiveAuto
3

4<h1>Counter</h1>
5<p role="status">Current count: @count</p>
6<button class="btn btn-primary" @onclick="Increment">Click me</button>
7<p><small>Running on: @(OperatingSystem.IsBrowser() ? "WebAssembly" : "Server")</small></p>
8

9@code {
10    private int count;
11    private void Increment() => count++;
12}

First visit: “Running on: Server” (instant). After the WebAssembly files are cached: “Running on: WebAssembly”, and clicks never touch the server.

This example is illustrative. Copy or download it and run it in your own environment; the expected output is shown above.

Step-by-step explanation#

1. The component is in the .Client project so it can run in the browser.
2. InteractiveAuto uses the server while WebAssembly downloads.
3. OperatingSystem.IsBrowser() shows where the code is running.
4. Later visits load from cache and run fully client-side.

Real-world use case#

Common mistakes#

Best practices#

• Use InteractiveAuto for a fast first load
• Call APIs with HttpClient for data
• Keep the client project small
• Validate everything again on the server

Remember this#

Practice exercise#

1In the .Client project (or a shared Razor class library referenced by it).

Mini quiz#

Want more? Practise all Blazor MCQs · Blazor interview questions

Related topics#

My notes#

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