Blazor WebAssembly and Blazor Server are two distinct hosting models for Blazor applications, and picking the right one is less about "better" and more about "appropriate for your scenario."
Let’s see Blazor Server in action. Imagine a simple counter component.
// Counter.razor
@page "/counter"
<h1>Counter</h1>
<p role="status">Current count: @currentCount</p>
<button class="btn btn-primary" @onclick="IncrementCount">Click me</button>
@code {
private int currentCount = 0;
private void IncrementCount()
{
currentCount++;
}
}
When a user navigates to /counter in a Blazor Server app, the browser doesn’t download the C# code. Instead, it establishes a SignalR connection back to the server. Each button click sends a message over this connection. The server executes the IncrementCount method, updates the currentCount variable, and then sends a small diff of the UI back to the browser. The browser then applies this diff, and the user sees the updated count. All the C# code runs on the server.
Now, consider Blazor WebAssembly. The same counter component looks identical in the .razor file.
// Counter.razor
@page "/counter"
<h1>Counter</h1>
<p role="status">Current count: @currentCount</p>
<button class="btn btn-primary" @onclick="IncrementCount">Click me</button>
@code {
private int currentCount = 0;
private void IncrementCount()
{
currentCount++;
}
}
However, when a user navigates to /counter in a Blazor WebAssembly app, the browser downloads the compiled C# code (as .NET IL, which is then JIT-compiled by the Mono runtime) along with the Blazor runtime itself. When the "Click me" button is clicked, the IncrementCount method executes directly in the browser’s tab. There’s no round trip to the server for the logic. The UI updates instantly because the code is running locally.
The core problem Blazor solves is allowing C# developers to build interactive web UIs without JavaScript. Both models achieve this, but they do so by fundamentally different means of execution and deployment.
Blazor Server is ideal when:
- Latency is not a primary concern for UI interactions: Since every UI update involves a network round trip, applications with extremely high-frequency UI updates or those targeting users with very high latency connections might feel sluggish.
- You need to access server-side resources directly: Your C# code can seamlessly call into your server’s APIs, databases, or file system without needing to expose them via HTTP endpoints.
- The application needs to be thin on the client: The browser only downloads a small amount of JavaScript for the SignalR connection and the Blazor DOM diffing, making the initial download very fast.
- You want to keep your application logic private: The C# code never leaves the server, meaning your intellectual property is more secure.
- You can manage server infrastructure effectively: Blazor Server apps can consume significant server resources (CPU, memory) due to running the application logic for all connected clients. Scaling requires careful server provisioning.
Blazor WebAssembly is ideal when:
- You need a rich, responsive client-side experience: UI interactions are as fast as native desktop applications because the code runs directly in the browser.
- You want to minimize server load: The server’s primary role becomes serving the initial application files and potentially acting as an API backend. The browser does the heavy lifting for UI logic.
- Offline capabilities are important: Blazor WebAssembly apps can be cached by the browser, allowing them to function even without an internet connection.
- You want to leverage existing .NET libraries on the client: You can use many .NET libraries that have been compiled for WebAssembly, expanding your development options.
- You can tolerate a larger initial download: The browser needs to download the .NET runtime and your application’s assemblies, which can be several megabytes. Techniques like ahead-of-time (AOT) compilation and removing unused assemblies can mitigate this.
When you’re working with Blazor WebAssembly, the browser’s cache becomes a crucial part of the user experience. If you deploy a new version of your application, users won’t automatically get the update on their next visit. They’ll continue to use the cached version until the browser’s cache is refreshed. You’ll typically need to implement cache-busting strategies, often by changing the filename of your index.html or the associated JavaScript/WASM files (e.g., app.1.2.3.js) to force browsers to download the new assets.
The choice between Blazor Server and Blazor WebAssembly boils down to where you want the C# code to execute: on the server or in the client’s browser. Each has significant trade-offs regarding performance, security, deployment, and resource consumption.
The next hurdle you’ll likely encounter is understanding how to integrate these Blazor applications with existing backend APIs, especially when considering security and authentication patterns.