Blazor Server vs. Blazor WebAssembly: A Comprehensive Guide

Blazor has revolutionized .NET web development by allowing developers to build interactive client-side web UIs with C# and Razor syntax. Two primary hosting models exist: Blazor Server and Blazor WebAssembly (WASM). While both leverage Blazor's component model, their underlying architecture and runtime environments are significantly different, leading to distinct advantages and disadvantages for various use cases.

This guide will delve into the core distinctions between Blazor Server and Blazor WebAssembly, helping you choose the right model for your next project.

Blazor Server: Server-Side Interactivity

In Blazor Server, your application logic runs on the server. When a user navigates to your Blazor app, the initial HTML is sent, and then a real-time SignalR connection is established between the browser and the server. All user interactions, such as button clicks or form submissions, are sent over this connection to the server, where they are processed. The server then computes the updated UI and sends the differences back to the browser, which updates the DOM accordingly.

Key Characteristics of Blazor Server:

Server-Centric: All C# code runs on the server.
SignalR Connection: Relies on a persistent SignalR connection for interactivity.
Minimal Client-Side Footprint: The browser only needs to download a small JavaScript file to manage the SignalR connection and DOM updates.
Faster Initial Load: Since the .NET runtime isn't downloaded to the browser, initial page load times are generally quicker.
Access to Server Resources: Can easily access server-side APIs, databases, and file systems.
State Management: Component state is maintained on the server, which can be advantageous for complex applications but also introduces scalability considerations.

When to Use Blazor Server:

Applications requiring access to server-side resources and APIs.
Internal business applications where low latency and consistent network connectivity are expected.
When minimizing client-side resource usage is critical.
Prototypes and initial development where rapid iteration is key.

Blazor WebAssembly (WASM): Client-Side Power

Blazor WebAssembly allows you to run your .NET code directly in the browser using WebAssembly. When a user visits your Blazor WASM app, the .NET runtime and your application's assemblies are downloaded to the browser. The application then executes entirely within the browser's sandbox, providing a native-like interactive experience without requiring a constant server connection for logic execution.

Key Characteristics of Blazor WebAssembly:

Client-Centric: All C# code runs in the browser via WebAssembly.
No Persistent Server Connection for Logic: Interactivity is handled client-side. Server is typically used for API calls.
Larger Initial Download: The .NET runtime and application assemblies need to be downloaded, which can lead to larger initial download sizes.
Offline Capabilities: Can be used to build Progressive Web Apps (PWAs) with offline support.
Leverages Browser Capabilities: Can directly interact with browser APIs.
Reduced Server Load: Once downloaded, the server is primarily used for serving static assets and API endpoints, reducing server computational load.

When to Use Blazor WebAssembly:

Public-facing web applications requiring rich client-side interactivity.
Applications that can benefit from offline capabilities or PWA features.
When migrating desktop applications or complex client-side logic to the web.
Scenarios where server costs need to be minimized.

Side-by-Side Comparison

Feature	Blazor Server	Blazor WebAssembly
Runtime Environment	Server (ASP.NET Core)	Browser (WebAssembly)
Execution Location	Server	Client
Initial Download Size	Small	Larger (includes .NET runtime)
Interactivity Model	SignalR (persistent connection)	WebAssembly (client-side execution)
Server Load	Higher (handles all logic)	Lower (serves static assets/APIs)
Client Resource Usage	Low	Higher (runs .NET runtime)
Access to Server Resources	Direct	Via APIs/HTTP requests
Offline Support	No	Yes (with PWA)
Latency Sensitivity	High (depends on connection)	Lower (once loaded)
Security	Logic is secured on the server	Logic is in browser (can be inspected)

Choosing the Right Path

The decision between Blazor Server and Blazor WebAssembly hinges on your project's specific requirements. Consider factors like network dependency, client resource constraints, the need for offline functionality, and the sensitivity of your application logic.

Technical Considerations

Blazor Server Code Example (Conceptual)

This is a simplified representation of how interactivity works. The actual component rendering and diffing are handled by Blazor.


// Server-side logic executed on the server
@page "/counter"

Counter

Current count: @currentCount



@code {
    private int currentCount = 0;

    private void IncrementCount()
    {
        currentCount++;
    }
}

Blazor WebAssembly Code Example (Conceptual)

The same component code in Blazor WASM would run entirely within the browser. The interaction is handled by the client-side .NET runtime.


// Client-side logic executed in the browser
@page "/counter"

Counter

Current count: @currentCount



@code {
    private int currentCount = 0;

    private void IncrementCount()
    {
        currentCount++;
    }
}

Notice how the C# code looks identical. The difference lies in where that code is compiled and executed.

Conclusion

Both Blazor Server and Blazor WebAssembly offer powerful ways to build modern, interactive web applications with .NET. Blazor Server excels in scenarios where server resources are readily available and a persistent connection is feasible, offering faster initial loads and direct server access. Blazor WebAssembly shines when client-side execution, offline capabilities, and reduced server load are priorities, enabling rich, PWA-like experiences.

By understanding the fundamental differences in their hosting models, you can make an informed decision to leverage the full potential of Blazor for your .NET web development projects.