Blazor

What Is Blazor?

Blazor is Microsoft's web-UI framework for building interactive client-side and server-side web applications using C# and .NET instead of JavaScript. Part of the ASP.NET Core ecosystem, Blazor lets .NET developers write their front-end logic, components, and even much of their interactivity in the same language they use on the server, sharing code, types, and tooling across the full stack. The answer-first definition is straightforward: Blazor is a framework for building web user interfaces with C# and .NET, and it runs in two principal flavors, one that executes in the browser via WebAssembly and one that runs on the server and streams UI updates to the browser.

Blazor is not a browser extension and not a hosted service. It is application code built on .NET, served from your own infrastructure, that produces the interactive web interface a visitor sees. For organizations already invested in the Microsoft stack, .NET on the back end, C# expertise, Visual Studio tooling, Blazor is compelling because it extends that investment to the front end, removing the need to maintain a separate JavaScript codebase and skill set for the UI.

The framework matters because it represents a genuinely different approach to building rich web applications. For most of the modern web's history, interactive front ends meant JavaScript, whether hand-written or produced by a framework like React or Angular. Blazor breaks that assumption by bringing .NET to the browser through WebAssembly, a binary instruction format that browsers can execute at near-native speed, and by offering a server-driven alternative for scenarios where shipping a runtime to the browser is undesirable. It is widely recognized as the flagship way to build .NET-based web UIs and has become a significant part of the ASP.NET Core story.

Understanding Blazor requires understanding its hosting models, because they behave quite differently and are detected differently. The two foundational models are Blazor WebAssembly, where the application and the .NET runtime download to the browser and run entirely client-side, and Blazor Server, where the application runs on the server and the browser maintains a real-time connection that carries UI updates and user events. Recent versions of .NET unified these under a flexible model that lets a single application mix render modes per component, but the underlying WebAssembly and Server mechanics remain the two pillars on which everything rests.

How Blazor Works

Blazor applications are built from components, written in a file format called Razor that combines HTML markup with embedded C#. A component encapsulates a piece of UI, its rendering logic, and its event handling. Components compose into pages and layouts, and they communicate through parameters and events much as components do in other modern UI frameworks. The crucial difference is that the logic inside those components is C# rather than JavaScript.

In the Blazor WebAssembly model, the application is compiled and, along with a .NET runtime compiled to WebAssembly, downloaded to the browser. From that point the application runs entirely client-side: the browser executes .NET code via WebAssembly, manipulates the DOM, and handles interactivity without round-trips to the server for rendering. This makes the app capable of running as a true single-page application, even offline as a progressive web app, at the cost of an initial download of the runtime and application assemblies. Calls to the back end happen over HTTP APIs, just as a JavaScript SPA would call a server.

In the Blazor Server model, the application runs on the server. When a user loads the page, they receive a small amount of HTML and a script that establishes a persistent SignalR connection (a real-time WebSocket-based channel) back to the server. User interactions, clicks, input, navigation, are sent over that connection to the server, which runs the component logic, computes the UI changes, and sends back only the differences to apply to the DOM. This keeps the client lightweight and the application code entirely on the server, but it requires a continuous connection and introduces network latency into every interaction.

A helpful way to internalize the difference is to follow a button click in each model. In Blazor WebAssembly, the click is handled entirely in the browser by C# running on WebAssembly; the DOM updates locally and no server round-trip is needed unless the app fetches data. In Blazor Server, the same click travels over the SignalR connection to the server, which runs the handler, calculates the DOM diff, and streams it back to the browser to apply. Both produce the same visible result, but the location of the logic, and therefore the performance characteristics and the external fingerprints, differ substantially.

The unified render-mode approach in recent .NET releases lets developers choose, per component, whether it renders statically on the server, interactively on the server via SignalR, interactively in the browser via WebAssembly, or with automatic switching. This flexibility means a single modern Blazor application can exhibit signals from more than one hosting model at once, which is worth keeping in mind when trying to identify it from the outside. Throughout, Blazor integrates tightly with the broader ASP.NET Core platform for routing, dependency injection, authentication, and configuration, so a Blazor front end and an ASP.NET Core back end share one cohesive framework.

How to Tell if a Website Uses Blazor

Blazor's detectability depends heavily on which hosting model an application uses, and the framework's .NET foundation provides some of the clearest signals. StackOptic analyzes the server response and the markup a browser would receive, and you can confirm the same signals manually. Because Blazor sits on ASP.NET Core, evidence of the .NET platform is often the first clue, after which model-specific markers narrow it to Blazor.

The Blazor framework script. The single strongest signal is the framework's bootstrapping script in the page source. Blazor WebAssembly applications load blazor.webassembly.js, while Blazor Server applications load blazor.server.js (and unified apps may load blazor.web.js). Finding any of these script references is close to definitive proof of Blazor and immediately tells you the hosting model.

WebAssembly assets. Blazor WebAssembly applications download a set of files from a _framework/ directory, including the .NET runtime as a .wasm file (such as dotnet.wasm) and the application's compiled assemblies (.dll files, often packaged as .dat or compressed). Requests to _framework/ for .wasm and assembly files in the Network tab are a hallmark of Blazor WebAssembly.

The app mount point and reconnection UI. Blazor renders into an element, commonly <app> or a <div id="app">, that fills in once the framework boots. Blazor Server apps include a distinctive reconnection UI (markup with an id like components-reconnect-modal) that appears when the SignalR connection drops, a recognizable Server-mode tell in the markup.

ASP.NET Core and SignalR signals. Because Blazor runs on ASP.NET Core, you may see headers consistent with the .NET platform, though these are frequently stripped or masked by a reverse proxy. Blazor Server's reliance on SignalR means a WebSocket connection (often to a _blazor endpoint) is established after load, visible in the Network tab as a long-lived WebSocket request.

Method	What to do	What Blazor reveals
View Source	"View Page Source" on the app	blazor.webassembly.js, blazor.server.js, or blazor.web.js; app mount element
Browser DevTools	Inspect the Network tab and Elements	_framework/ requests for .wasm/.dll, SignalR WebSocket to _blazor, reconnect modal markup
curl -s	`curl -s https://example.com	grep -i "blazor"`
curl -I	curl -I https://example.com	ASP.NET Core/.NET headers when present, though often masked by a proxy
Wappalyzer	Run the extension on the live page	Identifies "Blazor" and often the underlying ASP.NET Core platform

A fast check is curl -s https://example.com | grep -i "blazor\.", which surfaces the bootstrap script and the hosting model in one step. For broader methodology, see our guides on how to find out what technology a website uses, how to read a website's HTTP headers, and how to find out what programming language a website uses.

The detectability of Blazor is unusually clear-cut compared with many server-side frameworks, because the framework script and, for WebAssembly, the downloaded runtime are so distinctive and hard to hide. That said, a few honest caveats apply. The ASP.NET Core and SignalR headers that point to the .NET platform are often stripped or rewritten by a reverse proxy or CDN, so their absence does not rule Blazor out. Blazor WebAssembly's signals, the _framework/ directory, the .wasm runtime, and the framework script, are very difficult to disguise because the browser genuinely needs to download them, making WebAssembly apps especially easy to confirm. Blazor Server is identified most reliably by its framework script and its SignalR WebSocket to the _blazor endpoint, which appears only after the page loads, so inspecting the live Network tab is more informative than View Source alone for that model. Combining the script-reference check with a look at the network activity yields a confident verdict and usually reveals which hosting model is in play. Pulling the raw server response, as a server-side scan does, makes the bootstrap script easy to spot without browser interference.

Key Features

• C# and .NET for the front end. Build interactive web UIs in C# instead of JavaScript, sharing code and types across client and server.
• Two hosting models. WebAssembly for client-side execution and Server for server-driven UI over SignalR, plus a unified per-component render-mode approach in recent .NET.
• Component-based architecture. Reusable Razor components that combine HTML markup with embedded C# logic.
• WebAssembly execution. Runs .NET in the browser at near-native speed without browser plugins.
• ASP.NET Core integration. Shared routing, dependency injection, authentication, and configuration with the broader .NET platform.
• JavaScript interop. Call JavaScript from C# and C# from JavaScript when integrating with existing web libraries.
• Strong tooling. First-class support in Visual Studio and the .NET CLI, with debugging across the stack.

Pros and Cons

Pros

• Lets .NET teams build full-stack web applications in a single language, reusing skills, code, and types.
• WebAssembly enables rich client-side apps, including offline-capable progressive web apps, without JavaScript frameworks.
• Tight integration with ASP.NET Core and excellent Microsoft tooling and long-term support.
• Component model and type safety make for maintainable, well-structured front-end code.

Cons

• Blazor WebAssembly has a larger initial download because it ships the .NET runtime to the browser.
• Blazor Server requires a persistent connection and is sensitive to network latency, with server resources consumed per active user.
• A smaller front-end ecosystem and component-library selection than the largest JavaScript frameworks.
• Most valuable to teams already in the .NET world; less compelling for shops with no C# investment.

Blazor vs Alternatives

Blazor competes with JavaScript front-end frameworks and, to a degree, with traditional server-rendered .NET approaches. The table clarifies its position.

Framework	Language	Execution model	Best for
Blazor	C# / .NET	WebAssembly (client) or Server (SignalR)	.NET teams building rich web UIs without JavaScript
React	JavaScript	Client-side (with SSR options)	Broadest ecosystem and adoption across the web
Angular	TypeScript	Client-side framework	Enterprise SPAs wanting an all-in-one platform
Vue.js	JavaScript	Client-side framework	Approachable, progressive front-end development
ASP.NET Core MVC/Razor Pages	C# / .NET	Server-rendered HTML	Traditional server-rendered .NET sites without rich client interactivity

If a site turns out to use a JavaScript framework instead, the same signals point to it; comparing Blazor with a dominant alternative like React highlights the contrast between a .NET, WebAssembly-or-server model and the JavaScript norm. Because Blazor sits on the .NET platform, our guide on how to find out what programming language a website uses helps confirm the underlying stack.

Use Cases

Blazor is the natural choice for organizations already invested in .NET that want to build rich, interactive web applications without maintaining a separate JavaScript front-end stack. Enterprise line-of-business applications, internal tools, and admin dashboards are especially common, because these teams typically have deep C# expertise, value type safety and tooling, and prioritize maintainability over chasing the latest front-end trends.

Blazor WebAssembly suits applications that benefit from client-side execution, including progressive web apps that work offline, interactive dashboards, and standalone front ends that talk to a separate API. Blazor Server suits applications where keeping logic on the server is preferable, perhaps for security, for simpler deployment, or to avoid a large client download, and where users have reliable, low-latency connections, such as internal corporate networks. The unified render-mode approach in recent .NET releases lets teams mix these as appropriate within one application.

Consider a few representative adopters. A company with an established ASP.NET Core back end might build its customer-facing portal in Blazor WebAssembly so its existing .NET developers can own the entire stack and share validation logic and data models between client and server. An enterprise IT team might build internal administrative tools in Blazor Server, accepting the persistent-connection requirement because users are on the corporate network and the team wants to keep all logic server-side. A software vendor that ships on-premises products might favor Blazor to consolidate on a single .NET toolchain across everything it builds. In each case the unifying factor is an existing commitment to .NET and C# that Blazor extends to the browser.

From a competitive-intelligence and technology-research standpoint, detecting Blazor is a strong, specific signal: it almost always indicates a Microsoft-stack organization with .NET and C# expertise. For vendors selling developer tools, infrastructure, or services into the .NET ecosystem, that is a high-value qualifying signal, and the hosting model (WebAssembly versus Server) adds further nuance about how the team builds and deploys. For analysts mapping technology landscapes, Blazor cleanly distinguishes .NET-centric shops from JavaScript-framework adopters. Surfacing that signal automatically across many domains, and inferring the hosting model from the framework script and network behavior, is precisely the kind of insight automated detection delivers. The broader value of using such stack data to qualify and prioritize prospects is covered in our overview of technographics and using tech-stack data to qualify leads.

Frequently Asked Questions

What is the difference between Blazor WebAssembly and Blazor Server?

Blazor WebAssembly downloads the application and the .NET runtime to the browser and runs entirely client-side, much like a JavaScript single-page application, at the cost of a larger initial download. Blazor Server runs the application on the server and uses a persistent SignalR connection to send UI updates to the browser and receive user events, keeping the client lightweight but requiring a continuous connection and being sensitive to latency. Recent .NET versions let a single app mix both models per component.

Can you reliably detect Blazor on a website?

Yes, comparatively reliably. The Blazor bootstrap script, blazor.webassembly.js, blazor.server.js, or blazor.web.js, is a near-definitive signal that also reveals the hosting model. Blazor WebAssembly additionally downloads recognizable _framework/ assets including a .wasm runtime, and Blazor Server establishes a SignalR WebSocket to a _blazor endpoint. While the underlying ASP.NET Core headers can be masked by a proxy, the framework script and network behavior are difficult to hide, making Blazor easier to confirm than many server-side frameworks.

Does Blazor replace JavaScript entirely?

For most application logic, yes, you can build interactive UIs in C# without writing JavaScript. However, Blazor provides JavaScript interop precisely because some scenarios still need it: integrating existing JavaScript libraries, calling certain browser APIs, or reusing established web components. In practice, many Blazor apps use a small amount of JavaScript through interop while keeping the bulk of their logic in C#. The framework reduces, rather than absolutely eliminates, the need for JavaScript.

Is Blazor suitable for public-facing websites or just internal apps?

Both, though the right hosting model matters. Blazor WebAssembly works well for public-facing single-page applications, including progressive web apps, since execution is client-side. Blazor Server can also serve public sites, but its persistent connection and per-user server resources make it most comfortable when connections are reliable and concurrency is manageable, which is why it is especially popular for internal tools. Static server-side rendering in recent .NET versions further broadens Blazor's suitability for content-oriented public pages.

Why would a team choose Blazor over React or Angular?

Teams choose Blazor primarily to build their web UI in C# and .NET rather than JavaScript, sharing code, models, validation logic, and tooling across the entire stack. For organizations with deep .NET expertise and an existing ASP.NET Core back end, this consolidation reduces context-switching, leverages existing skills, and benefits from Microsoft's tooling and long-term support. React and Angular remain the better choice for teams without a .NET investment or those needing the largest possible front-end ecosystem and talent pool.

Want to detect Blazor, its hosting model, and the rest of a site's stack automatically? Run any URL through StackOptic at https://stackoptic.com.