How to Use Blazor for Building Interactive Web Applications in .NET

For decades, JavaScript held an unchallenged monopoly over browser-side interactivity. Blazor, a framework by Microsoft, fundamentally disrupts this paradigm by allowing developers to build rich, interactive Single-Page Applications (SPAs) using C# and .NET instead of JavaScript. This means a full-stack .NET team can now share models, validation logic, and even entire libraries between the server-side API and the client-side UI, eliminating the cognitive overhead and code duplication inherent in maintaining separate JavaScript and C# codebases.

Blazor Server executes C# code on the server and uses a persistent SignalR WebSocket connection to push real-time UI diffs to the browser. It offers fast initial load times and full access to server resources, making it ideal for intranet enterprise dashboards. Blazor WebAssembly (WASM) downloads the entire .NET runtime and application DLLs to the browser, enabling fully offline-capable PWAs. In .NET 8+, Blazor United merges both models, allowing developers to choose Server or WASM rendering on a per-component basis within the same application.

Getting started requires the .NET SDK. For a server-side app, run `dotnet new blazor -o MyBlazorApp`. For a standalone WASM app, run `dotnet new blazorwasm -o MyWasmApp`. The generated project includes `Program.cs` (the entry point), `App.razor` (the root component), and a `Pages/` directory containing Razor components (`.razor` files). Each Razor component combines HTML markup with C# logic in an `@code {}` block. Launch the development server with `dotnet watch run`, which provides hot-reload capabilities for rapid iterative development.

Blazor's component model is its core strength. A Razor component is a self-contained unit of UI with its own rendering logic, parameters, and lifecycle methods. Components accept data via `[Parameter]` attributes (similar to React props) and communicate upward via `EventCallback`. For complex state, you can inject shared services using `@inject` and the built-in Dependency Injection (DI) container. Blazor provides lifecycle hooks like `OnInitializedAsync`, `OnParametersSetAsync`, and `OnAfterRenderAsync` for managing component behavior during rendering and state changes.

Blazor provides a powerful forms framework centered around the `` component and `DataAnnotationsValidator`. You define a C# model class with `[Required]`, `[StringLength]`, `[EmailAddress]`, and other Data Annotations attributes. The `` component binds to this model and automatically validates user input on both the client (for instant feedback) and the server (for security). Use ``, ``, ``, and `` components for type-safe, two-way data binding. `` and `` render error messages inline.

While Blazor aims to minimize JavaScript usage, real-world applications often need to integrate existing JS libraries (e.g., Chart.js, Leaflet maps, or payment SDKs like Stripe Elements). Blazor provides IJSRuntime for invoking JavaScript functions from C# (`await JSRuntime.InvokeVoidAsync("initializeMap", elementRef)`) and JSInvokable for calling C# methods from JavaScript. This bidirectional interop ensures that Blazor is never a dead-end; any capability available in the browser's JavaScript ecosystem can be seamlessly incorporated into a Blazor application.

Rendering thousands of rows in a table can cripple any SPA. Blazor provides the `` component, which only renders the DOM elements currently visible in the viewport, drastically reducing memory usage and rendering time. For Blazor WASM, the initial download size of the .NET runtime (~2MB) can be mitigated using lazy loading of assemblies: by configuring `WebAssemblyLazyLoadingAssemblies` in the project file, specific DLLs are downloaded on-demand only when the user navigates to a route that requires them. AOT (Ahead-of-Time) compilation further boosts WASM execution speed.

Blazor Server apps are deployed like any ASP.NET Core application—to Azure App Service, Docker containers, or IIS. Blazor WASM apps, being static files, can be hosted on Azure Static Web Apps, GitHub Pages, or any CDN. The critical challenge for WASM is SEO: search engine crawlers may not fully execute the WebAssembly runtime. The solution is prerendering, configured in .NET 8+ by using `rendermode="InteractiveWebAssembly"` on components, which serves initial HTML from the server for crawlers and hydrates the WASM runtime on the client for interactive users.