Introduction to ASP.NET Core Blazor

Welcome to Blazor!

Blazor is a framework for building interactive client-side web UI with .NET:

  • Create rich interactive UIs using C# instead of JavaScript.
  • Share server-side and client-side app logic written in .NET.
  • Render the UI as HTML and CSS for wide browser support, including mobile browsers.
  • Integrate with modern hosting platforms, such as Docker.

Using .NET for client-side web development offers the following advantages:

  • Write code in C# instead of JavaScript.
  • Leverage the existing .NET ecosystem of .NET libraries.
  • Share app logic across server and client.
  • Benefit from .NET's performance, reliability, and security.
  • Stay productive with Visual Studio on Windows, Linux, and macOS.
  • Build on a common set of languages, frameworks, and tools that are stable, feature-rich, and easy to use.

Components

Blazor apps are based on components. A component in Blazor is an element of UI, such as a page, dialog, or data entry form.

Components are .NET C# classes built into .NET assemblies that:

The component class is usually written in the form of a Razor markup page with a .razor file extension. Components in Blazor are formally referred to as Razor components. Razor is a syntax for combining HTML markup with C# code designed for developer productivity. Razor allows you to switch between HTML markup and C# in the same file with IntelliSense programming support in Visual Studio. Razor Pages and MVC also use Razor. Unlike Razor Pages and MVC, which are built around a request/response model, components are used specifically for client-side UI logic and composition.

Blazor uses natural HTML tags for UI composition. The following Razor markup demonstrates a component (Dialog.razor) that displays a dialog and processes an event when the user selects a button:

<div class="card" style="width:22rem">
    <div class="card-body">
        <h3 class="card-title">@Title</h3>
        <p class="card-text">@ChildContent</p>
        <button @onclick="OnYes">Yes!</button>
    </div>
</div>

@code {
    [Parameter]
    public RenderFragment ChildContent { get; set; }

    [Parameter]
    public string Title { get; set; }

    private void OnYes()
    {
        Console.WriteLine("Write to the console in C#! 'Yes' button selected.");
    }
}

In the preceding example, OnYes is a C# method triggered by the button's onclick event. The dialog's text (ChildContent) and title (Title) are provided by the following component that uses this component in its UI.

The Dialog component is nested within another component using an HTML tag. In the following example, the Index component (Pages/Index.razor) uses the preceding Dialog component. The tag's Title attribute passes a value for the title to the Dialog component's Title property. The Dialog component's text (ChildContent) are set by the content of the <Dialog> element. When the Dialog component is added to the Index component, IntelliSense in Visual Studio speeds development with syntax and parameter completion.

@page "/"

<h1>Hello, world!</h1>

<p>
    Welcome to your new app.
</p>

<Dialog Title="Learn More">
    Do you want to <i>learn more</i> about Blazor?
</Dialog>

The dialog is rendered when the Index component is accessed in a browser. When the button is selected by the user, the browser's developer tools console shows the message written by the OnYes method:

Dialog component rendered in the browser nested inside of the Index component. The browser developer tools console shows the message written by C# code when the user selects the Yes! button in the UI.

Components render into an in-memory representation of the browser's Document Object Model (DOM) called a render tree, which is used to update the UI in a flexible and efficient way.

Blazor WebAssembly

Blazor WebAssembly is a single-page app (SPA) framework for building interactive client-side web apps with .NET. Blazor WebAssembly uses open web standards without plugins or recompiling code into other languages. Blazor WebAssembly works in all modern web browsers, including mobile browsers.

Running .NET code inside web browsers is made possible by WebAssembly (abbreviated wasm). WebAssembly is a compact bytecode format optimized for fast download and maximum execution speed. WebAssembly is an open web standard and supported in web browsers without plugins.

WebAssembly code can access the full functionality of the browser via JavaScript, called JavaScript interoperability, often shortened to JavaScript interop or JS interop. .NET code executed via WebAssembly in the browser runs in the browser's JavaScript sandbox with the protections that the sandbox provides against malicious actions on the client machine.

Blazor WebAssembly runs .NET code in the browser with WebAssembly.

When a Blazor WebAssembly app is built and run in a browser:

  • C# code files and Razor files are compiled into .NET assemblies.
  • The assemblies and the .NET runtime are downloaded to the browser.
  • Blazor WebAssembly bootstraps the .NET runtime and configures the runtime to load the assemblies for the app. The Blazor WebAssembly runtime uses JavaScript interop to handle DOM manipulation and browser API calls.

The size of the published app, its payload size, is a critical performance factor for an app's usability. A large app takes a relatively long time to download to a browser, which diminishes the user experience. Blazor WebAssembly optimizes payload size to reduce download times:

  • Unused code is stripped out of the app when it's published by the Intermediate Language (IL) Trimmer.
  • HTTP responses are compressed.
  • The .NET runtime and assemblies are cached in the browser.

Blazor Server

Blazor Server provides support for hosting Razor components on the server in an ASP.NET Core app. UI updates are handled over a SignalR connection.

The runtime stays on the server and handles:

  • Executing the app's C# code.
  • Sending UI events from the browser to the server.
  • Applying UI updates to a rendered component that are sent back by the server.

The connection used by Blazor Server to communicate with the browser is also used to handle JavaScript interop calls.

Blazor Server runs .NET code on the server and interacts with the Document Object Model on the client over a SignalR connection

Blazor Server apps render content differently than traditional models for rendering UI in ASP.NET Core apps using Razor views or Razor Pages. Both models use the Razor language to describe HTML content for rendering, but they significantly differ in how markup is rendered.

When a Razor Page or view is rendered, every line of Razor code emits HTML in text form. After rendering, the server disposes of the page or view instance, including any state that was produced. When another request for the page occurs, the entire page is rerendered to HTML again and sent to the client.

Blazor Server produces a graph of components to display similar to an HTML or XML Document Object Model (DOM). The component graph includes state held in properties and fields. Blazor evaluates the component graph to produce a binary representation of the markup, which is sent to the client for rendering. After the connection is made between the client and the server, the component's static prerendered elements are replaced with interactive elements. Prerendering the content on the server makes the app feel more responsive on the client.

After the components are interactive on the client, UI updates are triggered by user interaction and app events. When an update occurs, the component graph is rerendered, and a UI diff (difference) is calculated. This diff is the smallest set of DOM edits required to update the UI on the client. The diff is sent to the client in a binary format and applied by the browser.

A component is disposed after the user navigates away from the component.

JavaScript interop

For apps that require third-party JavaScript libraries and access to browser APIs, components interoperate with JavaScript. Components are capable of using any library or API that JavaScript is able to use. C# code can call into JavaScript code, and JavaScript code can call into C# code.

Code sharing and .NET Standard

Blazor implements the .NET Standard, which enables Blazor projects to reference libraries that conform to .NET Standard specifications. .NET Standard is a formal specification of .NET APIs that are common across .NET implementations. .NET Standard class libraries can be shared across different .NET platforms, such as Blazor, .NET Framework, .NET Core, Xamarin, Mono, and Unity.

APIs that aren't applicable inside of a web browser (for example, accessing the file system, opening a socket, and threading) throw a PlatformNotSupportedException.

Additional resources