Blazor

Blazor Guide

Applies to: Blazor (.NET 8+), C# 12+, WebAssembly, Server, Blazor United, Interactive Web Apps

Core Principles

1. Component-Based: Build encapsulated .razor components that own their state and rendering
2. Hosting Flexibility: Choose WebAssembly, Server, or Blazor United (hybrid) per-component
3. C# Everywhere: Share models, validation, and logic between client and server
4. Type Safety: Leverage nullable reference types and strong parameter typing
5. Lifecycle Awareness: Understand component lifecycle to avoid leaks and race conditions

Hosting Models

Guardrails

Component Rules

• Keep components under 200 lines (split with child components)
• Use [Parameter, EditorRequired] for required parameters
• Never mutate [Parameter] properties directly -- use local state instead
• Use EventCallback<T> for parent-child communication (not Action<T>)
• Implement IDisposable when subscribing to events, timers, or state changes
• Use @key directive on list-rendered items for DOM diffing performance
• Wrap component trees in <ErrorBoundary> for graceful error handling
• Use RenderFragment and RenderFragment<T> for templated components

Lifecycle Rules

• Do NOT call JS interop in OnInitialized/OnInitializedAsync -- use OnAfterRender
• Use OnParametersSet for reacting to parameter changes (not property setters)
• Call StateHasChanged() only when necessary (avoid triggering excessive re-renders)
• Use ShouldRender() override to prevent unnecessary renders on hot paths
• Always use CancellationToken with async lifecycle methods to handle disposal

Data Binding

• Use @bind-Value for two-way binding with input components
• Use @bind:event to control when binding updates (e.g., oninput vs onchange)
• Use EditForm with DataAnnotationsValidator for form validation
• Never use @bind and @onchange on the same element (they conflict)

Security

• Use [Authorize] attribute on pages requiring authentication
• Use <AuthorizeView> for conditional UI based on auth state
• Validate all inputs server-side (client validation is for UX, not security)
• Never trust Blazor WASM for authorization decisions -- enforce on API
• Use antiforgery tokens for form submissions in SSR mode

Performance

• Use <Virtualize> for large lists instead of @foreach
• Use @rendermode appropriately (static SSR by default, interactive only when needed)
• Use [StreamRendering] for pages with async data loading
• Minimize JS interop calls (batch operations when possible)
• Use IMemoryCache or Blazored.LocalStorage for client-side caching
• Lazy-load assemblies for large WASM apps with LazyAssemblyLoader

Project Structure

MyApp/
├── MyApp.Client/                    # Blazor WebAssembly / interactive
│   ├── Pages/                       # Routable components (@page)
│   │   ├── Home.razor
│   │   └── Users/
│   │       ├── UserList.razor
│   │       ├── UserDetail.razor
│   │       └── UserForm.razor
│   ├── Components/                  # Reusable components
│   │   ├── Layout/
│   │   │   ├── MainLayout.razor
│   │   │   └── NavMenu.razor
│   │   ├── Common/
│   │   │   ├── LoadingSpinner.razor
│   │   │   ├── ErrorBoundary.razor
│   │   │   └── Modal.razor
│   │   └── Users/
│   │       └── UserCard.razor
│   ├── Services/                    # Client-side services
│   │   ├── IUserService.cs
│   │   └── UserService.cs
│   ├── State/                       # State management
│   │   └── AppState.cs
│   ├── wwwroot/                     # Static assets
│   ├── _Imports.razor               # Global using directives
│   ├── App.razor                    # Root component
│   └── Program.cs                   # DI and startup
├── MyApp.Server/                    # API / server host
│   ├── Controllers/
│   └── Program.cs
├── MyApp.Shared/                    # Shared models and DTOs
│   ├── Models/
│   └── DTOs/
├── tests/
│   ├── MyApp.Client.Tests/          # bUnit component tests
│   └── MyApp.Server.Tests/          # API integration tests
└── MyApp.sln

Layer Responsibilities

Component Patterns

Page Component with Data Loading

@page "/users"
@attribute [Authorize]
@inject IUserService UserService

<PageTitle>Users</PageTitle>

@if (loading)
{
    <LoadingSpinner />
}
else if (error is not null)
{
    <div class="alert alert-danger">
        <p>@error</p>
        <button class="btn btn-outline-danger" @onclick="LoadUsers">Retry</button>
    </div>
}
else if (users is null || users.Count == 0)
{
    <div class="alert alert-info">No users found.</div>
}
else
{
    @foreach (var user in users)
    {
        <UserCard @key="user.Id" User="user" => Edit(user.Id)" />
    }
}

@code {
    private List<UserResponse>? users;
    private bool loading = true;
    private string? error;

    protected override async Task OnInitializedAsync()
    {
        await LoadUsers();
    }

    private async Task LoadUsers()
    {
        loading = true;
        error = null;
        try
        {
            users = await UserService.GetUsersAsync();
        }
        catch (Exception ex)
        {
            error = ex.Message;
        }
        finally
        {
            loading = false;
        }
    }

    private void Edit(long id) => Navigation.NavigateTo($"/users/{id}/edit");
}

Reusable Component with Parameters

<div class="card h-100">
    <div class="card-body">
        <h5 class="card-title">@User.FirstName @User.LastName</h5>
        <small class="text-muted">@User.Email</small>
        <span class="badge @(User.Active ? "bg-success" : "bg-secondary")">
            @(User.Active ? "Active" : "Inactive")
        </span>
    </div>
    <div class="card-footer">
        <button class="btn btn-sm btn-outline-primary" @onclick="OnEdit">Edit</button>
        <button class="btn btn-sm btn-outline-danger" @onclick="OnDelete">Delete</button>
    </div>
</div>

@code {
    [Parameter, EditorRequired]
    public UserResponse User { get; set; } = default!;

    [Parameter]
    public EventCallback OnEdit { get; set; }

    [Parameter]
    public EventCallback OnDelete { get; set; }
}

Templated Component (RenderFragment)

@code {
    [Parameter] public string Title { get; set; } = "Modal";
    [Parameter] public RenderFragment? BodyContent { get; set; }
    [Parameter] public RenderFragment? FooterContent { get; set; }
    [Parameter] public EventCallback OnClose { get; set; }
}

Use RenderFragment for content slots, RenderFragment<T> for typed template parameters.

Data Binding & Forms

EditForm with Validation

<EditForm Model="model"
    <DataAnnotationsValidator />
    <ValidationSummary class="alert alert-danger" />

    <div class="mb-3">
        <label for="email" class="form-label">Email</label>
        <InputText id="email" class="form-control" @bind-Value="model.Email" />
        <ValidationMessage For="@(() => model.Email)" class="text-danger" />
    </div>

    <button type="submit" class="btn btn-primary" disabled="@submitting">
        @(submitting ? "Saving..." : "Save")
    </button>
</EditForm>

Routing

Route Parameters and Constraints

@page "/users/{Id:long}"
@page "/users/{Id:long}/edit"

@code {
    [Parameter]
    public long Id { get; set; }
}

Navigation

@inject NavigationManager Navigation

Navigation.NavigateTo("/users");                    // Client-side navigation
Navigation.NavigateTo("/users/1", forceLoad: true); // Full page reload

Use <NavLink> with Match="NavLinkMatch.Prefix" or NavLinkMatch.All for active CSS state.

State Management

Observable State Service

public class AppState
{
    private UserResponse? _currentUser;

    public UserResponse? CurrentUser
    {
        get => _currentUser;
        set { _currentUser = value; NotifyStateChanged(); }
    }

    public event Action? OnChange;
    private void NotifyStateChanged() => OnChange?.Invoke();
}

Consuming State in Components

@inject AppState State
@implements IDisposable

<span>@State.CurrentUser?.FirstName</span>

@code {
    protected override void OnInitialized()
    {
        State.OnChange += StateHasChanged;
    }

    public void Dispose()
    {
        State.OnChange -= StateHasChanged;
    }
}

CascadingValue for Shared Data

Use <CascadingValue Value="@theme"> in layouts. Consume with [CascadingParameter] in children. Good for theme, auth state, and locale. Avoid for frequently changing data (triggers full subtree re-render).

JavaScript Interop

Calling JS from C#

@inject IJSRuntime JS

@code {
    // Only call in OnAfterRenderAsync, not OnInitializedAsync
    protected override async Task OnAfterRenderAsync(bool firstRender)
    {
        if (firstRender)
        {
            var width = await JS.InvokeAsync<int>("blazorInterop.getWindowWidth");
        }
    }
}

Encapsulate JS interop behind service classes registered in DI. Use InvokeVoidAsync for calls without return values, InvokeAsync<T> for typed returns.

.NET 8 Blazor United

Interactive Render Modes

@* Static SSR (default -- no interactivity) *@
<StaticComponent />

@* Interactive Server (SignalR) *@
<InteractiveComponent @rendermode="InteractiveServer" />

@* Interactive WebAssembly *@
<InteractiveComponent @rendermode="InteractiveWebAssembly" />

@* Interactive Auto (starts Server, switches to WASM when downloaded) *@
<InteractiveComponent @rendermode="InteractiveAuto" />

Streaming Rendering

Add @attribute [StreamRendering] to pages with async data loading. The page renders immediately with placeholder content, then streams updates as OnInitializedAsync completes. Ideal for SSR pages loading slow data.

Testing Overview

Standards

• Use bUnit for component unit tests
• Use xUnit as the test runner
• Use Moq or NSubstitute for service mocking
• Use FluentAssertions for readable assertions
• Test rendered markup, parameter behavior, and event callbacks
• Coverage target: >80% for business logic components

Basic Component Test

using Bunit;
using FluentAssertions;

public class UserCardTests : TestContext
{
    [Fact]
    public void UserCard_DisplaysUserInfo()
    {
        var user = new UserResponse(1, "test@example.com", "John", "Doe",
            "User", true, DateTime.UtcNow);

        var cut = RenderComponent<UserCard>(p => p.Add(x => x.User, user));

        cut.Find(".card-title").TextContent.Should().Contain("John Doe");
        cut.Find("small.text-muted").TextContent.Should().Contain("test@example.com");
    }

    [Fact]
    public void UserCard_EditButton_InvokesCallback()
    {
        var user = new UserResponse(1, "t@e.com", "J", "D", "User", true, DateTime.UtcNow);
        var clicked = false;

        var cut = RenderComponent<UserCard>(p => p
            .Add(x => x.User, user)
            .Add(x => x.OnEdit, EventCallback.Factory.Create(this, () => clicked = true)));

        cut.Find("button.btn-outline-primary").Click();
        clicked.Should().BeTrue();
    }
}

Tooling

Essential Commands

dotnet new blazorwasm -o MyApp.Client     # New Blazor WASM app
dotnet new blazorserver -o MyApp.Server   # New Blazor Server app
dotnet new blazor -o MyApp                # New Blazor Web App (.NET 8 United)
dotnet watch run --project MyApp.Client   # Dev server with hot reload
dotnet build                              # Build
dotnet publish -c Release                 # Publish
dotnet test                               # Run tests
dotnet add package bunit                  # Add bUnit testing

Key Dependencies

Best Practices Summary

DO

• Use @key on list items for efficient DOM diffing
• Use EventCallback over Action for component events
• Implement IDisposable for event handler and timer cleanup
• Use <Virtualize> for rendering large collections
• Use typed HttpClient for API calls
• Use CascadingValue for widely shared state (theme, auth)
• Use @rendermode per-component in .NET 8 United

DO NOT

• Call JS interop in OnInitialized (use OnAfterRender)
• Mutate [Parameter] properties directly
• Use synchronous HTTP calls
• Ignore component lifecycle (leaks events and timers)
• Overuse JS interop (prefer C# solutions)
• Skip loading/error state handling in data-fetching components

Advanced Topics

For detailed patterns and examples, see:

• references/patterns.md -- Forms, SignalR, authentication, testing, performance, error handling patterns

External References

• Blazor Documentation
• Blazor University
• bUnit Testing
• Blazored Libraries
• Awesome Blazor