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Blazor Framework - Quick Reference (SKILL.md)

Version: 1.0.0 Target: .NET 8.0+ with Blazor Server/WebAssembly UI Library: Microsoft Fluent UI Blazor Components Purpose: Fast lookup for common Blazor patterns and best practices

1. Blazor Hosting Models

Blazor Server

Characteristics:

• Server-side rendering with SignalR connection
• Stateful connection to server required
• Small initial download (~2MB), fast startup
• UI events sent to server via SignalR

Setup:

// Program.cs
builder.Services.AddServerSideBlazor();
app.MapBlazorHub();
app.MapFallbackToPage("/_Host");

Use Cases: Internal apps, intranet, enterprise applications

Blazor WebAssembly

Characteristics:

• Client-side execution in browser via WebAssembly
• Full .NET runtime in browser
• Larger initial download (~10MB), but offline capable
• No server connection after initial load

Setup:

// Program.cs (Client)
builder.Services.AddBlazorWebAssembly();
await builder.Build().RunAsync();

Use Cases: Public-facing apps, PWAs, offline-first applications

Render Modes (.NET 8+)

@* Server-side with SignalR *@
@rendermode InteractiveServer

@* Client-side WebAssembly *@
@rendermode InteractiveWebAssembly

@* Auto: Server initially, then WebAssembly after download *@
@rendermode InteractiveAuto

@* Static: Server-side rendering without interactivity *@
@rendermode @(new Microsoft.AspNetCore.Components.Web.RenderMode.Static)

2. Component Architecture

Basic Component Structure

@page "/counter"
@using MyApp.Services

<h3>@Title</h3>
<p>Current count: @currentCount</p>
<button @onclick="IncrementCount">Click me</button>

@code {
    [Parameter]
    public string Title { get; set; } = "Counter";

    [Parameter]
    public EventCallback<int> OnCountChanged { get; set; }

    private int currentCount = 0;

    private async Task IncrementCount()
    {
        currentCount++;
        await OnCountChanged.InvokeAsync(currentCount);
    }
}

Component Parameters

// Required parameter
[Parameter, EditorRequired]
public string Name { get; set; } = "";

// Optional parameter with default
[Parameter]
public int MaxCount { get; set; } = 10;

// EventCallback (parent-child communication)
[Parameter]
public EventCallback<string> OnValueChanged { get; set; }

// Cascading parameter (from ancestor)
[CascadingParameter]
public AppState AppState { get; set; } = default!;

// Capture unmatched attributes
[Parameter(CaptureUnmatchedValues = true)]
public Dictionary<string, object>? AdditionalAttributes { get; set; }

Component Lifecycle

// 1. Parameters set (before initialization)
public override void SetParametersAsync(ParameterView parameters)
{
    base.SetParametersAsync(parameters);
}

// 2. Component initialization (once)
protected override void OnInitialized()
{
    // Synchronous initialization
}

protected override async Task OnInitializedAsync()
{
    // Async initialization (preferred for data loading)
    data = await DataService.GetDataAsync();
}

// 3. Parameters set (every time parameters change)
protected override void OnParametersSet()
{
    // React to parameter changes
}

protected override async Task OnParametersSetAsync()
{
    // Async parameter processing
}

// 4. After rendering
protected override void OnAfterRender(bool firstRender)
{
    if (firstRender)
    {
        // One-time setup after first render
    }
}

protected override async Task OnAfterRenderAsync(bool firstRender)
{
    if (firstRender)
    {
        // JS interop, focus management
        await JS.InvokeVoidAsync("initializeComponent");
    }
}

// 5. Determine if re-render needed
protected override bool ShouldRender()
{
    // Return false to skip re-render
    return true;
}

// 6. Disposal (cleanup)
public void Dispose()
{
    // Unsubscribe from events, dispose resources
    AppState.OnChange -= StateHasChanged;
}

public async ValueTask DisposeAsync()
{
    // Async disposal
    if (hubConnection is not null)
    {
        await hubConnection.DisposeAsync();
    }
}

3. Fluent UI Components

Layout Components

@using Microsoft.FluentUI.AspNetCore.Components

@* Vertical stack with gap *@
<FluentStack Orientation="Orientation.Vertical" VerticalGap="16">
    <h1>Title</h1>
    <p>Content</p>
</FluentStack>

@* Horizontal stack *@
<FluentStack Orientation="Orientation.Horizontal" HorizontalGap="8">
    <FluentButton>Cancel</FluentButton>
    <FluentButton Appearance="Appearance.Accent">Save</FluentButton>
</FluentStack>

@* Grid layout *@
<FluentGrid Spacing="16">
    <FluentGridItem xs="12" sm="6" md="4">
        <FluentCard>Column 1</FluentCard>
    </FluentGridItem>
    <FluentGridItem xs="12" sm="6" md="4">
        <FluentCard>Column 2</FluentCard>
    </FluentGridItem>
</FluentGrid>

Input Components

@* Text input *@
<FluentTextField
    @bind-Value="name"
    Label="Name"
    Placeholder="Enter your name"
    Required="true" />

@* Text area *@
<FluentTextArea
    @bind-Value="description"
    Label="Description"
    Rows="4" />

@* Number input *@
<FluentNumberField
    @bind-Value="age"
    Label="Age"
    Min="0"
    Max="150" />

@* Select dropdown *@
<FluentSelect
    @bind-Value="selectedOption"
    Label="Choose option"
    Items="@options"
    OptionText="@(x => x.Name)"
    OptionValue="@(x => x.Id)" />

@* Checkbox *@
<FluentCheckbox
    @bind-Value="accepted"
    Label="I accept the terms" />

@* Switch toggle *@
<FluentSwitch
    @bind-Value="isEnabled"
    Label="Enable feature" />

@* Date picker *@
<FluentDatePicker
    @bind-Value="selectedDate"
    Label="Select date" />

Buttons

@* Primary button *@
<FluentButton Appearance="Appearance.Accent"
    Submit
</FluentButton>

@* Secondary button *@
<FluentButton Appearance="Appearance.Neutral">
    Cancel
</FluentButton>

@* Icon button *@
<FluentIconButton
    Icon="@(new Icons.Regular.Size20.Add())"
    aria-label="Add item"
    />

@* Split button *@
<FluentSplitButton Text="Actions">
    <FluentMenuItem
    <FluentMenuItem
</FluentSplitButton>

Data Display

@* Card *@
<FluentCard>
    <h3>Card Title</h3>
    <p>Card content goes here.</p>
</FluentCard>

@* Data grid with sorting *@
<FluentDataGrid Items="@users" Virtualize="true">
    <PropertyColumn Property="@(u => u.Name)" Sortable="true" />
    <PropertyColumn Property="@(u => u.Email)" />
    <PropertyColumn Property="@(u => u.CreatedAt)" Format="yyyy-MM-dd" Sortable="true" />
    <TemplateColumn Title="Actions">
        <FluentButton => EditUser(context))">Edit</FluentButton>
    </TemplateColumn>
</FluentDataGrid>

@* Accordion *@
<FluentAccordion>
    <FluentAccordionItem Heading="Section 1">
        Content for section 1
    </FluentAccordionItem>
    <FluentAccordionItem Heading="Section 2">
        Content for section 2
    </FluentAccordionItem>
</FluentAccordion>

Feedback Components

@* Dialog *@
<FluentDialog @bind-Hidden="hideDialog" Modal="true">
    <h2>Confirmation</h2>
    <p>Are you sure you want to delete this item?</p>
    <FluentButton Appearance="Appearance.Accent"
        Yes, Delete
    </FluentButton>
    <FluentButton => hideDialog = true)">
        Cancel
    </FluentButton>
</FluentDialog>

@* Message bar (notification) *@
<FluentMessageBar Intent="MessageIntent.Success" Visible="@showSuccess">
    Item saved successfully!
</FluentMessageBar>

@* Progress ring (loading spinner) *@
<FluentProgressRing Visible="@isLoading" />

@* Toast notification *@
@inject IToastService ToastService

@code {
    private void ShowToast()
    {
        ToastService.ShowSuccess("Operation completed successfully!");
    }
}

4. State Management

Component State

@code {
    // Private field for component-local state
    private int count = 0;
    private string message = "";
    private List<Item> items = new();

    // State change triggers re-render automatically
    private void UpdateState()
    {
        count++;
        // Component re-renders automatically
    }
}

Service-Based State

// AppState.cs
public class AppState
{
    private string currentUser = "";

    public string CurrentUser
    {
        get => currentUser;
        set
        {
            if (currentUser != value)
            {
                currentUser = value;
                NotifyStateChanged();
            }
        }
    }

    public event Action? OnChange;

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

// Program.cs
builder.Services.AddScoped<AppState>();

// Component
@inject AppState AppState
@implements IDisposable

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

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

    private void UpdateUser()
    {
        AppState.CurrentUser = "John Doe";
        // All subscribed components re-render
    }
}

Cascading Values

@* App.razor or parent component *@
<CascadingValue Value="@currentTheme">
    <Router AppAssembly="@typeof(Program).Assembly">
        @* Child components *@
    </Router>
</CascadingValue>

@code {
    private string currentTheme = "light";
}

@* Child component *@
@code {
    [CascadingParameter]
    public string Theme { get; set; } = "";

    // Access Theme without prop drilling
}

5. Forms & Validation

Basic EditForm

<EditForm Model="@person"
    <DataAnnotationsValidator />
    <ValidationSummary />

    <FluentTextField @bind-Value="person.Name" Label="Name" />
    <ValidationMessage For="@(() => person.Name)" />

    <FluentNumberField @bind-Value="person.Age" Label="Age" />
    <ValidationMessage For="@(() => person.Age)" />

    <FluentButton Type="ButtonType.Submit" Appearance="Appearance.Accent">
        Submit
    </FluentButton>
</EditForm>

@code {
    private Person person = new();

    private async Task HandleValidSubmit()
    {
        // Form is valid
        await SavePersonAsync(person);
    }

    public class Person
    {
        [Required(ErrorMessage = "Name is required")]
        [MaxLength(100)]
        public string Name { get; set; } = "";

        [Range(0, 150, ErrorMessage = "Age must be between 0 and 150")]
        public int Age { get; set; }

        [Required]
        [EmailAddress]
        public string Email { get; set; } = "";
    }
}

FluentValidation

// Install: FluentValidation.Blazor
using FluentValidation;

public class PersonValidator : AbstractValidator<Person>
{
    public PersonValidator()
    {
        RuleFor(x => x.Name)
            .NotEmpty().WithMessage("Name is required")
            .MaximumLength(100);

        RuleFor(x => x.Age)
            .InclusiveBetween(0, 150);

        RuleFor(x => x.Email)
            .NotEmpty()
            .EmailAddress();
    }
}

// Component
<EditForm Model="@person"
    <FluentValidationValidator />
    <ValidationSummary />
    @* Form fields *@
</EditForm>

Custom Validation

// Custom attribute
public class FutureDateAttribute : ValidationAttribute
{
    protected override ValidationResult? IsValid(object? value, ValidationContext validationContext)
    {
        if (value is DateTime date && date > DateTime.Now)
        {
            return ValidationResult.Success;
        }
        return new ValidationResult("Date must be in the future");
    }
}

// Usage
public class Event
{
    [FutureDate]
    public DateTime EventDate { get; set; }
}

6. Routing & Navigation

Route Declaration

@* Basic route *@
@page "/products"

@* Multiple routes *@
@page "/products"
@page "/items"

@* Route parameter *@
@page "/product/{id}"

@code {
    [Parameter]
    public string Id { get; set; } = "";
}

@* Optional parameter *@
@page "/product/{id?}"

@* Route constraint *@
@page "/product/{id:int}"
@page "/user/{userId:guid}"

@* Catch-all parameter *@
@page "/docs/{*path}"

Navigation

@inject NavigationManager Navigation

@code {
    private void NavigateToProduct(int id)
    {
        Navigation.NavigateTo($"/product/{id}");
    }

    private void NavigateExternal()
    {
        Navigation.NavigateTo("https://example.com", forceLoad: true);
    }

    private void Refresh()
    {
        Navigation.NavigateTo(Navigation.Uri, forceLoad: true);
    }

    // Get current URL
    var currentUrl = Navigation.Uri;
    var baseUrl = Navigation.BaseUri;

    // Query string
    var uri = new Uri(Navigation.Uri);
    var query = System.Web.HttpUtility.ParseQueryString(uri.Query);
    var searchTerm = query["search"];
}

NavLink Component

<FluentNavMenu>
    @* Exact match (active only for exact path) *@
    <FluentNavLink Href="/" Match="NavLinkMatch.All">
        Home
    </FluentNavLink>

    @* Prefix match (active for path and descendants) *@
    <FluentNavLink Href="/products" Match="NavLinkMatch.Prefix">
        Products
    </FluentNavLink>
</FluentNavMenu>

7. JavaScript Interop

Calling JavaScript from Blazor

@inject IJSRuntime JS

@code {
    protected override async Task OnAfterRenderAsync(bool firstRender)
    {
        if (firstRender)
        {
            // Call void function
            await JS.InvokeVoidAsync("alert", "Hello from Blazor!");

            // Call function with return value
            var result = await JS.InvokeAsync<string>("localStorage.getItem", "key");

            // Call custom function
            await JS.InvokeVoidAsync("myApp.initialize", elementRef);
        }
    }
}

JavaScript file (wwwroot/js/app.js):

window.myApp = {
    initialize: function(element) {
        console.log("Initializing", element);
    },

    getData: function() {
        return { value: 42 };
    }
};

Calling Blazor from JavaScript

// Component
public class InteropComponent : ComponentBase
{
    private DotNetObjectReference<InteropComponent>? dotNetHelper;

    protected override void OnInitialized()
    {
        dotNetHelper = DotNetObjectReference.Create(this);
    }

    [JSInvokable]
    public void CallMeFromJS(string message)
    {
        Console.WriteLine($"Called from JS: {message}");
        StateHasChanged(); // Re-render if needed
    }

    [JSInvokable]
    public Task<string> GetDataFromBlazor()
    {
        return Task.FromResult("Data from Blazor");
    }

    public void Dispose()
    {
        dotNetHelper?.Dispose();
    }
}

JavaScript:

// Call static method
DotNet.invokeMethodAsync('MyApp', 'StaticMethod', 'arg1');

// Call instance method
dotNetHelper.invokeMethodAsync('CallMeFromJS', 'Hello');

8. Accessibility (WCAG 2.1 AA)

Semantic HTML

<nav aria-label="Main navigation">
    <FluentNavMenu>
        <FluentNavLink Href="/">Home</FluentNavLink>
        <FluentNavLink Href="/about">About</FluentNavLink>
    </FluentNavMenu>
</nav>

<main>
    <h1>Page Title</h1>
    <article>
        <h2>Article Heading</h2>
        <p>Content...</p>
    </article>
</main>

<aside aria-label="Sidebar">
    <h3>Related Links</h3>
    <ul>
        <li><a href="/link1">Link 1</a></li>
    </ul>
</aside>

ARIA Attributes

@* Button with accessible label *@
<FluentIconButton
    Icon="@(new Icons.Regular.Size20.Delete())"
    aria-label="Delete item"
    />

@* Form with descriptive text *@
<FluentTextField
    @bind-Value="email"
    Label="Email"
    aria-describedby="email-help" />
<span id="email-help">We'll never share your email.</span>

@* Hidden decorative icon *@
<FluentIcon Icon="Icons.Regular.Size20.Info" aria-hidden="true" />

@* Live region for dynamic updates *@
<div aria-live="polite" aria-atomic="true">
    @statusMessage
</div>

Keyboard Navigation

@* All Fluent UI components support keyboard navigation *@

@* Custom component with keyboard support *@
<div
    tabindex="0"
    @onkeydown="HandleKeyDown"
    role="button"
    aria-label="Custom button">
    Click or press Enter
</div>

@code {
    private void HandleKeyDown(KeyboardEventArgs e)
    {
        if (e.Key == "Enter" || e.Key == " ")
        {
            // Activate on Enter or Space
            Activate();
        }
        else if (e.Key == "Escape")
        {
            // Close on Escape
            Close();
        }
    }
}

9. SignalR & Real-Time (Blazor Server)

Hub Definition

// NotificationHub.cs
using Microsoft.AspNetCore.SignalR;

public class NotificationHub : Hub
{
    public async Task SendNotification(string user, string message)
    {
        await Clients.All.SendAsync("ReceiveNotification", user, message);
    }

    public async Task JoinGroup(string groupName)
    {
        await Groups.AddToGroupAsync(Context.ConnectionId, groupName);
    }

    public async Task SendToGroup(string groupName, string message)
    {
        await Clients.Group(groupName).SendAsync("ReceiveMessage", message);
    }
}

// Program.cs
builder.Services.AddSignalR();
app.MapHub<NotificationHub>("/notificationhub");

Component with SignalR

@inject NavigationManager Navigation
@implements IAsyncDisposable

<h3>Real-Time Notifications</h3>

@foreach (var notification in notifications)
{
    <FluentCard>@notification</FluentCard>
}

@code {
    private HubConnection? hubConnection;
    private List<string> notifications = new();

    protected override async Task OnInitializedAsync()
    {
        hubConnection = new HubConnectionBuilder()
            .WithUrl(Navigation.ToAbsoluteUri("/notificationhub"))
            .WithAutomaticReconnect()
            .Build();

        hubConnection.On<string, string>("ReceiveNotification", (user, message) =>
        {
            notifications.Add($"{user}: {message}");
            StateHasChanged();
        });

        await hubConnection.StartAsync();
    }

    private async Task SendNotification()
    {
        if (hubConnection is not null)
        {
            await hubConnection.SendAsync("SendNotification", "User", "Hello!");
        }
    }

    public async ValueTask DisposeAsync()
    {
        if (hubConnection is not null)
        {
            await hubConnection.DisposeAsync();
        }
    }
}

10. Testing with bUnit

Basic Component Test

using Bunit;
using FluentAssertions;
using Xunit;

public class CounterTests : TestContext
{
    [Fact]
    public void Counter_StartsAtZero()
    {
        // Arrange & Act
        var cut = RenderComponent<Counter>();

        // Assert
        cut.Find("p").TextContent.Should().Contain("count: 0");
    }

    [Fact]
    public void Counter_Increments_OnButtonClick()
    {
        // Arrange
        var cut = RenderComponent<Counter>();
        var button = cut.Find("button");

        // Act
        button.Click();

        // Assert
        cut.Find("p").TextContent.Should().Contain("count: 1");
    }
}

Testing with Parameters

[Fact]
public void Component_Renders_WithParameters()
{
    // Arrange & Act
    var cut = RenderComponent<MyComponent>(parameters => parameters
        .Add(p => p.Title, "Test Title")
        .Add(p => p.Count, 5));

    // Assert
    cut.Find("h3").TextContent.Should().Be("Test Title");
    cut.Find("p").TextContent.Should().Contain("5");
}

Testing EventCallbacks

[Fact]
public void Component_Invokes_Callback()
{
    // Arrange
    var callbackInvoked = false;
    var cut = RenderComponent<MyComponent>(parameters => parameters
        .Add(p => p.OnClick, () => callbackInvoked = true));

    // Act
    cut.Find("button").Click();

    // Assert
    callbackInvoked.Should().BeTrue();
}

Testing with Services

[Fact]
public void Component_Uses_InjectedService()
{
    // Arrange
    var mockService = new Mock<IDataService>();
    mockService.Setup(s => s.GetData()).Returns(new List<Item> { new() { Name = "Test" } });

    Services.AddSingleton(mockService.Object);

    // Act
    var cut = RenderComponent<DataComponent>();

    // Assert
    cut.FindAll("li").Should().HaveCount(1);
    cut.Find("li").TextContent.Should().Be("Test");
}

Common Patterns Summary

Best Practices

1. Use Fluent UI components for consistent, accessible UI
2. Dispose resources in Dispose()/DisposeAsync()
3. Async all the way - Prefer OnInitializedAsync over synchronous methods
4. EventCallback for events - Not regular events
5. Services for shared state - With change notification pattern
6. Semantic HTML - Use proper elements for accessibility
7. ARIA labels - For icon buttons and dynamic content
8. Test components - Use bUnit for component logic tests
9. SignalR reconnection - Use WithAutomaticReconnect()
10. Route constraints - Validate route parameters at routing level

Next: See REFERENCE.md for comprehensive guides, advanced patterns, and production deployment strategies.