Dotnet Blazor Components

dotnet-blazor-components

Blazor component architecture: lifecycle methods, state management (cascading values, DI, browser storage), JavaScript interop (AOT-safe), EditForm validation, and QuickGrid. Covers per-render-mode behavior differences where relevant.

Scope boundary: This skill owns component implementation patterns. Hosting model selection and render mode configuration are owned by [skill:dotnet-blazor-patterns]. Authentication components (AuthorizeView, CascadingAuthenticationState) are owned by [skill:dotnet-blazor-auth].

Out of scope: bUnit testing -- see [skill:dotnet-blazor-testing]. Standalone SignalR hub patterns -- see [skill:dotnet-realtime-communication]. E2E testing -- see [skill:dotnet-playwright]. UI framework selection -- see [skill:dotnet-ui-chooser].

Cross-references: [skill:dotnet-blazor-patterns] for hosting models and render modes, [skill:dotnet-blazor-auth] for authentication, [skill:dotnet-blazor-testing] for bUnit testing, [skill:dotnet-realtime-communication] for standalone SignalR, [skill:dotnet-playwright] for E2E testing, [skill:dotnet-ui-chooser] for framework selection, [skill:dotnet-accessibility] for accessibility patterns (ARIA, keyboard nav, screen readers).

Component Lifecycle

Lifecycle Methods

@code {
    // 1. Called when parameters are set/updated
    public override async Task SetParametersAsync(ParameterView parameters)
    {
        // Access raw parameters before they are applied
        await base.SetParametersAsync(parameters);
    }

    // 2. Called after parameters are assigned (sync)
    protected override void OnInitialized()
    {
        // One-time initialization (runs once per component instance)
    }

    // 3. Called after parameters are assigned (async)
    protected override async Task OnInitializedAsync()
    {
        // Async initialization (data fetching, service calls)
        products = await ProductService.GetProductsAsync();
    }

    // 4. Called every time parameters change
    protected override void OnParametersSet()
    {
        // React to parameter changes
    }

    // 5. Called after each render
    protected override void OnAfterRender(bool firstRender)
    {
        if (firstRender)
        {
            // JS interop safe here -- DOM is available
        }
    }

    // 6. Async version of OnAfterRender
    protected override async Task OnAfterRenderAsync(bool firstRender)
    {
        if (firstRender)
        {
            await JSRuntime.InvokeVoidAsync("initializeChart", chartElement);
        }
    }

    // 7. Cleanup
    public void Dispose()
    {
        // Unsubscribe from events, dispose resources
    }

    // 8. Async cleanup
    public async ValueTask DisposeAsync()
    {
        // Async cleanup (dispose JS object references)
        if (module is not null)
        {
            await module.DisposeAsync();
        }
    }
}

Lifecycle Behavior per Render Mode

Gotcha: In Static SSR, OnAfterRender never executes because there is no persistent connection. Do not place critical logic in OnAfterRender for Static SSR pages.

State Management

Cascading Values

Cascading values flow data down the component tree without explicit parameter passing.

<!-- Parent: provide a cascading value -->
<CascadingValue Value="@theme" Name="AppTheme">
    <Router AppAssembly="typeof(App).Assembly">
        <!-- All descendants can receive AppTheme -->
    </Router>
</CascadingValue>

@code {
    private ThemeSettings theme = new() { IsDarkMode = false, AccentColor = "#0078d4" };
}

<!-- Child: consume the cascading value -->
@code {
    [CascadingParameter(Name = "AppTheme")]
    public ThemeSettings? Theme { get; set; }
}

Fixed cascading values (.NET 8+): For values that never change after initial render, use IsFixed="true" to avoid re-render overhead:

<CascadingValue Value="@config" IsFixed="true">
    <ChildComponent />
</CascadingValue>

Dependency Injection

// Register services in Program.cs
builder.Services.AddScoped<IProductService, ProductService>();
builder.Services.AddSingleton<AppState>();

// Inject in components
@inject IProductService ProductService
@inject AppState State

DI lifetime behavior per render mode:

Gotcha: In Blazor Server, Scoped services live for the entire circuit duration (not per-request like in MVC). A circuit persists until the user navigates away or the connection drops. Long-lived scoped services may accumulate state -- use OwningComponentBase<T> for component-scoped DI.

Browser Storage

// ProtectedBrowserStorage -- encrypted, per-user storage
// Available in InteractiveServer only (not WASM -- server encrypts/decrypts)
@inject ProtectedSessionStorage SessionStorage
@inject ProtectedLocalStorage LocalStorage

protected override async Task OnAfterRenderAsync(bool firstRender)
{
    if (firstRender)
    {
        // Session storage (cleared when tab closes)
        await SessionStorage.SetAsync("cart", cartItems);
        var result = await SessionStorage.GetAsync<List<CartItem>>("cart");
        if (result.Success) { cartItems = result.Value!; }

        // Local storage (persists across sessions)
        await LocalStorage.SetAsync("preferences", userPrefs);
    }
}

For InteractiveWebAssembly, use JS interop to access browser storage directly:

// WASM: Direct browser storage via JS interop
await JSRuntime.InvokeVoidAsync("localStorage.setItem", "key",
    JsonSerializer.Serialize(value, AppJsonContext.Default.UserPrefs));

var json = await JSRuntime.InvokeAsync<string?>("localStorage.getItem", "key");
if (json is not null)
{
    value = JsonSerializer.Deserialize(json, AppJsonContext.Default.UserPrefs);
}

Gotcha: ProtectedBrowserStorage is not available during prerendering. Always access it in OnAfterRenderAsync(firstRender: true), never in OnInitializedAsync.

JavaScript Interop

Calling JavaScript from .NET

@inject IJSRuntime JSRuntime

// Invoke a global JS function
await JSRuntime.InvokeVoidAsync("console.log", "Hello from Blazor");

// Invoke and get a return value
var width = await JSRuntime.InvokeAsync<int>("getWindowWidth");

// With timeout (important for Server to avoid hanging circuits)
var result = await JSRuntime.InvokeAsync<string>(
    "expensiveOperation",
    TimeSpan.FromSeconds(10),
    inputData);

JavaScript Module Imports (AOT-Safe)

// Import a JS module -- trim-safe, no reflection
private IJSObjectReference? module;

protected override async Task OnAfterRenderAsync(bool firstRender)
{
    if (firstRender)
    {
        module = await JSRuntime.InvokeAsync<IJSObjectReference>(
            "import", "./js/interop.js");
        await module.InvokeVoidAsync("initialize", elementRef);
    }
}

// Always dispose module references
public async ValueTask DisposeAsync()
{
    if (module is not null)
    {
        await module.DisposeAsync();
    }
}

// wwwroot/js/interop.js
export function initialize(element) {
    // Set up the element
}

export function getValue(element) {
    return element.value;
}

Calling .NET from JavaScript

// Instance method callback
private DotNetObjectReference<MyComponent>? dotNetRef;

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

[JSInvokable]
public void OnJsEvent(string data)
{
    message = data;
    StateHasChanged();
}

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

// Call .NET from JS
export function registerCallback(dotNetRef) {
    document.addEventListener('custom-event', (e) => {
        dotNetRef.invokeMethodAsync('OnJsEvent', e.detail);
    });
}

JS Interop per Render Mode

Gotcha: In InteractiveServer, all JS interop calls travel over SignalR, adding network latency. Minimize round trips by batching operations into a single JS function call.

EditForm Validation

Basic EditForm with Data Annotations

<EditForm Model="product" FormName="product-form">
    <DataAnnotationsValidator />
    <ValidationSummary />

    <div>
        <label for="name">Name:</label>
        <InputText id="name" @bind-Value="product.Name" />
        <ValidationMessage For="() => product.Name" />
    </div>

    <div>
        <label for="price">Price:</label>
        <InputNumber id="price" @bind-Value="product.Price" />
        <ValidationMessage For="() => product.Price" />
    </div>

    <div>
        <label for="category">Category:</label>
        <InputSelect id="category" @bind-Value="product.Category">
            <option value="">Select...</option>
            <option value="Electronics">Electronics</option>
            <option value="Clothing">Clothing</option>
        </InputSelect>
        <ValidationMessage For="() => product.Category" />
    </div>

    <button type="submit">Save</button>
</EditForm>

@code {
    private ProductModel product = new();

    private async Task HandleSubmit()
    {
        await ProductService.CreateAsync(product);
        Navigation.NavigateTo("/products");
    }
}

Model with Validation Attributes

public sealed class ProductModel
{
    [Required(ErrorMessage = "Product name is required")]
    [StringLength(200, MinimumLength = 1)]
    public string Name { get; set; } = "";

    [Range(0.01, 1_000_000, ErrorMessage = "Price must be between {1} and {2}")]
    public decimal Price { get; set; }

    [Required(ErrorMessage = "Category is required")]
    public string Category { get; set; } = "";
}

EditForm with Enhanced Form Handling (.NET 8+)

Static SSR forms require FormName and use [SupplyParameterFromForm]:

@page "/products/create"

<EditForm Model="product" FormName="create-product" Enhance>
    <DataAnnotationsValidator />
    <!-- form fields -->
    <button type="submit">Create</button>
</EditForm>

@code {
    [SupplyParameterFromForm]
    private ProductModel product { get; set; } = new();

    private async Task HandleSubmit()
    {
        await ProductService.CreateAsync(product);
        Navigation.NavigateTo("/products");
    }
}

The Enhance attribute enables enhanced form handling -- the form submits via fetch and patches the DOM without a full page reload.

Gotcha: FormName must be unique across all forms on the page. Duplicate FormName values cause ambiguous form submission errors.

QuickGrid

QuickGrid is a high-performance grid component built into Blazor (.NET 8+). It supports sorting, filtering, pagination, and virtualization.

Basic QuickGrid

@using Microsoft.AspNetCore.Components.QuickGrid

<QuickGrid Items="products">
    <PropertyColumn Property="p => p.Name" Sortable="true" />
    <PropertyColumn Property="p => p.Price" Format="C2" Sortable="true" />
    <PropertyColumn Property="p => p.Category" Sortable="true" />
    <TemplateColumn Title="Actions">
        <button @onclick="() => Edit(context)">Edit</button>
    </TemplateColumn>
</QuickGrid>

@code {
    private IQueryable<Product> products = Enumerable.Empty<Product>().AsQueryable();

    protected override async Task OnInitializedAsync()
    {
        var list = await ProductService.GetAllAsync();
        products = list.AsQueryable();
    }

    private void Edit(Product product) => Navigation.NavigateTo($"/products/{product.Id}/edit");
}

QuickGrid with Pagination

<QuickGrid Items="products" Pagination="pagination">
    <PropertyColumn Property="p => p.Name" Sortable="true" />
    <PropertyColumn Property="p => p.Price" Format="C2" />
</QuickGrid>

<Paginator State="pagination" />

@code {
    private PaginationState pagination = new() { ItemsPerPage = 20 };
    private IQueryable<Product> products = default!;
}

QuickGrid with Virtualization

For large datasets, virtualization renders only visible rows:

<QuickGrid Items="products" Virtualize="true" ItemSize="50">
    <PropertyColumn Property="p => p.Name" />
    <PropertyColumn Property="p => p.Price" Format="C2" />
</QuickGrid>

QuickGrid OnRowClick (.NET 11 Preview)

.NET 11 adds OnRowClick to QuickGrid for row-level click handling without template columns:

<QuickGrid Items="products"
    <PropertyColumn Property="p => p.Name" />
    <PropertyColumn Property="p => p.Price" Format="C2" />
</QuickGrid>

@code {
    private void HandleRowClick(GridRowClickEventArgs<Product> args)
    {
        Navigation.NavigateTo($"/products/{args.Item.Id}");
    }
}

Fallback (net10.0): Use a TemplateColumn with a click handler or wrap each row in a clickable element.

Source: ASP.NET Core .NET 11 Preview - QuickGrid enhancements

.NET 11 Preview Features

EnvironmentBoundary Component

EnvironmentBoundary conditionally renders content based on the hosting environment (Development, Staging, Production):

<EnvironmentBoundary Include="Development">
    <p>Debug panel -- only visible in Development</p>
    <DebugToolbar />
</EnvironmentBoundary>

<EnvironmentBoundary Exclude="Production">
    <p>Testing controls -- hidden in Production</p>
</EnvironmentBoundary>

Fallback (net10.0): Inject IWebHostEnvironment and use conditional rendering in @code.

Source: ASP.NET Core .NET 11 Preview - EnvironmentBoundary

Label and DisplayName Support

.NET 11 adds [DisplayName] support for input components, automatically generating <label> elements:

<EditForm Model="model" FormName="contact">
    <!-- Automatically renders <label> from [DisplayName] -->
    <InputText @bind-Value="model.FullName" />
    <InputText @bind-Value="model.EmailAddress" />
</EditForm>

@code {
    private ContactModel model = new();
}

// Model
public sealed class ContactModel
{
    [DisplayName("Full Name")]
    [Required]
    public string FullName { get; set; } = "";

    [DisplayName("Email Address")]
    [EmailAddress]
    public string EmailAddress { get; set; } = "";
}

Fallback (net10.0): Add explicit <label for="..."> elements manually.

Source: ASP.NET Core .NET 11 Preview - Label/DisplayName

IHostedService in WebAssembly

.NET 11 allows IHostedService implementations to run in Blazor WebAssembly, enabling background tasks in the browser:

// Register in WASM Program.cs
builder.Services.AddHostedService<DataSyncService>();

public sealed class DataSyncService : BackgroundService
{
    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        while (!stoppingToken.IsCancellationRequested)
        {
            await SyncDataFromServer();
            await Task.Delay(TimeSpan.FromMinutes(5), stoppingToken);
        }
    }
}

Fallback (net10.0): Use a Timer in a component or inject a singleton service that starts background work on first use.

Source: ASP.NET Core .NET 11 Preview - IHostedService in WASM

SignalR ConfigureConnection

.NET 11 adds ConfigureConnection to the Blazor Server circuit hub, allowing customization of the SignalR connection (e.g., adding custom headers, configuring reconnection):

// Program.cs
app.MapBlazorHub(options =>
{
    options.ConfigureConnection = connection =>
    {
        connection.Metadata["tenant"] = "default";
    };
});

Fallback (net10.0): Use IHubFilter or middleware to inspect/modify connections at the hub level.

Source: ASP.NET Core .NET 11 Preview - SignalR ConfigureConnection

Agent Gotchas

1. Do not call JS interop in OnInitializedAsync. The DOM is not available yet. Use OnAfterRenderAsync(firstRender: true) for JS calls that need DOM elements.
2. Do not forget StateHasChanged() after external state changes. When state changes from a non-Blazor context (timer, event handler, JS callback), call StateHasChanged() or InvokeAsync(StateHasChanged) to trigger re-render.
3. Do not use ProtectedBrowserStorage during prerendering. It throws because no interactive circuit exists yet. Access it only in OnAfterRenderAsync.
4. Do not forget FormName on Static SSR forms. Without it, form submissions in Static SSR mode are not routed to the correct handler.
5. Do not dispose DotNetObjectReference before JS is done with it. Premature disposal causes JSException when JavaScript tries to invoke the callback. Dispose in Dispose() or DisposeAsync().
6. Do not assume Scoped services are per-request in Blazor Server. Scoped services live for the entire circuit. Use OwningComponentBase<T> when you need component-scoped service lifetimes.

Prerequisites

• .NET 8.0+ (QuickGrid, enhanced form handling, cascading values with IsFixed)
• Microsoft.AspNetCore.Components.QuickGrid package for QuickGrid
• .NET 11 preview for EnvironmentBoundary, Label/DisplayName, QuickGrid OnRowClick, IHostedService in WASM

Knowledge Sources

Blazor component patterns in this skill are grounded in guidance from:

• Damian Edwards -- Razor and Blazor component design patterns, render mode architecture, and performance best practices. Principal architect on the ASP.NET team.

These sources inform the patterns and rationale presented above. This skill does not claim to represent or speak for any individual.

References

• Blazor Component Lifecycle
• Blazor State Management
• Blazor JS Interop
• Blazor Forms and Validation
• QuickGrid Component
• Cascading Values and Parameters