fetch-and-send-data

license	MIT
name	fetch-and-send-data
description	Call APIs, load data into components, and handle the async lifecycle in Blazor. USE FOR fetching data from a backend, submitting data to an API, displaying loading/error states, registering HttpClient, building service abstractions for Auto/WebAssembly render modes. DO NOT USE for form validation (see collect-user-input), prerendering persistence (see support-prerendering), or project scaffolding (see create-blazor-project).

Fetch and Send Data

Step 1 — Read AGENTS.md

Check Interactivity Mode and Scope:

Mode	Data access
None (Static SSR)	Server-side: inject services/DbContext. Use [StreamRendering] for loading UX.
Server	Server-side: inject services/DbContext. Guard prerender with ??= + [PersistentState].
WebAssembly	Browser-side: HttpClient only. No direct server access.
Auto	Both server and browser. Always go through an API.

Step 2 — Register HttpClient

Only needed when calling external APIs from Server, or always for WebAssembly/Auto. Server components accessing their own database should inject DbContext or a service directly.

// Named client — requires Microsoft.Extensions.Http NuGet
builder.Services.AddHttpClient("CatalogAPI", client =>
{
    client.BaseAddress = new Uri("https://api.example.com/");
});

// Typed client
builder.Services.AddHttpClient<CatalogClient>(client =>
    client.BaseAddress = new Uri("https://api.example.com/"));

For WebAssembly/Auto with prerendering, register in both server and .Client Program.cs.

Step 3 — Fetch Data

Simple load

@page "/products"
@inject CatalogClient Catalog

@if (products is null)
{
    <p>Loading…</p>
}
else
{
    @foreach (var p in products)
    {
        <p>@p.Name — @p.Price.ToString("C")</p>
    }
}

@code {
    private Product[]? products;

    protected override async Task OnInitializedAsync()
    {
        products = await Catalog.GetProductsAsync();
    }
}

No error handling needed in the simplest case — wrap the component usage in <ErrorBoundary> at the parent/layout level to catch unhandled exceptions.

Static SSR — StreamRendering

Without [StreamRendering], the user sees nothing until OnInitializedAsync completes:

@attribute [StreamRendering]

Only affects Static SSR. No effect on interactive components.

Prerendering guard

Prerendering calls OnInitializedAsync twice. Skip the duplicate:

[PersistentState] private Product[]? products;

protected override async Task OnInitializedAsync()
{
    products ??= await Catalog.GetProductsAsync();
}

See the support-prerendering skill for details.

Step 4 — Handle Errors

Use <ErrorBoundary> as the default error strategy. It provides a consistent error experience across all components without any per-component catch logic. Wrap component usage at the layout or parent level:

<ErrorBoundary>
    <ChildContent>
        <ProductList />
    </ChildContent>
    <ErrorContent>
        <div class="alert alert-danger">Something went wrong. Please refresh.</div>
    </ErrorContent>
</ErrorBoundary>

Non-cancellation exceptions (HttpRequestException, etc.) propagate to ErrorBoundary automatically — no catch blocks needed in the component.

Cancellation is special

ComponentBase silently swallows all OperationCanceledException — both self-initiated (disposal, parameter change) and external (HttpClient timeout). ErrorBoundary never sees them. This means:

• Self-cancellation → silently ignored. Correct behavior, no action needed.
• External cancellation (timeout) → also silently swallowed. Component gets stuck in loading state. Usually acceptable — timeouts are rare.

When to add in-component error handling

Only add catch blocks when the component needs behavior ErrorBoundary can't provide — typically retries or timeout-specific messages. Even then, only catch what you need:

// Catch only external cancellation (timeouts) — everything else flows to ErrorBoundary
catch (OperationCanceledException ex) when (!cancellationToken.IsCancellationRequested)
{
    Logger.LogWarning(ex, "Request timed out for category {CategoryId}", CategoryId);
    error = "The request timed out. Please try again.";
}

If the component also needs to handle general errors with a retry button instead of letting ErrorBoundary take over:

catch (OperationCanceledException ex) when (!cancellationToken.IsCancellationRequested)
{
    Logger.LogWarning(ex, "Request timed out for category {CategoryId}", CategoryId);
    error = "The request timed out. Please try again.";
}
catch (Exception ex)
{
    Logger.LogError(ex, "Failed to load products for category {CategoryId}", CategoryId);
    error = "Unable to load products. Please try again.";
}

Rules

• Never display exception.Message — it may contain PII, connection strings, or internal details. Use hardcoded user-friendly messages.
• Always log through ILogger — the real exception goes to the logging pipeline.
• Services must accept CancellationToken — pass it to every async call so work stops when the component cancels.

Step 5 — Parameter-Driven Reloading

When data depends on a route or query parameter that changes (e.g., navigating between /products/1 and /products/2), use OnParametersSetAsync with a guard to skip reloads for parameters that don't affect data.

Pattern: cancel-and-reload with stale data overlay

@page "/products/{CategoryId:int}"
@implements IAsyncDisposable
@inject ProductService ProductService
@inject ILogger<Products> Logger

@if (error is not null)
{
    <div class="alert alert-danger">
        <p>@error</p>
        <button @onclick="LoadAsync">Retry</button>
    </div>
}
else if (products is null)
{
    <p>Loading…</p>
}
else
{
    @if (isLoading)
    {
        <p><em>Refreshing…</em></p>
    }
    @foreach (var p in products)
    {
        <p>@p.Name — @p.Price.ToString("C")</p>
    }
}

@code {
    [Parameter] public int CategoryId { get; set; }
    [SupplyParameterFromQuery] public string? ViewMode { get; set; } // UI-only

    private CancellationTokenSource? cts;
    private int? loadedCategoryId;
    private List<Product>? products;
    private bool isLoading;
    private string? error;

    protected override async Task OnParametersSetAsync()
    {
        if (CategoryId == loadedCategoryId)
        {
            return; // Only ViewMode changed — no reload
        }

        loadedCategoryId = CategoryId;
        await LoadAsync();
    }

    private async Task LoadAsync()
    {
        if (cts is not null)
        {
            await cts.CancelAsync();
            cts.Dispose();
        }

        cts = new CancellationTokenSource();
        var cancellationToken = cts.Token; // Capture locally before await

        error = null;
        isLoading = true;

        try
        {
            var result = await ProductService.GetByCategoryAsync(CategoryId, cancellationToken);
            products = result;
        }
        catch (OperationCanceledException ex) when (!cancellationToken.IsCancellationRequested)
        {
            Logger.LogWarning(ex, "Timed out loading category {CategoryId}", CategoryId);
            error = "The request timed out. Please try again.";
        }
        finally
        {
            isLoading = false;
        }
    }

    public async ValueTask DisposeAsync()
    {
        if (cts is not null)
        {
            await cts.CancelAsync();
            cts.Dispose();
        }
    }
}

Key details:

• Guard with tracked value: loadedCategoryId skips reloads when only UI parameters change.
• Capture the token locally before the await — the CTS field may be replaced by a concurrent parameter change.
• Don't null out products on subsequent loads — keep existing data visible with an isLoading overlay.
• IAsyncDisposable cancels pending work when the user navigates away.

Step 6 — Send Data

var response = await http.PostAsJsonAsync("products", newProduct);
response.EnsureSuccessStatusCode();

var response = await http.PutAsJsonAsync($"products/{id}", updated);
response.EnsureSuccessStatusCode();

var response = await http.DeleteAsync($"products/{id}");
response.EnsureSuccessStatusCode();

Disable the submit button while saving to prevent duplicate requests. Show a saving indicator.

Step 7 — Service Abstraction for Auto or WebAssembly with Prerendering

When components run in both server and browser (Auto mode, or WebAssembly with prerendering), abstract data access behind an abstract base class:

public abstract class ProductServiceBase
{
    public abstract Task<Product[]> GetAllAsync(CancellationToken ct = default);
}

// Server — direct database access
public class ServerProductService(AppDbContext db) : ProductServiceBase
{
    public override async Task<Product[]> GetAllAsync(CancellationToken ct = default) =>
        await db.Products.ToArrayAsync(ct);
}

// Client — calls API
public class ClientProductService(HttpClient http) : ProductServiceBase
{
    public override async Task<Product[]> GetAllAsync(CancellationToken ct = default) =>
        await http.GetFromJsonAsync<Product[]>("api/products", ct) ?? [];
}

Register the appropriate implementation in each project's Program.cs. Components inject the abstract base class.

Don'ts

• Don't call APIs in constructors — use OnInitializedAsync.
• Don't use OnParametersSetAsync unless data depends on a changing parameter. Use OnInitializedAsync for initial loads.
• Don't inject DbContext in WebAssembly/Auto components — no database in the browser.
• Don't call your own server via HttpClient — inject the service directly.
• Don't display exception.Message to users — PII risk. Log it, show a generic message.
• Don't catch OperationCanceledException for self-cancellation — ComponentBase handles it.