version: "1.0.1" name: blazor description: "Build and review Blazor applications across server, WebAssembly, web app, and hybrid scenarios with correct component design, state flow, rendering, and hosting choices. USE FOR: building interactive web UIs with C# instead of JavaScript; choosing between Server, WebAssembly, or Auto render modes; designing component hierarchies and state. DO NOT USE FOR: unrelated stacks; generic tasks that do not need this specific guidance. INVOKES: inspect the repository context, edit targeted files, and run relevant build, test, lint, or validation commands when changes are made." compatibility: "Requires Blazor project (.NET 6+, preferably .NET 8+ for unified model)."

Blazor

Trigger On

• building interactive web UIs with C# instead of JavaScript
• choosing between Server, WebAssembly, or Auto render modes
• designing component hierarchies and state management
• handling prerendering and hydration
• integrating with JavaScript when necessary

Documentation

• Blazor Overview
• Render Modes
• Performance Best Practices
• State Management
• JS Interop

References

• patterns.md - Detailed component patterns, state management strategies, and JS interop techniques
• anti-patterns.md - Common Blazor mistakes and how to avoid them

Render Modes (.NET 8+)

Applying Render Modes

@* Per-component *@
@rendermode InteractiveServer
 
@* Or in App.razor for global *@
<Routes @rendermode="InteractiveAuto" />

InteractiveAuto Architecture

First Request:
  Browser → Server (Interactive Server) → Fast response
 
Subsequent Requests:
  Browser → WASM (downloaded in background) → No server needed

Workflow

1. Choose render mode based on requirements:

• Need SEO? Start with Static or prerendering
• Need real-time? Use InteractiveServer
• Need offline? Use InteractiveWebAssembly
• Want both? Use InteractiveAuto

1. Design components for reusability:

• Small, focused components
• Parameters for customization
• Events for communication

1. Handle state correctly:

• Component state lives in component
• Shared state via services (DI)
• Persist state across prerender with [PersistentState]

1. Validate in both environments (for Auto mode)

Current Upstream Notes

• Treat dotnet/aspnetcore v10.0.11 as servicing. It fixes restoration of expired client-persisted circuit state; re-run reconnect, persisted-state expiry, render-mode, and interactive-auto client/server split tests when upgrading.
• The August 2026 ASP.NET Core overview still positions Blazor as the component UI path. A servicing update does not change the component or render-mode architecture by itself.

Component Patterns

Basic Component

@* Counter.razor *@
<button @onclick="IncrementCount">
    Clicked @count times
</button>
 
@code {
    private int count = 0;
 
    [Parameter]
    public int InitialCount { get; set; } = 0;
 
    protected override void OnInitialized()
    {
        count = InitialCount;
    }
 
    private void IncrementCount() => count++;
}

Parameter and Event Callbacks

@* Parent.razor *@
<ChildComponent Value="@value" ValueChanged="@OnValueChanged" />
 
@* ChildComponent.razor *@
@code {
    [Parameter] public string Value { get; set; } = "";
    [Parameter] public EventCallback<string> ValueChanged { get; set; }
 
    private async Task UpdateValue(string newValue)
    {
        await ValueChanged.InvokeAsync(newValue);
    }
}

State Persistence (.NET 8+)

@* Prevents double-fetch during prerender + hydration *@
@code {
    [PersistentState]
    public List<Product> Products { get; set; } = [];
 
    protected override async Task OnInitializedAsync()
    {
        // Only fetches once, persisted across prerender
        Products ??= await Http.GetFromJsonAsync<List<Product>>("api/products");
    }
}

Data Access Pattern for Auto Mode

// Shared interface
public interface IProductService
{
    Task<List<Product>> GetProductsAsync();
}
 
// Server implementation (direct DB access)
public class ServerProductService : IProductService
{
    private readonly AppDbContext _db;
    public async Task<List<Product>> GetProductsAsync()
        => await _db.Products.ToListAsync();
}
 
// Client implementation (HTTP call)
public class ClientProductService : IProductService
{
    private readonly HttpClient _http;
    public async Task<List<Product>> GetProductsAsync()
        => await _http.GetFromJsonAsync<List<Product>>("api/products");
}
 
// Registration
// Server: builder.Services.AddScoped<IProductService, ServerProductService>();
// Client: builder.Services.AddScoped<IProductService, ClientProductService>();

Anti-Patterns to Avoid

Performance Best Practices

1. Virtualize large lists:

``razor <Virtualize Items="@products" Context="product"> <ProductCard Product="@product" /> </Virtualize> ``

1. Use `@key` for list diffing:

``razor @foreach (var item in items) { <ItemComponent @key="item.Id" Item="@item" /> } ``

1. Debounce rapid events:

```csharp private Timer? _debounceTimer;

private void OnInput(ChangeEventArgs e) { _debounceTimer?.Dispose(); _debounceTimer = new Timer(_ => InvokeAsync(DoSearch), null, 300, Timeout.Infinite); } ```

1. Lazy load assemblies (WASM):

``csharp var assemblies = await LazyAssemblyLoader .LoadAssembliesAsync(["MyHeavyFeature.wasm"]); ``

JS Interop

Calling JavaScript from C#

@inject IJSRuntime JS
 
await JS.InvokeVoidAsync("alert", "Hello from Blazor!");
var result = await JS.InvokeAsync<string>("prompt", "Enter name:");

Calling C# from JavaScript

[JSInvokable]
public static string GetMessage() => "Hello from C#!";

DotNet.invokeMethodAsync('MyAssembly', 'GetMessage')
    .then(result => console.log(result));

Deliver

• interactive Blazor components with appropriate render mode
• efficient state management and data flow
• proper handling of prerendering scenarios
• performant list rendering with virtualization

Validate

• components render correctly in chosen mode
• state persists correctly across prerender/hydration
• no unnecessary re-renders (check with browser tools)
• JS interop works in both Server and WASM
• error boundaries catch component failures
• Auto mode works in both environments