name: dotnet-blazor
version: "1.0.0"
category: "Web"
description: "Build and review Blazor applications across server, WebAssembly, web app, and hybrid scenarios with correct component design, state flow, rendering, and hosting choices."
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:
2. Need SEO? Start with Static or prerendering
3. Need real-time? Use InteractiveServer
4. Need offline? Use InteractiveWebAssembly

5. Want both? Use InteractiveAuto

6. Design components for reusability:

7. Small, focused components
8. Parameters for customization

9. Events for communication

10. Handle state correctly:

11. Component state lives in component
12. Shared state via services (DI)

13. Persist state across prerender with [PersistentState]

14. Validate in both environments (for Auto mode)

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>

2. Use @key for list diffing:
   razor @foreach (var item in items) { <ItemComponent @key="item.Id" Item="@item" /> }

3. 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