Blazor

---
name: blazor
description: Guidelines for Blazor development including component lifecycle, state management, and performance optimization
---

# Blazor Development Guidelines

You are an expert in Blazor development with deep knowledge of both Blazor Server and Blazor WebAssembly.

## Component Architecture

### Component Design
- Create small, focused components
- Use component parameters for input
- Use EventCallback for output/events
- Implement IDisposable for cleanup
- Use cascading parameters sparingly

### Component Structure
```razor
@page "/users/{Id:int}"
@inject IUserService UserService

<h1>@User?.Name</h1>

@code {
    [Parameter]
    public int Id { get; set; }

    private User? User { get; set; }

    protected override async Task OnInitializedAsync()
    {
        User = await UserService.GetUserAsync(Id);
    }
}
```

## Component Lifecycle

### Lifecycle Methods
- `OnInitialized`/`OnInitializedAsync`: Initial setup
- `OnParametersSet`/`OnParametersSetAsync`: When parameters change
- `OnAfterRender`/`OnAfterRenderAsync`: After DOM updates
- `Dispose`: Cleanup resources

### Best Practices
- Use `OnInitializedAsync` for data loading
- Check `firstRender` in `OnAfterRenderAsync`
- Dispose subscriptions and timers
- Avoid long-running synchronous code

## Data Binding

### One-Way Binding
```razor
<p>@message</p>
<input value="@inputValue" />
```

### Two-Way Binding
```razor
<input @bind="inputValue" />
<input @bind="inputValue" @bind:event="oninput" />
```

### Event Handling
```razor
<button @onclick="HandleClick">Click</button>
<button @onclick="() => HandleClickWithParam(id)">Click</button>
<button @onclick="HandleClickAsync">Async Click</button>
```

## Render Optimization

### Prevent Unnecessary Renders
- Use `@key` for list items
- Implement `ShouldRender()` when appropriate
- Use `StateHasChanged()` judiciously
- Avoid inline handlers in loops

### Virtualization
```razor
<Virtualize Items="@items" Context="item">
    <ItemContent>
        <div>@item.Name</div>
    </ItemContent>
</Virtualize>
```

## State Management

### Component State
- Use private fields for component state
- Call `StateHasChanged()` when state changes externally
- Use `InvokeAsync` for thread-safe updates

### Cascading Parameters
```razor
<CascadingValue Value="@currentTheme">
    <ChildComponent />
</CascadingValue>

<!-- In child component -->
[CascadingParameter]
public Theme CurrentTheme { get; set; }
```

### State Containers
- Create injectable state services
- Use events for state change notifications
- Consider Fluxor for complex state management

## Blazor Server vs WebAssembly

### Blazor Server
- State lives on server
- Real-time connection via SignalR
- Faster initial load
- Requires stable connection
- Better for internal apps

### Blazor WebAssembly
- Runs entirely in browser
- Larger initial download
- Works offline (PWA capable)
- No server resources per user
- Better for public apps

## API Integration

### HTTP Client
```csharp
@inject HttpClient Http

private async Task LoadData()
{
    users = await Http.GetFromJsonAsync<List<User>>("api/users");
}
```

### Error Handling
```csharp
try
{
    users = await Http.GetFromJsonAsync<List<User>>("api/users");
}
catch (HttpRequestException ex)
{
    errorMessage = "Failed to load users";
}
```

## Error Handling

### Error Boundaries
```razor
<ErrorBoundary>
    <ChildContent>
        <RiskyComponent />
    </ChildContent>
    <ErrorContent Context="ex">
        <p>An error occurred: @ex.Message</p>
    </ErrorContent>
</ErrorBoundary>
```

### Global Error Handling
- Implement `IErrorBoundary` for custom handling
- Log errors to server
- Show user-friendly messages

## Testing

### bUnit Testing
```csharp
[Fact]
public void ComponentRendersCorrectly()
{
    using var ctx = new TestContext();
    var cut = ctx.RenderComponent<Counter>();

    cut.Find("p").MarkupMatches("<p>Current count: 0</p>");
    cut.Find("button").Click();
    cut.Find("p").MarkupMatches("<p>Current count: 1</p>");
}
```

## Authentication

### Setup
- Use `AuthenticationStateProvider`
- Use `AuthorizeView` for conditional UI
- Use `[Authorize]` attribute on pages
- Implement custom auth state provider for JWT

### AuthorizeView
```razor
<AuthorizeView>
    <Authorized>
        <p>Welcome, @context.User.Identity?.Name!</p>
    </Authorized>
    <NotAuthorized>
        <p>Please log in.</p>
    </NotAuthorized>
</AuthorizeView>
```

## Performance Tips

- Use `@key` for dynamic lists
- Implement virtualization for large lists
- Lazy load components with `@if`
- Minimize JavaScript interop calls
- Use streaming rendering in .NET 8+