Blazor

Overview

Blazor is a .NET web framework for building interactive client-side web UIs using C# and Razor syntax. It supports multiple hosting models: Blazor Server (SignalR-based), Blazor WebAssembly (client-side .NET runtime), and the unified model introduced in .NET 8 that combines static server-side rendering with per-component interactivity. Blazor components are reusable .razor files that encapsulate markup, logic, and state. The framework integrates natively with ASP.NET Core for authentication, dependency injection, and middleware.

Component Fundamentals

Blazor components are the building blocks of the UI. Each component is a .razor file with markup and C# code.

@page "/customers"
@using Microsoft.AspNetCore.Components
@using MyApp.Services
@inject ICustomerService CustomerService
@inject NavigationManager Navigation

<PageTitle>Customers</PageTitle>

<h1>Customer Directory</h1>

@if (_customers is null)
{
    <p><em>Loading customers...</em></p>
}
else if (_customers.Count == 0)
{
    <p>No customers found.</p>
}
else
{
    <table class="table table-striped">
        <thead>
            <tr>
                <th>Name</th>
                <th>Email</th>
                <th>Joined</th>
                <th></th>
            </tr>
        </thead>
        <tbody>
            @foreach (var customer in _customers)
            {
                <tr @key="customer.Id">
                    <td>@customer.Name</td>
                    <td>@customer.Email</td>
                    <td>@customer.JoinedDate.ToShortDateString()</td>
                    <td>
                        <button class="btn btn-sm btn-primary"
                                @onclick="() => EditCustomer(customer.Id)">
                            Edit
                        </button>
                    </td>
                </tr>
            }
        </tbody>
    </table>
}

@code {
    private List<Customer>? _customers;

    protected override async Task OnInitializedAsync()
    {
        _customers = await CustomerService.GetAllAsync();
    }

    private void EditCustomer(int id)
    {
        Navigation.NavigateTo($"/customers/{id}/edit");
    }
}

Render Modes (.NET 8+)

The unified Blazor model in .NET 8+ lets you set render modes per-component: static SSR, interactive Server, interactive WebAssembly, or Auto.

// Program.cs - Configure render modes
using Microsoft.AspNetCore.Components;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddRazorComponents()
    .AddInteractiveServerComponents()
    .AddInteractiveWebAssemblyComponents();

builder.Services.AddScoped<ICustomerService, CustomerService>();

var app = builder.Build();

app.UseStaticFiles();
app.UseAntiforgery();

app.MapRazorComponents<App>()
    .AddInteractiveServerRenderMode()
    .AddInteractiveWebAssemblyRenderMode()
    .AddAdditionalAssemblies(typeof(MyApp.Client._Imports).Assembly);

app.Run();

Apply render modes to individual components:

@* Static SSR by default, interactive where needed *@
@page "/dashboard"
@rendermode InteractiveServer

<h1>Dashboard</h1>
<LiveChart Data="_chartData" />
<NotificationPanel @rendermode="InteractiveWebAssembly" />

@code {
    private ChartData[] _chartData = Array.Empty<ChartData>();

    protected override async Task OnInitializedAsync()
    {
        _chartData = await DashboardService.GetChartDataAsync();
    }
}

Reusable Component with Parameters and EventCallbacks

Build composable components using parameters and event callbacks for parent-child communication.

@* Components/ConfirmDialog.razor *@
@namespace MyApp.Components

<div class="modal @(_isVisible ? "show d-block" : "d-none")" tabindex="-1">
    <div class="modal-dialog">
        <div class="modal-content">
            <div class="modal-header">
                <h5 class="modal-title">@Title</h5>
            </div>
            <div class="modal-body">
                <p>@Message</p>
            </div>
            <div class="modal-footer">
                <button class="btn btn-secondary" @onclick="Cancel">Cancel</button>
                <button class="btn btn-danger" @onclick="Confirm">@ConfirmText</button>
            </div>
        </div>
    </div>
</div>

@code {
    private bool _isVisible;

    [Parameter, EditorRequired]
    public string Title { get; set; } = string.Empty;

    [Parameter, EditorRequired]
    public string Message { get; set; } = string.Empty;

    [Parameter]
    public string ConfirmText { get; set; } = "Delete";

    [Parameter]
    public EventCallback OnConfirmed { get; set; }

    [Parameter]
    public EventCallback OnCancelled { get; set; }

    public void Show() { _isVisible = true; StateHasChanged(); }
    public void Hide() { _isVisible = false; StateHasChanged(); }

    private async Task Confirm()
    {
        Hide();
        await OnConfirmed.InvokeAsync();
    }

    private async Task Cancel()
    {
        Hide();
        await OnCancelled.InvokeAsync();
    }
}

Forms and Validation

Blazor provides built-in form handling with EditForm, model binding, and DataAnnotations validation.

@page "/customers/create"
@using System.ComponentModel.DataAnnotations
@inject ICustomerService CustomerService
@inject NavigationManager Navigation

<EditForm Model="_model" OnValidSubmit="HandleSubmit" FormName="CreateCustomer">
    <DataAnnotationsValidator />
    <ValidationSummary class="text-danger" />

    <div class="mb-3">
        <label class="form-label">Name</label>
        <InputText class="form-control" @bind-Value="_model.Name" />
        <ValidationMessage For="() => _model.Name" />
    </div>

    <div class="mb-3">
        <label class="form-label">Email</label>
        <InputText class="form-control" @bind-Value="_model.Email" />
        <ValidationMessage For="() => _model.Email" />
    </div>

    <div class="mb-3">
        <label class="form-label">Tier</label>
        <InputSelect class="form-select" @bind-Value="_model.Tier">
            <option value="">Select tier...</option>
            <option value="Free">Free</option>
            <option value="Pro">Pro</option>
            <option value="Enterprise">Enterprise</option>
        </InputSelect>
    </div>

    <button type="submit" class="btn btn-primary" disabled="@_isSubmitting">
        @(_isSubmitting ? "Saving..." : "Create Customer")
    </button>
</EditForm>

@code {
    private CustomerFormModel _model = new();
    private bool _isSubmitting;

    private async Task HandleSubmit()
    {
        _isSubmitting = true;
        try
        {
            await CustomerService.CreateAsync(_model);
            Navigation.NavigateTo("/customers");
        }
        finally
        {
            _isSubmitting = false;
        }
    }

    public class CustomerFormModel
    {
        [Required, StringLength(100, MinimumLength = 2)]
        public string Name { get; set; } = string.Empty;

        [Required, EmailAddress]
        public string Email { get; set; } = string.Empty;

        [Required]
        public string Tier { get; set; } = string.Empty;
    }
}

Hosting Model Comparison

Best Practices

1. Add @key directives to every element inside @foreach loops using a stable unique identifier (such as a database ID), not the loop index; without @key, Blazor's diffing algorithm reuses DOM elements incorrectly when items are inserted, removed, or reordered.

2. Mark required component parameters with [EditorRequired] so that consuming components get compile-time warnings when mandatory parameters are omitted, catching integration bugs before runtime.

3. Avoid calling StateHasChanged() inside OnInitializedAsync or OnParametersSetAsync because Blazor automatically re-renders after these lifecycle methods complete; redundant calls double the render work and cause visible flicker on Server render mode.

4. Extract @code blocks exceeding 40 lines into a partial class code-behind file (e.g., MyComponent.razor.cs) to keep markup readable, enable better IntelliSense, and allow the C# code to be unit-tested without Razor compilation.

5. Use CascadingValue with IsFixed="true" for values that never change (such as theme configuration or feature flags) so that Blazor skips change-detection on every render cycle for all descendant components that consume the value.

6. Implement IDisposable on components that register event handlers, timers, or JS interop callbacks and unsubscribe in Dispose(); Blazor Server circuits can leak memory for each connected user if handlers accumulate over the circuit lifetime.

7. Wrap IJSRuntime.InvokeAsync calls in OnAfterRenderAsync guarded by firstRender for DOM-dependent initialization (e.g., chart libraries, focus management); calling JS interop during OnInitializedAsync will throw during server-side prerendering because no DOM exists yet.

8. Scope CSS to components using MyComponent.razor.css isolation files and reference the generated {Assembly}.styles.css bundle in the layout rather than writing global CSS rules that collide across component libraries.

9. Configure Blazor Server's CircuitOptions.DetailedErrors to true only in Development and set CircuitOptions.DisconnectedCircuitRetentionPeriod to a bounded timespan (e.g., 3 minutes) in production to free server memory when users close browser tabs without disconnecting.

10. Use StreamRendering attribute on pages that perform slow data fetches so the initial HTML shell renders immediately with a loading placeholder and the content streams in as the async operation completes, improving perceived performance without requiring interactive mode.