Google Login ext:asp Explained: How ASP.NET Handles OAuth Sign-In

Step 4: Update Program.cs

In the latest .NET 8 project structure, all configuration happens in Program.cs.

Here’s a working example:

using Microsoft.AspNetCore.Authentication.Cookies;
using Microsoft.AspNetCore.Authentication.Google;

var builder = WebApplication.CreateBuilder(args);

// Add authentication services
builder.Services.AddAuthentication(options =>
{
options.DefaultScheme = CookieAuthenticationDefaults.AuthenticationScheme;
options.DefaultChallengeScheme = GoogleDefaults.AuthenticationScheme;
})
.AddCookie()
.AddGoogle(options =>
{
options.ClientId = builder.Configuration[“Authentication:Google:ClientId”];
options.ClientSecret = builder.Configuration[“Authentication:Google:ClientSecret”];
options.CallbackPath = “/signin-google”; // default redirect URI
});

builder.Services.AddControllersWithViews();

var app = builder.Build();

// Middleware pipeline
app.UseHttpsRedirection();
app.UseStaticFiles();
app.UseRouting();

app.UseAuthentication();
app.UseAuthorization();

app.MapDefaultControllerRoute();
app.Run();

Step 5: Add Login and Logout in Controller

Create a controller named AccountController.cs:

using Microsoft.AspNetCore.Authentication;
using Microsoft.AspNetCore.Authentication.Cookies;
using Microsoft.AspNetCore.Mvc;

public class AccountController : Controller
{
[HttpGet(“login”)]
public IActionResult Login()
{
return Challenge(new AuthenticationProperties
{
RedirectUri = “/home/index”
}, “Google”);
}

[HttpGet(“logout”)]
public async Task<IActionResult> Logout()
{
await HttpContext.SignOutAsync(CookieAuthenticationDefaults.AuthenticationScheme);
return RedirectToAction(“Index”, “Home”);
}
}

Step 6: Add Sign-In Button in Your View

In Views/Shared/_Layout.cshtml or any page:

@if (User.Identity.IsAuthenticated)
{
<p>Hello, @User.Identity.Name</p>
<a href=”/logout”>Logout</a>
}
else
{
<a href=”/login” class=”btn btn-primary”>Sign in with Google</a>
}

6. How ASP.NET Core Handles OAuth Internally

ASP.NET Core abstracts OAuth through middleware. When a user initiates Google Login:

1. The Challenge() method triggers Google’s authentication handler.

2. A redirect to Google’s OAuth consent screen occurs.

3. After approval, Google sends an authorization code to your callback URI.

4. ASP.NET exchanges the code for an access token and ID token.

5. The middleware validates the token, creates a ClaimsPrincipal, and signs the user in using cookie authentication.

This seamless handling is why ASP.NET Core remains a top choice for OAuth-based systems.

7. Testing Google Login

When testing locally:

• Use https://localhost:5001 as your base URL.

• Ensure the same redirect URI is added in Google Cloud Console.

• If using Kestrel, update launchSettings.json accordingly.

To test in production:

• Deploy your app to a secure (HTTPS) domain.

• Update the redirect URIs in Google Cloud Console to match your live URL.

9. Best Security Practices

1. Always use HTTPS for OAuth redirects.

2. Never hardcode credentials in the source code.

3. Store secrets securely (e.g., Azure Key Vault or environment variables).

4. Implement Logout to clear cookies after sign-out.

5. Use strong session expiration to prevent unauthorized access.

10. Advanced Customizations

• Custom Login UI
  Replace the default link with a branded “Sign in with Google” button and load Google’s official SVG icon.

• Multiple OAuth Providers
  Add Facebook, GitHub, or Microsoft authentication by chaining additional .AddFacebook(), .AddMicrosoftAccount() methods in Program.cs.

• Combine with ASP.NET Core Identity
  For deeper user management (roles, claims, profile data), integrate Google Login with ASP.NET Core Identity.

services.AddDefaultIdentity<IdentityUser>()
.AddEntityFrameworkStores<ApplicationDbContext>();

Then add .AddGoogle() to the authentication configuration.

11. Real-World Use Cases

• Corporate Dashboards:
  Allow employees to log in with their Google Workspace accounts.

• SaaS Platforms:
  Simplify user onboarding by offering Google sign-in instead of custom registration.

• Educational Portals:
  Let students access learning dashboards using their Google accounts securely.

Each of these benefits from the simplicity, trust, and scalability of Google OAuth 2.0.