You receive an HTTP 404 ERROR with an ASP.NET Core API or Website Print

Troubleshooting HTTP 404 Errors in ASP.NET Core

Common Causes of 404 Errors

1. Application not running or failed to start
2. Missing or incorrect route configuration
3. Controller/endpoint not registered
4. Incorrect Application Startup File in Plesk
5. Files not uploaded correctly

Step 1: Verify Application is Running

Check if your app started successfully:

1. In Plesk, go to Files → httpdocs/logs
2. Check the latest stdout log for errors
3. If no logs exist, enable stdout logging in web.config

Step 2: Verify Published Files

Ensure these files exist in httpdocs:

• YourApp.dll (main application)
• YourApp.deps.json
• YourApp.runtimeconfig.json
• web.config

Step 3: Check Route Configuration

Modern Pattern: Program.cs (.NET 6, 7, 8, 9, 10)

var builder = WebApplication.CreateBuilder(args);

// Add services
builder.Services.AddControllers();  // For API controllers
// OR
builder.Services.AddControllersWithViews();  // For MVC
// OR
builder.Services.AddRazorPages();  // For Razor Pages

var app = builder.Build();

// Configure middleware
app.UseRouting();
app.UseAuthorization();

// Map endpoints
app.MapControllers();  // For API with [Route] attributes
// OR
app.MapControllerRoute(
    name: "default",
    pattern: "{controller=Home}/{action=Index}/{id?}");  // For MVC
// OR
app.MapRazorPages();  // For Razor Pages

app.Run();

Legacy Pattern: Startup.cs (.NET Core 3.1, 5.0)

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddControllers();
    }

    public void Configure(IApplicationBuilder app)
    {
        app.UseRouting();
        app.UseEndpoints(endpoints =>
        {
            endpoints.MapControllers();
        });
    }
}

Step 4: Verify Controller Routes

For Web API controllers, ensure routes are defined:

[ApiController]
[Route("api/[controller]")]
public class ProductsController : ControllerBase
{
    [HttpGet]
    public IActionResult GetAll() => Ok(new[] { "Product1", "Product2" });

    [HttpGet("{id}")]
    public IActionResult GetById(int id) => Ok($"Product {id}");
}

This creates endpoints at /api/products and /api/products/{id}

Step 5: Check Plesk Settings

1. Go to Websites & Domains → Hosting Settings
2. Verify Application Startup File matches your DLL name exactly
3. Verify .NET Core Version matches your target framework
4. Click OK and test again

Step 6: Restart Application

1. Go to Websites & Domains
2. Find your domain and click Restart App in .NET Core section
3. Or recycle the Application Pool via Dedicated IIS Application Pool

Common Mistakes

• Missing [ApiController] attribute on API controllers
• Missing [Route] attribute when using attribute routing
• Calling app.Run() before app.MapControllers()
• Not calling AddControllers() in service configuration
• Case sensitivity in route URLs

Testing Your Routes

Use browser developer tools or Postman to test:

• Check the exact URL being requested
• Verify HTTP method (GET, POST, etc.) matches your endpoint
• Check response headers for clues