Top 10 ASP.NET Core Errors and How to Fix Them

Niotechone Marketing Team
April 25, 2026
10:30 am

Introduction

ASP.NET Core is a modern, cross‑platform framework for building fast and scalable web applications. It’s widely adopted by leading .NET development companies and trusted by businesses worldwide for enterprise‑grade solutions.

Whether you’re working with an ASP.NET Development company or planning to hire .NET developers must understand how to correct common errors. We will discuss the 10 most common ASP.NET Core errors in this blog, describe them in a nutshell, and demonstrate to you easy solutions to ensure your applications are reliable.

Top 10 ASP.NET Core Errors and Their Fixes

1. HTTP Error 500 – Internal Server Error

This is an error that indicates that the server has crashed during your request. It does not provide details, making debugging difficult. Typically, it is due to unmanaged exceptions or improperly configured middleware.

Causes:

Unhandled exceptions in controllers or services
Misconfigured middleware
Database connection issues

Fix:

🔷

app.UseDeveloperExceptionPage();

Check logs using ILogger or Application Insights
Wrap risky code in try-catch blocks

2. 404 Not Found

This is a mistake that occurs when the page or API endpoint requested is not found. It can be frequently due to improper routing or absent actions in your application.

Causes:

Wrong routing configuration
Missing controller/action
Incorrect URL

Fix:

🔷

app.MapControllerRoute(
    name: "default",
    pattern: "{controller=Home}/{action=Index}/{id?}");

Make sure that the controller and action are present
Double-check the URL spelling

3. CSRF Token Validation Failed

This happens when form submissions don’t include the required anti-forgery token. ASP.NET Core uses tokens to protect against cross-site request forgery.

Causes:

Missing @Html.AntiForgeryToken() in Razor views
Token mismatch due to caching

Fix:

🌐


    @Html.AntiForgeryToken()
    <button type="submit">Submit</button>

4. Database Connection Errors

Your application does not connect to the database. This normally occurs because of wrong connection strings or server problems. Many ASP.NET development services are based on the correct database configuration to provide a smooth performance.

Causes:

Wrong connection string in appsettings.json
Database not running
Firewall blocking connection

Fix:

📋

"ConnectionStrings": {
  "DefaultConnection": "Server=.;Database=MyDb;Trusted_Connection=True;"
}

Test the connection in SQL Server Management Studio
Apply migrations:

💻

dotnet ef database update

5. NullReferenceException

This occurs when you try to use an object that hasn’t been initialized. It’s one of the most common runtime errors in ASP.NET Core.

Causes:

Forgetting to inject dependencies
Missing await in asynchronous calls
Accessing null values

Fix:

🔷

public class HomeController {
    private readonly IMyService _service;
    public HomeController(IMyService service) {
        _service = service ?? throw new ArgumentNullException(nameof(service));
    }
}

Use null checks (?.) to avoid crashes

6. CORS Policy Error

This mistake prevents frontend applications from accessing your API because of cross-origin limitations. If you hire .NET developers to create a React frontend and a backend with ASP.NET Core, this is one of the initial challenges that they’ll solve.

Causes:

CORS is not activated in the API
Wrong domain configuration

Fix:

🔷

builder.Services.AddCors(options =>
{
    options.AddPolicy("AllowAll",
        policy => policy.AllowAnyOrigin().AllowAnyMethod().AllowAnyHeader());
});
app.UseCors("AllowAll");

7. Invalid Model State

This happens when the submitted data doesn’t match the validation rules. ASP.NET Core validates models automatically during binding.

Causes:

Missing required fields
Wrong data types
Validation attributes not matching input

Fix:

🔷

public class User {
    [Required]
    public string Name { get; set; }
}

Check ModelState.IsValid in controllers.

8. Static Files Not Loading

CSS, JS, or images are not loaded as the middleware for static files is not configured.

Causes:

Middlewares are not turned on to serve static files
Wrong file path

Fix:

🔷

app.UseStaticFiles();

Place files in wwwroot
Use correct paths in HTML

9. Session Not Working

Session data is not carried over between requests. This happens when session middleware isn’t properly configured.

Causes:

Session middleware not configured
Cookies disabled

Fix:

🔷

builder.Services.AddSession();
app.UseSession();

Store session data:

🔷

HttpContext.Session.SetString("UserName", "John");

10. SSL/TLS Issues

HTTPS errors occur when certificates are invalid or not configured. This is common during deployment.

Causes:

Invalid SSL certificate
HTTPS not enforced

Fix:

🔷

app.UseHttpsRedirection();

Install a valid SSL certificate.
For development:

💻

dotnet dev-certs https --trust

Real-World Example: Building a FinTech Application

Let’s say you’re creating a FinTech web app using ASP.NET Core for the backend and React for the frontend. The app allows users to:

Log in securely
View their account balance
Track transactions in real time

Now imagine you hit a CORS error when the React frontend tries to call the ASP.NET Core API. Here’s how it plays out:

Problem:

The frontend (http://localhost:3000) is blocked from accessing the backend API (https://localhost:5001)
Users see blank screens or error messages instead of their financial data

Impact:

Customers cannot view balances or transactions
Trust is damaged because financial apps must be reliable and secure

Solution:

Configure CORS in ASP.NET Core to allow requests from your frontend domain

Example fix in Program.cs:

🔷

builder.Services.AddCors(options =>
{
    options.AddPolicy("AllowFrontend",
        policy => policy.WithOrigins("http://localhost:3000")
                        .AllowAnyMethod()
                        .AllowAnyHeader());
});
app.UseCors("AllowFrontend");

Result:

The frontend can now fetch account details smoothly
Users see updated balances, transaction charts, and dashboards without interruptions

This is exactly the type of issue a .NET Core development company solves daily. By fixing these, an .NET application development company ensures smooth financial transactions and user trust.

Conclusion

ASP.NET Core is powerful, but even small errors can cause big issues. Whether you’re working with an ASP.NET development company or planning to hire .NET developers, those who understand how to correct these issues will save time and enhance reliability.

Collaborating with a reputable .NET development services provider makes sure that your applications are scalable, secure, and user-friendly.