Common Errors and Its Solution

.NET Core applications, like any other development environment, are prone to common errors. Below are some frequent issues and their solutions:

1. NullReferenceException

Error: Attempting to access an object that is null.

• Cause: Accessing a member on an object that hasn’t been initialized.
• Solution:
  • Always check for null using conditional operators or if statements before accessing object properties.
  • Use nullable reference types or ?. (null-conditional operator) to avoid exceptions.
  • Initialize objects properly in constructors or use dependency injection.

2. Unhandled Exception: System.IO.FileNotFoundException

Error: The application fails to find a required file.

• Cause: Incorrect file path or missing file in deployment.
• Solution:
  • Ensure the file exists at the specified path.
  • Use relative paths or configuration files to manage file paths.
  • If the file is part of the project, ensure it is marked as "Copy to Output Directory" in the file properties.

3. InvalidOperationException: Unable to resolve service

Error: Dependency Injection (DI) fails to resolve a service.

• Cause: The service is not registered or improperly registered in the Startup.cs file.
• Solution:
  • Ensure the service is registered in Startup.cs under the correct lifetime (AddScoped, AddSingleton, or AddTransient).
  • Double-check the constructor parameters for the class that requires the service.

4. 500 Internal Server Error

Error: The server responds with a general error.

• Cause: This can occur due to various issues such as misconfigurations, missing middleware, or uncaught exceptions.
• Solution:
  • Check the application logs to identify the root cause.
  • Use proper error handling and exception logging mechanisms (e.g., try-catch blocks, logging libraries like Serilog).
  • Implement a custom error page or middleware to provide more information on the error.

5. CORS (Cross-Origin Resource Sharing) Issues

Error: Errors related to cross-origin requests, typically when calling APIs from a different domain.

• Cause: CORS not configured properly in the API.
• Solution:
  • In Startup.cs, enable CORS in the ConfigureServices and Configure methods:

    services.AddCors(options => {
        options.AddPolicy("AllowAll", builder => builder.AllowAnyOrigin().AllowAnyMethod().AllowAnyHeader());
    });
     
    app.UseCors("AllowAll");
  • Be specific in your CORS policy to avoid security vulnerabilities.

6. Entity Framework Core Errors

a. Migration Errors

• Error: Issues when applying migrations, such as "No migrations were found" or "Multiple constructors accepting all given argument types were found."
• Cause: Either migrations haven’t been added or there are constructor issues in DbContext.
• Solution:
  • Use the Add-Migration and Update-Database commands to manage migrations.
  • Ensure that DbContext has a constructor accepting DbContextOptions<T>.

b. Lazy Loading Errors

• Error: Lazy loading throws exceptions if the related entity is not loaded.
• Cause: Lazy loading is not configured properly.
• Solution:
  • Enable lazy loading by installing the Microsoft.EntityFrameworkCore.Proxies package and configuring it in Startup.cs:

    services.AddDbContext<AppDbContext>(options =>
        options.UseLazyLoadingProxies().UseSqlServer(connectionString));

7. HTTP 404 Not Found

Error: Resources such as pages or APIs cannot be found.

• Cause: Incorrect routing, missing route parameters, or controller/action not registered correctly.
• Solution:
  • Ensure that correct routes are defined in controllers (use [Route] attributes).
  • Check for typos in route URLs.
  • For Razor pages, verify the @page directive in .cshtml files.

8. OutOfMemoryException

Error: The application runs out of memory.

• Cause: Memory leaks, large object allocations, or long-lived objects in memory.
• Solution:
  • Use memory profilers (e.g., JetBrains dotMemory, Visual Studio Diagnostic Tools) to identify memory leaks.
  • Dispose of unmanaged resources properly by implementing IDisposable.
  • Avoid holding references to large objects for long periods, and prefer streaming large files rather than loading them fully into memory.

9. Configuration Issues

Error: Configuration settings are not being picked up, or the application crashes due to missing configuration keys.

• Cause: Incorrect or missing configuration files (appsettings.json, environment variables).
• Solution:
  • Check the configuration files for missing or misspelled keys.
  • Use the IConfiguration interface in your services to ensure settings are correctly injected.
  • Set default values in case a configuration value is missing.

10. ASP.NET Core Middleware Order

Error: Middleware components don’t behave as expected.

• Cause: Incorrect middleware order in Startup.cs.
• Solution:
  • Ensure that middleware components like UseRouting, UseAuthentication, UseAuthorization, UseCors, etc., are placed in the correct order. For example:

    app.UseRouting();
    app.UseAuthentication();
    app.UseAuthorization();
    app.UseEndpoints(endpoints => {
        endpoints.MapControllers();
    });

By addressing these common errors with appropriate solutions, you can ensure a more stable and performant .NET Core application. These solutions are a starting point; debugging logs, profilers, and documentation should always be referenced when specific issues arise.