Running a .NET MVC website in production can sometimes result in unexpected errors that affect performance, functionality, or user experience. Whether you encounter runtime exceptions, deployment failures, or configuration glitches, diagnosing and resolving errors effectively is a must-have skill for any ASP.NET MVC developer.

This article provides a step-by-step guide to troubleshoot errors in a .NET MVC application, covering the most common causes, tools, and best practices.

✅ Step 1: Enable Detailed Error Logs During Development

In development mode, allow detailed error messages to identify the issue quickly.

web.config

<system.web>
  <customErrors mode="Off"/>
  <compilation debug="true"/>
</system.web>

⚠️ Do NOT enable these in production.

✅ Step 2: Check the Event Viewer Logs

If your MVC site is hosted on IIS, Windows Event Viewer is a great place to find exception logs.

  1. Open Event Viewer

  2. Navigate to Windows Logs > Application

  3. Look for .NET Runtime or Application Error

This helps identify runtime crashes and application pool failures.

✅ Step 3: Use Try-Catch and Global Error Handling

Catch unhandled errors globally using Application_Error in Global.asax or a custom filter.

Global.asax

protected void Application_Error()
{
    var exception = Server.GetLastError();
    // Log the exception
    System.Diagnostics.Trace.TraceError(exception.ToString());
}

Custom HandleErrorAttribute

public class CustomErrorHandler : HandleErrorAttribute
{
    public override void OnException(ExceptionContext filterContext)
    {
        // Log exception here
        base.OnException(filterContext);
    }
}

Register in FilterConfig.cs:

filters.Add(new CustomErrorHandler());

✅ Step 4: Check IIS Configuration and Permissions

Common IIS issues:

IssueSolution
HTTP 500 Internal Server ErrorCheck .NET version and pipeline mode
HTTP 404 Not FoundEnable MVC routing & extensionless URLs
Access Denied ErrorsGrant folder permissions to IIS_IUSRS
Application Pool Stopped AutomaticallyEnable "Always Running" and check logs

✅ Step 5: Debug Route Errors

Wrong URL routing can lead to 404 errors.

Use this debugging helper:

routes.IgnoreRoute("{resource}.axd/{*pathInfo}");
routes.MapRoute(
    name: "Default",
    url: "{controller}/{action}/{id}",
    defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
);

Also check:

✅ Step 6: Troubleshoot Dependency and DLL Issues

Example runtime error:

Could not load file or assembly 'Newtonsoft.Json'

✅ Solutions:

✅ Step 7: Log Exceptions Using Logging Tools

Instead of relying on console logs, use structured logging:

Example with NLog:

private static readonly Logger logger = LogManager.GetCurrentClassLogger();

try
{
    // some logic
}
catch(Exception ex)
{
    logger.Error(ex);
}

✅ Step 8: Debug Using Browser Developer Tools

Use F12 Developer Tools for:

✅ Step 9: Use Remote Debugging (Advanced)

Attach Visual Studio debugger to the live IIS process:

  1. Install Remote Debugging Tools on the server

  2. From Visual Studio → Debug → Attach to Process

  3. Select w3wp.exe

Great for live issue reproduction.

✅ Step 10: Enable Friendly Error Pages in Production

Use custom error pages to show user-friendly messages.

<customErrors mode="On" defaultRedirect="~/Error">
  <error statusCode="404" redirect="~/Error/NotFound"/>
  <error statusCode="500" redirect="~/Error/ServerError"/>
</customErrors>

🔧 Common MVC Error Checklist

Error TypeQuick Fix
Configuration ErrorCheck web.config
Dependency DLL MissingReinstall NuGet package
Database Connection FailedTest connection string
Unauthorized Error (401/403)Check IIS authentication
NullReferenceExceptionValidate objects
View Not FoundVerify view name/path
Route Not MatchingFix RouteConfig

✅ Conclusion

Troubleshooting a .NET MVC application becomes much easier when you follow a systematic approach. Always:

✔ Enable logging
✔ Capture error details
✔ Check IIS & permissions
✔ Validate configuration
✔ Test routing and dependencies

With the right techniques, you’ll diagnose and resolve most issues quickly and professionally.

✅ How to pick the right tool

Here are some criteria and matching considerations:

🛠 Practical tips for using these tools in your troubleshooting workflow