Unexpected issues that impact functionality, performance, or user experience might occasionally arise when a ASP.NET MVC website is run in production. Any ASP.NET MVC developer must be able to diagnose and fix mistakes efficiently, regardless of whether they are runtime exceptions, deployment failures, or configuration issues. This article covers the most typical causes, tools, and recommended practices for troubleshooting issues in ASP.NET MVC applications.

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

• customErrors = Off shows full error details.
• debug = true provides a stack trace.
• 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.

Open Event Viewer

• Navigate to Windows Logs > Application
• 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:

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:

• Missing controllers/actions
• Incorrect parameter names
• Route conflicts

Step 6: Troubleshoot Dependency and DLL Issues
Example runtime error:
Could not load file or assembly 'Newtonsoft.Json'

Solutions:

• Check bin folder for missing DLLs
• Install via NuGet:
• Install-Package Newtonsoft.Json
• Ensure Copy Local = true for references

Step 7: Log Exceptions Using Logging Tools
Instead of relying on console logs, use structured logging:

• NLog
• Serilog
• Elmah (popular for MVC)

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:

• Network call failures (500/404 status)
• JavaScript runtime errors
• Invalid AJAX responses
• Missing CSS/JS files

Step 9: Use Remote Debugging (Advanced)
Attach Visual Studio debugger to the live IIS process:

• Install Remote Debugging Tools on the server
• From Visual Studio → Debug → Attach to Process

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

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.