🛠️ ASP.NET Error Handling & Debugging – Catch, Trace, and Fix Web App Errors

🧲 Introduction – Why Robust Error Handling Matters in ASP.NET

In any real-world web application, errors are inevitable—but crashes don’t have to be. ASP.NET provides a comprehensive error-handling mechanism that allows developers to capture, log, and gracefully respond to unexpected failures. Coupled with modern debugging techniques, you can fix issues quickly and maintain user trust.

🎯 In this guide, you’ll learn:

• How to handle runtime errors in ASP.NET (Web Forms & Core)
• How to use try-catch, global.asax, customErrors, and middleware
• How to debug ASP.NET apps using breakpoints, trace, and logs
• How to configure user-friendly error pages

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📘 Topics Covered

🔹 Topic	📖 Description
❗ ASP.NET – Error Handling	Capture and manage application-level and page-level exceptions
🐞 ASP.NET – Debugging	Use Visual Studio, Trace, and built-in tools to troubleshoot issues

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❗ ASP.NET – Error Handling

🔹 Try-Catch Example (Code-Level)

try {
    int result = 10 / int.Parse("0");
} catch (DivideByZeroException ex) {
    lblError.Text = "Cannot divide by zero!";
}

✅ Handles specific exceptions gracefully and prevents page crashes.

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🔹 Page-Level Error Handling

Use Page_Error() in code-behind:

protected void Page_Error(object sender, EventArgs e) {
    Exception ex = Server.GetLastError();
    Server.ClearError();
    Response.Redirect("ErrorPage.aspx");
}

✅ Great for redirecting users and logging details silently.

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🔹 Application-Level Errors (Global.asax)

void Application_Error(object sender, EventArgs e) {
    Exception ex = Server.GetLastError();
    // Log error to file or DB
    Server.ClearError();
    Response.Redirect("~/Error.aspx");
}

✅ Centralized place to catch unhandled exceptions.

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🔹 Web.config Custom Errors

<customErrors mode="On" defaultRedirect="Error.aspx">
  <error statusCode="404" redirect="NotFound.aspx" />
</customErrors>

Mode	Behavior
On	Custom errors shown to users
Off	Full stack trace shown (for dev)
RemoteOnly	Shown to remote users only

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🔹 ASP.NET Core – Error Handling Middleware

app.UseExceptionHandler("/Error");

In Error.cshtml, show a friendly message:

<h2>Oops! Something went wrong.</h2>
<p>Please try again later.</p>

✅ DeveloperExceptionPage shows full trace during development:

app.UseDeveloperExceptionPage();

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🐞 ASP.NET – Debugging Techniques

🐛 Debug with Visual Studio:

• ✅ Breakpoints: Pause execution at a specific line
• 🔍 Watch/Locals: Inspect variable values
• 🔄 Step Into/Over: Traverse logic flow

🔹 Real-Time Trace Debugging:

<%@ Page Trace="true" %>

✅ Appends trace information at the bottom of the page.

You can also enable in web.config:

<trace enabled="true" pageOutput="false" requestLimit="40" />

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🧾 Log to File or DB (Manual Debugging)

File.AppendAllText(Server.MapPath("~/logs/errors.txt"), ex.ToString());

✅ Useful in production when Trace is disabled.

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🔎 Remote Debugging in Production (Safe)

• Use tools like Application Insights, ELMAH, or Sentry
• Mask sensitive data in logs
• Avoid showing raw stack traces to users

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📌 Summary – Recap & Next Steps

With ASP.NET’s error handling and debugging tools, you can build resilient, user-friendly, and developer-friendly applications. Handle exceptions at every level, log intelligently, and debug interactively for faster troubleshooting.

🔍 Key Takeaways:

• Use try-catch to handle known error conditions
• Configure customErrors for user-friendly error pages
• Use Application_Error or middleware for global exception logging
• Debug using Visual Studio breakpoints, trace output, or log files

⚙️ Real-World Applications:

• Custom 404/500 pages to retain user experience
• Audit trail of errors for post-deployment analysis
• Error recovery flows for payment gateways, forms, and API calls

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❓ Frequently Asked Questions

❓ What is the difference between customErrors and Application_Error?
✅ customErrors configures UI-level error redirection. Application_Error lets you handle and log exceptions globally in Global.asax.

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❓ Can I see detailed error messages on remote server?
✅ Set <customErrors mode="Off"/>, but only in development—never expose stack traces in production.

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❓ What’s the best way to log errors in ASP.NET?
✅ Use System.Diagnostics.Trace, NLog, Serilog, or manual file/DB logging. ASP.NET Core supports built-in logging interfaces.

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❓ How to debug a deployed ASP.NET app?
✅ Use Application Insights, log files, or attach remote debugger using Visual Studio Enterprise.

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