This is a post about code refactoring, the long journey of removing technical debts in the system I have built at work.

Since late 2015, I have been working on a system which is supposed to be extensible and scalable to support our branches in different countries. With the frequent change of user requirements and business strategies, mistakes and system-wide code refactoring are things that I can’t avoid.

Today, I’m going to list down some key lessons and techniques I learned in this one year plus of journey of clearing technical debts.

…we are not living in a frozen world of requirements and changes. So you can’t and shouldn’t avoid refactoring at all.

The risk in avoiding would definitely bring up messy codes, and tough maintenance of your software, you can leave it behind but somebody else would suffer.

— Nail Yuce, Author of the redbook “Collaborative Application Lifecycle Management”

Databases Organization

Originally, we have three branches in three countries. The branches are selling the same product, laptop. It’s business decision that these three branches should behave individually with different names so that customers generally can’t tell that these three branches are operated by one company.

Let’s say these three branches have names as such, Alpha, Beta, and Gamma. Instead of setting up different databases for the branches, I put the tables of these three branches in the same database for two reasons:

  1. Save cost;
  2. Easy to maintain because there will be only one database and one connection string.

These two points turn out to be invalid few months after I designed the system in such a way.

I’m using Azure SQL, actually separating databases won’t incur higher cost because of the introduction of Elastic Database Pool in Microsoft Azure. It’s also not easy to maintain because to put three business entities in one database, I have two ways to do it.

  1. Have a column in each table specifying the record is from/for which branch;
  2. Prefix the table names.

I chose the 2nd way. Prefix the table names. Hence, instead of a simple table name such as Suppliers, I now have three tables, APSupplier, BTSupplier, and GMSupplier, where AP, BT, and GM are the abbreviation of the branch names.

This design decision leads to the second problem that I am going to share with you next.

Problem of Prefixing Table Names

My senior once told me that experience was what made a software developer valuable. The “experience” here is not about technical experience because technology simply moves forward at a very fast pace, especially in web development.

The experience that makes a software developer valuable is more about system design and decision making in the software development process. For example, now I know prefixing table names in my case is a wrong move.

Experience helps in building a better system which is not “too simple and naive”.

There are actually a few obvious reasons of not prefixing.

For those who are using Entity Framework and love intellisense feature in Visual Studio IDE, they will know my pain. When I’m going to search for Supplier table, I have to type the two-letter branch abbreviation first then search for the Supplier table. Hence in the last one year, our team spent lots of man hours going through all these AP, BT, and GM things. Imaging the company starts to grow to 20 countries, we will then have AP, BT, GM, DT (for Delta), ES (for Epsilon), etc.

To make things worse, remember that three branches are actually just selling the laptops with the similar business models? So what would we get when we use inheritance for our models? Many meaningless empty sub-classes.

public abstract class BaseSupplier
    public int Id { get; set; }
    public string Name { get; set; }
    public string PersonInCharge { get; set; }
    public string Email { get; set; }
    public bool Status { get; set; }

public class APSupplier : BaseSupplier { }

public class BTSupplier : BaseSupplier { }

public class GMSupplier : BaseSupplier { }

So if we have branches in 20 countries (which is merely 10% of the total number of countries in the world), then our software developers’ life is going to be miserable because they need to maintain all these meaningless empty classes.

Factory Design Pattern and Template Methods

However, the design above actually at the same time also makes our system to be flexible. Now, imagine that one of the branches requests for a system change which requires addition of columns in its own Supplier table, we can simply change the corresponding sub-class without affecting the rest.

Design Patterns are good if we know how to use them properly. (Image Source: Rewrite)

This leads us to the Factory Design Pattern. Factory Design Pattern allows us to standardize the system design for each of the branch in the same system while at the same time allowing for individual branch to define their own business models.

public abstract class SupplierFactory
    public static SupplierFactory GetInstance(string portal)
        return Activator.CreateInstance(Type.GetType($"Lib.Factories.Supplier.{portal}SupplierFactory")) as SupplierFactory;

    protected abstract BaseSupplier CreateInstanceOfSupplier();

    protected abstract void InsertSupplierToDatabase(BaseSupplier newSupplier);

    public abstract IQueryable RetrieveSuppliers();

    public async Task AddNewSupplierAsync(SupplierManageViewModel manageVM)
        var newSupplier = CreateInstanceOfSupplier();
        newSupplier.Name = manageVM.Name;
        newSupplier.PersonInCharge = manageVM.PersonInCharge;
        newSupplier.Email= manageVM.Email;
        newSupplier.Status = manageVM.Status;


For each of the branch, then I define their own SupplierFactory which inherits from this abstract class SupplierFactory.

public class AlphaSupplierFactory : SupplierFactory
    private AlphaDbContext db = new AlphaDbContext();

    public override IQueryable RetrieveSuppliers()
        return db.APSuppliers;

    protected override void InsertSupplierToDatabase(BaseSupplier newSupplier)


As shown in the code above, firstly, I no longer use the abbreviation for the prefix of the class name. Yup, having abbreviation hurts.

Secondly, I have also split the big database to different smaller databases which store each branch’s info separately.

The standardization of the workflow is done using Template Method such as AddNewSupplier method shown above. The creation of new supplier will be using the same workflow for all branches.


public static SupplierFactory GetInstance(string portal)
    return Activator.CreateInstance(Type.GetType($"Lib.Factories.Supplier.{portal}SupplierFactory")) as SupplierFactory;

For those who wonder what I am doing with the Activator.CreateInstance in the GetInstance method, I use it to create an instance of the specified type that type’s default constuctor with portal acts as an indicator on which sub-class the code should pick using reflection with the Type.GetType method. So the values for portal will be Alpha, Beta, Gamma, etc. in my case.

This unfortunately adds one more burden to our development team to pay attention to the naming convention of the classes.

Fluent API: ToTable

All these unnecessary complexity is finally coming to an end after my team found out how to make use of the Fluent API. Fluent API provides several important methods to configure entities which help to override Code First conventions. ToTable method is one of them.

ToTable helps mapping entity to the actual table name. Hence, now we can fix our naming issues in our databases with the codes below in each of the branch’s database context class.

protected override void OnModelCreating(DbModelBuilder modelBuilder)

    ... // mappings for other tables

With this change, we can furthermore standardize the behavior and workflow of the system for each of our branch. However, since we only start to apply this after we have expanded our business to about 10 countries, there will be tons of changes waiting for us if we are going to apply Fluent API to refactor our codes.

Technical Debts

I made a few mistakes in the beginning of system design because of lacking of experience building extensible and scalable systems and lack of sufficient time and spec given to do proper system design.

This naming issue is just one of the debts we are going to clear in the future. Throughout the one year plus of system development, the team also started to realize many other ways to refactor our codes to make it more robust.

As a young team in a non-software-development environment, we need to be keen on self-learning but at the same time understand that we can’t know everything. We will keep on fighting to solve the crucial problems in the system and at the same time improving the system to better fit the changing business requirements.

I will discuss more about my journey of code refactoring in the future posts. Coming soon!

Reading my old codes sometimes makes me slap myself. (Image Source: Rewrite)

Burger and Cheese


As a web developer, I don’t have many chances to play with mobile app projects. So rather than limit myself to just one field, I love to explore other technologies, especially mobile app development.

Burger Project: My First Xamarin App

Last month, I attended a Xamarin talk at Microsoft Singapore office with my colleague. The talk was about authentication and authorization with social networks such as Facebook and Twitter via Azure App Service: Mobile App.

Ben Ishiyama-Levy is talking about how Xamarin and Microsoft Azure works together.
Ben Ishiyama-Levy is talking about how Xamarin and Microsoft Azure works together.

The speaker is Ben Ishiyama-Levy, a Xamarin evangelist. His talk inspired me to further explore how I could retrieve user info from social network after authenticating the users.

Because I am geek-first and I really want to find out more, so I continue to read more about this topic. With the help from my colleague, I developed a simple Xamarin.Android app to demonstrate the Authentication and logged-in user’s info retrieval.

The demo app is called Burger and it can be found on my Github repository:

Challenges in Burger Project

Retrieving user's info from social network.
Retrieving user’s info from social network.

In Burger project, the first big challenge is to understand how Azure App Service: Mobile App works in Xamarin. Luckily, with the material and tutorial given in the Xamarin talk from Ben, I was able to get a quick start on this.

My colleague also shared another tutorial which is about getting authenticated user’s personal details on Universal Windows Platform (UWP). It helps me a lot to understand about how mobile app and Azure App Service can work together.

My second challenge in this project is to understand Facebook Graph API. I still remember that I spent quite some time finding out why I could not retrieve the friend list of a logged-in Facebook user. With the introduction of the Facebook Graph API 2.0, access to a user’s friends list via /me/friends is limited to just friends using the same app. Hence after reading a few other online tutorials, I finally somehow able to get another subset of a user’s friends via /me/taggable_friends.

In this project, it’s also the first time I apply Reflection in my personal project. It helps me easily get the according social network login class with a neat and organized code.

Microsoft Developer Day at NUS, Singapore in May 2016

Cheese Project: When Google Speech Meets MS LUIS on Android

Few months ago, I’m fortunate to represent my company to attend Microsoft Developer Day 2016 in National University of Singapore (NUS).

The day is the first time Microsoft CEO Satya Nadella comes to Singapore. It’s also my first time learn about the powerful Cognitive Services and LUIS (Language Understanding Intelligence Service) in Microsoft Azure in Riza’s talk.

Riza’s presentation about Microsoft Cognitive APIs during Microsoft Developer Day.

Challenges in Cheese Project

Everyday, it takes about one hour for me to reach home from office. Hence, I will only have two to three hours every night to work on personal projects and learning. During weekends, when people are having fun out there, I will spend time on researching about some exciting new technologies.

There are many advance topics in LUIS. I still remember that when I was learning how LUIS works, my friend was actually playing the Rise of the Tomb Raider beside me. So while he was there phew-phew-phew, I was doing data training on LUIS web interface.

Microsoft LUIS (Language Understanding Intelligence Service) and Intents

Currently, I only worked on some simple intents, such as returning me current date and time as well as understanding which language I want to translate to.

My first idea in Cheese project is to build an Android app such that if I say “Please translate blah-blah to xxx language”, the app will understand and do the translation accordingly. This can be quite easily done with the help of both LUIS and Google Translate.

After showing this app to my colleagues, we realized one problem in the app. It’s too troublesome for users to keep saying “Please translate blah-blah to xxx language” every time they need to translate something. Hence, recently I have changed it to use GUI to provide language selection. This, however, reduces the role played by LUIS in this project.

VoiceText provides a range of speakers and voices with emotions!

To make the project even more fun, I implemented the VoiceText Web API from Japanese in the Android app. The cool thing about this TTS (Text-To-Speech) API is that it allows developers to specify the mood and characteristic of the voice. The challenge, of course, is to read the API written in Japanese. =P

Oh ya, this is the link to my Cheese repository on Github: I will continue to work on this project while exploring more about LUIS. Stay tuned.

languagelist    googlespeech    SuccessfullyTranslated.png

After-Work Personal Projects

There are still more things in mobile app development for me to learn. Even though most of the time I feel exhausted after long work day, working on new and exciting technologies helps me getting energized again in the evening.

I’m not as hardworking as my friends who are willing to sacrifice their sleep for their hobby projects and learning, hence the progress of my personal project development is kind of slow. Oh well, at least now I have my little app to help me talking to people when I travel to Hong Kong and Japan next year!

Protect Your ASP .NET Applications

ASP .NET MVC - Entity Framework - reCAPTCHA - OWASP - JSON

Here is a just a few items that I learnt on how to protect and secure my web applications in recent ASP .NET projects.

reCAPTCHA in Razor

CAPTCHA is an acronym for “Completely Automated Public Turing test to tell Computers and Humans Apart”.

CAPTCHA is a program to find out if the current user is whether a human or a robot by asking the user to do some challenge-response tests. This feature is important in some websites to prevent machine to, for example, auto login to the websites, to do online transactions, to register as members, and so on. Luckily, it’s now very easy to include CAPTCHA in our ASP .NET MVC web applications.

Register your website here to use reCAPTCHA:
Register your website here to use reCAPTCHA:

reCAPTCHA is a free Google CAPTCHA service that comes in the form of widget that can be added to websites easily. So, how do we implement reCAPTCHA in our ASP .NET MVC sites?

The library that I use is called ReCaptcha for MVC5, which can be downloaded from Codeplex. With the help of it, I am able to easily plugin reCAPTCHA in my MVC5 web applications.

After adding ReCaptcha.Mvc5.dll in my project, I will need to import its namespace to the Razor view of the page which needs to have reCAPTCHA widget.

@using ReCaptcha.Mvc5;

To render the reCAPTCHA widget in, for example, a form, we will do the following.

< div class="form-group">
    @Html.LabelFor(model => model.recaptcha_response_field, new { @class = "control-label col-md-2" })
    < div class="col-md-10">
        <!--Render the recaptcha-->
        @Html.reCAPTCHA("<public key here>")
    < /div>
 < /div>

The public key can be retrieved from the official reCAPTCHA page after you register your website there.

reCAPTCHA Widget on Website
reCAPTCHA Widget on Website

In the code above, there is a field called recaptcha_response_field, which will be added in our model class as demonstrated below.

public class RegistrationViewModel : ReCaptchaViewModel

    [Display(Name = "Word Verification")]
    public override string recaptcha_response_field { get; set; }

To do verification in the controller, we will have the following code to help us.

public async Task<ActionResult> Register(RegistrationViewModel registrationVM)

    if (!string.IsNullOrEmpty(checkoutVM.recaptcha_response_field))
        // Verify the recaptcha response.
        ReCaptchaResponse response = 
            await this.verifyReCAPTCHA(registrationVM, "<private key here>", true);

        if (response.Success)
            // Yay, the user is human!
            ModelState.AddModelError("", "Please enter correct verification word.");

The private key can also be found in the official reCAPTCHA page after you have submitted your website.

After doing all these, you are now be able to have a working reCAPTCHA widget in your website.

XSRF: Cross-Site Request Forgery

In the controller code above, there is one attribute called ValidateAntiForgeryToken. The purpose of this attribute is to prevent XSRF by adding anti-forgery tokens in both a hidden form field and the cookies to the server.

I draw a graph for me to better explain about what XSRF is.

XSRF (Cross-Site Request Forgery)
XSRF (Cross-Site Request Forgery)

Steps are as follows.

  1. The user logs in to, for example, a bank website.
  2. The response header from the bank site will contain the user’s authentication cookie. Since authentication cookie is a session cookie, it will only be cleared when the process ends. Thus, until that time, the browser will always include the cookie with each request to the same bank website.
  3. The attacker sends to the user a link and somehow encourage the user to click on it. This causes sending a request to the attacker’s server.
  4. The attacker website has the following form.
    <body onload="document.getElementById('attack-form').submit()">
        <form id="fm1" action="" method="post">
            <input name="transferTo" value="attackerAccount" />
            <input name="currency" value="USD" />
            <input name="money" value="7,000,000,000" />
  5. Because of Step 4, the user will be forced to send a request to the bank website to transfer money to attacker’s account with the user’s authentication cookie.

Hence, the attribute ValidateAntiForgeryToken helps to avoid XSRF by checking both the cookie and form have anti-forgery tokens and their values match.

Mass-Assignment Vulnerability and Over-Posting Attack

Few years ago, Github was found to have Mass-Assignment Vulnerability. The vulnerability allows people to perform Over-Posting Attack to the site so that the attackers can modify data items which are not normally allowed to access. Due to the fact that ASP .NET MVC web application is using Model Binding, the same vulnerability can happen in ASP .NET MVC environment as well.

You want to control what is being passed into the binder.
You want to control what is being passed into the binder.

There are two my personal favourite solutions to avoid Over-Posting Attack.

One is using Bind attribute in the controller method. For example, in order to prevent users editing the value of isAdmin when they update their profile, I can do something as follows.

public ViewResult Edit([Bind(Exclude = "IsAdmin")] User user)

Alternatively, we can also use “Include” to define those fields that should be included in the binding.

Second solution is using view model. For example, the following class will not contain properties such as IsAdmin which are not allowed to be edited in the form post of profile edit.

public class UserProfileUpdateViewModel
    public string FirstName { get; set; }
    public string LastName { get; set; }

XSS: Cross-Site Scripting

According to OWASP (Open Web Application Security Project), XSS attacks

…are a type of injection, in which malicious scripts are injected into otherwise benign and trusted web sites… Flaws that allow these attacks are quite widespread and occur anywhere a web application uses input from a user within the output it generates without validating or encoding it.

Kirill Saltanov from NUS is explaining to guests about XSS during 5th STePS event.
Kirill Saltanov from NUS is explaining to guests about XSS during 5th STePS event.

Currently, by default ASP .NET will throw exception if potentially dangerous content is detected in the request. In addition, the Razor view engine protect us against most of the XSS attacks by encoding data which is displayed to web page via the @ tag.

In View, we also need to encode any user-generated data that we are putting into our JavaScript code. Starting from ASP .NET 4.0, we can call HttpUtility.JavaScriptStringEncode. HttpUtility.JavaScriptStringEncode helps to encode a string so that it is safe to display and characters are escaped in a way that JavaScript can understand.

In order to avoid our database to have malicious markup and script, we need to encode the user inputs in the Controller as well using Server.HtmlEncode.


There are some cases where our web application should accept HTML tags. For example, we have a <textarea> element in our blogging system where user can write the content of post, then we need to skip the default checking of ASP .NET.

To post HTML back to our Model, we can simply add the [AllowHtml] attribute to the corresponding property in the Model, for example

public class BlogPost {
    public int ID { get; set; }
    public string Content { get; set; }

Then in the View, we will need to use @Html.Raw to tell Razor not to encode the HTML markup.


Wait… Won’t this make XSS attack possible in our website? Yup, of course. So, we must be very careful whenever we are trying to bypass the Razor encoding. The solution will then be using AntiXSS encoding library from Microsoft.

AntiXSS uses a safe list approach to encoding. With its help, we will then able to remove any malicious script from the user input in the Controller, as demonstrated below.

public ActionResult CreatePost(BlogPost post)
    if (ModelState.IsValid)
        post.Content = Sanitizer.GetSafeHtmlFragment(post.Content);
        return RedirectToAction("Index");
    return View(post);

ASP .NET Request Validation

Previously in the discussion of XSS, we know that by default ASP .NET throws exception if potentially dangerous content is detected in the request. This is because of the existence of ASP .NET Request Validation.

However, according to OWASP, Request Validation should not be used as our only method of XSS protection because it does not guarantee to catch every type of invalid input.

HttpOnly Cookies

In order to reduce the risk of XSS, popular modern browsers have added a new attribute to cookie called HttpOnly Cookie. This new attribute specifies that a cookie is not accessible through script. Hence, it prevents the sensitive data contained in the cookie can be sent to attacker’s side via malicious JavaScript in XSS attack.

When a cookie is labelled as HttpOnly, it tells the browser that the cookie should only be accessed by the server. It is very easy to check which cookies are HttpOnly in the developer tool of modern browsers.

Microsoft Edge F12 Developer Tools can tell which are the HttpOnly cookies.
Microsoft Edge F12 Developer Tools can tell which are the HttpOnly cookies.

So, how do we create HttpOnly cookies in ASP .NET web applications? Just add a new line to set HttpOnly attribute of the cookie to true is fine.

HttpCookie myCookie = new HttpCookie("MyHttpOnlyCookie");
myCookie["Message"] = "Hello, world!";
myCookie.Expires = DateTime.Now.AddDays(30);
myCookie.HttpOnly = true;

Alternatively, HttpOnly attribute can be set in web.config.

<httpCookies httpOnlyCookies="true" ...>

However, as pointed out in OWASP, if a browser is too old to support HttpOnly cookie, the attribute will be ignored by the browser and thus the cookies will be vulnerable to XSS attack. Also according to MSDN, HttpOnly does not prevent attacker with access to the network channel from accessing the cookie directly, so it recommends the use of SSL in addition of HttpOnly attribute.

HttpOnly Cookie was introduced in 2002 in IE6. Firefox only supported HttpOnly attribute in 2007, 5 years later. However, soon people realized that in Firefox, there was still a bug in the HttpOnly implementation. Firefox allowed attackers to do an XMLHttpRequest to get the cookie values from the HTTP Response headers. 2 years later, in 2009, Mozilla finally fixed the bug. Since then, the XMLHttpRequest can no longer access the Set-Cookie and Set-Cookie2 headers of any response no matter the HttpOnly attribute is set to true or not.

Browserscope provides a good overview about the security functionalities in major browsers.
Browserscope provides a good overview about the security functionalities in major browsers.

SQL Injection and Entity SQL

When I first learned SQL in university, I always thought escaping user inputs helped to prevent SQL Injection. This approach doesn’t work actually. I just read an article written by Steve Friedl regarding how escaping the input strings does not protect our applications from being attacked by SQL Injection. The following is the example Steve gave.

SELECT fieldlist
FROM table
WHERE id = 23 OR 1=1;  -- Boom! Always matches!

When I was working in the Summer Fellowship Programme, I started to use Parameterized SQL.

SqlConnection conn = new SqlConnection(connectionString); 
string sql = "SELECT fieldlist FROM table WHERE id = @id";
SqlCommand cmd = new SqlCommand(sql); 
cmd.Parameters.Add("@id", SqlDbType.Int, id); 
SqlDataReader reader = cmd.ExecuteReader();

This approach provides a huge security performance benefits.

In January, I started to learn Entity Framework. In Entity Framework, there are three types of queries:

  • Native SQL
  • Entity SQL
  • LINQ to Entity

In the first two types, there is a risk of allowing SQL Injection if the developers are not careful enough. Hence, it’s recommended to use parameterized queries. In addition, we can also use Query Builder Methods to safely construct Entity SQL, for example

ObjectQuery<Flight> query =
    .Where("it.FlightCode = @code",
    new ObjectParameter("code", flightCode));

However, if we choose to use LINQ to Entity, which does not compose queries by using string manipulation and concatenation, we will not have the problem of being attacked by traditional SQL Injection.

JsonResult and JSON Hijacking

Using the MVC JsonResult, we are able to make our controller in ASP .NET MVC application to return Json. However, by default, ASP .NET MVC does not allow us to response to an HTTP GET request with a JSON payload (Book: Professional ASP .NET MVC 5). Hence, if we test the controller by just typing the URL directly in the browser, we will receive the following error message.

This request has been blocked because sensitive information could be disclosed to third party web sites when this is used in a GET request. To allow GET requests, set JsonRequestBehavior to AllowGet.

Since the method only accepts POST requests, unless Cross-Origin Resource Sharing (CORS) is implemented, the browser will be able to protect our data from returning the Json result to other domains.

This is actually a feature introduced by ASP .NET MVC team in order to mitigate a security threat known as JSON Hijacking. JSON Hijacking is an attack similar to XSRF where attacker can access cross-domain JSON data which is returned as array literals.

The reason why “returning JSON data as array” is dangerous is that although browsers nowadays stop us from making cross domain HTTP request via JavaScript, we are still able to use a <script> tag to make the browser load a script from another domain.

<script src=""></script>

Due to the fact that a JSON array will be treated as a valid JavaScript script and can thus be executed. So, we need to wrap the JSON result in an object, just like what ASP .NET and WCF do. The ASP.NET AJAX library, for example, automatically wraps JSON data with { d: [] } construct to make the returned value to become an invalid JavaScript statement which cannot be executed:

{"d" : ["balance", "$7,000,000,000.00"] }

So, to avoid JSON Hijacking, we need to

  1. never return JSON array
  2. not allow HTTP GET request to get the sensitive data

Nowadays, even though JSON Hijacking is no longer a known problem in modern browsers, it is still a concern because “you shouldn’t stop plugging a security hole just because it isn’t likely to be exploited“.

By the way, GMail was successfully exploited via JSON Hijacking. =)

Summer 2015 Self-Learning Project

This article is part of my Self-Learning in this summer. To read the other topics in this project, please click here to visit the project overview page.

Summer Self-Learning Banner

App_offline.htm: The Super Weapon to Take ASP .NET Website Down


When I am doing maintenance work for my ASP .NET web application, I will always take down the web application temporarily. This is because the application domain will always restart when I deploy a new version of the application. Hence, in order to prevent online users to make request when the website is still under deployment, there is a need to take the website down and then show the online users a friendly message that my website is currently unavailable.

Two Steps to Take Website Down

Luckily, there is a very convenient way of doing that in ASP .NET 2.0 or later.

Firstly, a file with the name “App_offline.htm” needs to be created. This is the only web page that will be shown to the online visitors when the website is down. Thus, we will put our friendly messages to notify the visitors that the website is currently under maintenance in the web page.

Secondly, we will put this file in the root of the website virtual directory.

Finally, visit the website now. You will realize that no matter which web page you visit in the website, you will always be redirected to App_offline.htm, the page telling you that the website is under maintenance.

Website is under maintenance. Sorry about that!
Website is under maintenance. Sorry about that!

Things to Take Note of

Current Request Still Be Processed

According to an interesting experiment shared on Stack Overflow, only new request will be redirected to the App_Offline.htm. Existing requests when the App_Offline.htm is uploaded will still be processed.

Minimum of 512 Bytes

It turns out that Internet Explorer will show its own generic status code message if the App_Offline.htm contains less than 512 bytes.

Permission of App_offline.htm

Yes, check the permission of the file under Properties -> Security because if the App_offline.html doesn’t have correct permissions given, it will not work as expected as well.


As demonstrated above, it is indeed very easy to take an ASP .NET website down. What a convenient App_offline.htm!