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HTTP 500 – Internal server error

Problem Statement:
This is another common error that troubles a lot of ASP.NET users. Generally, the questions are framed like:

I receive a HTTP 500 – Internal Server Error  exception while trying to browse my hosted web application. Exception message says “This error (HTTP 500 Internal Server Error) means that the website you are visiting had a server problem which prevented the webpage from displaying”. Please help, how to resolve it?

Assessment:
Any HTTP request made to web application running on IIS returns the status of the response. This HTTP status code indicates about the request success or failure. If the request was unsuccessful, it might provide the reason why so.

Out of various status code, 5xx are the codes related to Server error. They indicate that the server failed to complete the request because server encountered an error.

The 500 Internal Server Error is a very general HTTP status code. It means something has gone wrong on the website and webserver is unable to specify what exactly, thus failing in fulfilling the request made by the client. This is not related to client and the fault is in the webpage/website requested that resides on server. This status code can be considered as a ‘catch-all’ server error of Web server.

More details around various HTTP status code: The HTTP status code in IIS 7.0, IIS 7.5, and IIS 8.0

Possible Resolutions:
Make sure that internally webserver maintains some kind of internal error logs that gives more detail of what went wrong and thus help in diagnosing the issue. Generally, it is logged into Windows Event Logs on the server. Thus, first thing while troubleshooting the error is to see Windows Event Logs on the server to find what went wrong.

Other useful thing to troubleshoot it would be to disable friendly HTTP error messages to see if the raw content can provide a direction to look more. Steps:

  • Go to menu Tools/Internet Options in your IE.
  • Click on the Advanced tab & then uncheck “Show friendly HTTP error messages” option & then click Ok.
  • Now, when on accessing the same web page, much more developer meaningful error message will be shown.

Moving on, following are most common:
Option #1: HRESULT: 0x80070035 – The page cannot be displayed because an internal server error has occurred.
This occurs because the server that is running IIS cannot access the configured root directory of the requested location.

Resolution would be to make sure that the server that is running IIS can access the configured root directory of the requested location.

Option #2: HRESULT: 0x800700c1 – The page cannot be displayed because an internal server error has occurred.
This occurs because a script mapping is not valid.

Resolution would be to make sure that the script mapping points to the ISAPI.dll file that can process the request.
To do this, follow these steps:

  1. Click Start, click Run, type inetmgr.exe, and then click OK.
  2. In IIS Manager, expand server name, expand Web sites, and then click the Web site that you want to modify.
  3. In Features view, double-click Handler Mappings.
  4. Make sure that the script mapping points to the correct ISAPI.dll file. (e.g: .asp files should map to the %windir%\system32\inetsrv\asp.dll file)

Option #3: HRESULT: 0x8007007f – There is a problem with the resource you are looking for, so it cannot be displayed.
This occurs because the handler mapping for the requested resource points to a .dll file that cannot process the request.

Resolution would be to edit the handler mapping for the requested resource to point to the .dll file that can process the request.
To do this, follow these steps:

  1. Click Start, click Run, type inetmgr.exe, and then click OK.
  2. In IIS Manager, expand server name, expand Web sites, and then click the Web site that you want to modify.
  3. In Features view, double-click Handler Mappings.
  4. Right-click the script mapping that you want to edit, and then click Edit.
  5. In the Edit Script Map dialog box, type the appropriate executable file in the Executable box, and then click OK.

Option #4:
One of the other possibilities could be an issue in the way web application is hosted. Some security configuration issue or conflict due to multiple config files.

Resolution would be to make sure application is hosted correctly by published the application as website and setting up the virtual directory as needed.

More details around the known issues and their resolution:
Error message when you visit a Web site that is hosted on IIS 7.0: “HTTP Error 500.0 – Internal Server Error”
Troubleshoot an “HTTP 500 – Internal Server Error” error message on IIS 4.0 or on IIS 5.0

Conclusion:
This is a server error and can only be solved by website admin who has access to files and the web-server. There can be one of/or multiple reasons to get this error. One has to track down the issue and handle accordingly.

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Posted by on February 12, 2013 in ASP.NET, IIS

 

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Extract node from XML having defined namespace using XPath in C#

Problem Statement:
At work, while working on one of the features, we had to get a specific node from an XML. Obvious choice was to use XPath. Ignoring specifics of XML, we directly put the XPath as expected and found that no node was found when we executed the path.

I had to look up on how to extract the deep embedded node we needed using XPath in such case.

Assessment:
We observed that the elements/nodes are bound to a namespace. When we tried the direct XPath, it was attempting to address elements that are bound to the default “no namespace” namespace, and thus nothing was retrieved.

Sample XML to work with:


<?xml version="1.0"?>
<SampleXmlRoot xmlns="http://www.samplesite.com/example/SampleXmlRoot">
<SampleXmlSubRoot someValue="21" xmlns="http://www.samplesite.com/example/SampleXmlSubRoot">
<SampleXmlLevel2Node id="101" xmlns="http://www.samplesite.com/example/SampleXmlLevel2Node">
<SampleXmlNode2Extract num="2">
<A1 type="money">
<value>1234.0</value>
</A1>
<A2 type="money">
<value>123.4</value>
</A2>
<A3 type="money">
<value>12.3</value>
</A3>
</SampleXmlNode2Extract>
<SampleXmlNodeOther num="2">
<B1 type="money">
<value>234.0</value>
</B1>
<B2 type="money">
<value>23.4</value>
</B2>
</SampleXmlNodeOther>
</SampleXmlLevel2Node>
</SampleXmlSubRoot>
</SampleXmlRoot>

Based on what we observed and quick reference we learned that we need to register the namespace for namespace prefix mapping that will be used in defining the XPath.

Using XmlNamespaceManager object, we can provide a collection of namespace definitions that will be  used by CLR to resolve the elements used in the XML documents.

Resolution:
Step 1: Define XmlNamespaceManager

// xmlDoc is XmlDocument in which full XML is loaded
XmlNamespaceManager xmlnsManager = new XmlNamespaceManager(xmlDoc.NameTable);

// Add the namespaces used in XML to the XmlNamespaceManager
xmlnsManager.AddNamespace("sxr", "http://www.samplesite.com/example/SampleXmlRoot");
xmlnsManager.AddNamespace("sxsr", "http://www.samplesite.com/example/SampleXmlSubRoot");
xmlnsManager.AddNamespace("sxl2", "http://www.samplesite.com/example/SampleXmlLevel2Node");

Step 2: Use select method that is overloaded to use XmlNamespaceManager along with XPath

In our case, we will select the first XmlNode that matches the XPath expression where any prefixes found in the XPath expression will be resolved using the supplied XmlNamespaceManager.

String xPath = "/sxr:SampleXmlRoot/sxsr:SampleXmlSubRoot/sxl2:SampleXmlLevel2Node/SampleXmlNode2Extract";
XmlNode extractedNode = xDoc.SelectSingleNode(xPath, xmlnsManager);

XmlNode extracted OuterXml will look something like:

<SampleXmlNode2Extract num="2">
<A1 type="money">
<value>1234.0</value>
</A1>
<A2 type="money">
<value>123.4</value>
</A2>
<A3 type="money">
<value>12.3</value>
</A3>
</SampleXmlNode2Extract>

Yep, that’s the exact one we needed!

Refer:
MSDN: XmlNamespaceManager Class
MSDN: XmlNode.SelectSingleNode Method (String, XmlNamespaceManager)

Conclusion:
XPath is a wonderful way to extract node(s) and we have defined methods in place to get them. We just need to know and learn about them. XmlNamespaceManager handles the namespace and helps in defining the XPath for a given XML.

 
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Posted by on February 2, 2013 in C#

 

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Browser Back button issue after logout

Problem description:
I have found a lot of people asking for a resolution to handle the browser’s back button once user has logged out. Typically, users reports something like:

I am facing issue in my application’s logout scenario. After the user login into website, he/she uses it(website) and when done, they do a logout, which leads them back to login page. But the problem is now, from this login page, if I click the browser back button then it again takes the user back to the previous visited page as if logged in. How can I stop user from viewing the previous page once logged out?

Assessment:
The browser Back button is an option to go back to previously visited pages. The back button can be considered as a pointer that is linked to the page previously visited by the user. Browser keeps a stack of the web pages visited as a doubly-linked list.

The back button works by stepping through the history of HTTP requests which is maintained by the browser itself. This history is stored in browsers cache that consists of the entire page content with resources like image and scripts. This enables browser to navigate backwards and forwards through the browser history and have each page displayed instantly from cache without the delay of having it retransmitted over the internet from the server.

Just to handle the scenario of getting page content from server, browsers have a Refresh button transmits the request to web server and get back the fresh copy of entire page. Internally, this also replaces the copy of the page in the browser’s cache.

Thus, basic reason behind the problem in discussion is Browser’s Cache!

Possible Resolutions:
On Logout, one does clear the session to make sure user related data no more exists. Now, browsers cache needs to be handled such that browser has no history (this will make back/forward button in browser grayed out disabled.) Here are various ways of how one can do it:

Option #1:  Set Response Cache settings in code-behind file for a page

// Code disables caching by browser.
Response.Cache.SetCacheability(HttpCacheability.NoCache);
Response.Cache.SetExpires(DateTime.UtcNow.AddHours(-1));
Response.Cache.SetNoStore();

Option #2: Set META tag for HTTP cache settings in your ASPX page header

<META Http-Equiv="Cache-Control" Content="no-cache"/>
<META Http-Equiv="Pragma" Content="no-cache"/>
<META Http-Equiv="Expires" Content="0"/>

Option #3: Clear browser’s history through JavaScript using script tag

//clears browser history and redirects url
<SCRIPT LANGUAGE="javascript">
function ClearHistory()
{
     var backlen = history.length;
     history.go(-backlen);
     window.location.href = loggedOutPageUrl
}
</SCRIPT>

Option #4: Clear browser’s history through JavaScript injecting through code-behind file via Response

protected void LogOut()
{
     Session.Abandon();
     string loggedOutPageUrl = "Logout.aspx";
     Response.Write("&lt;script language='javascript'&gt;");
     Response.Write("function ClearHistory()");
     Response.Write("{");
     Response.Write(" var backlen=history.length;");
     Response.Write(" history.go(-backlen);");
     Response.Write(" window.location.href='" + loggedOutPageUrl + "'; ");
     Response.Write("}");
     Response.Write("&lt;/script&gt;");
}

Option #5: Clear browser’s history through JavaScript injecting through code-behind file via Page.ClientScript

Page.ClientScript.RegisterStartupScript(this.GetType(),"clearHistory","ClearHistory();",true);

Conclusion:
Any one or combination of above can be used to handle the scenario.

Though, I would like to call out that it’s not a good thing to mess with browser’s history. One should implement it, only if they really need it and are prepared to accept that it is not a good practice. Lastly, few would not work if one disables JavaScript.

 
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Posted by on January 31, 2013 in ASP.NET, JavaScript

 

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Unable to evaluate expression because the code is optimized or a native frame is on top of the call stack

Problem description:
This is one of the common runtime error reported by a lot of ASP.NET users. A successfully compiled code at runtime throws the following error during an operation:

“Unable to evaluate expression because the code is optimized or a native frame is on top of the call stack.”

Typically, this happens when the Response.EndResponse.Redirect, or Server.Transfer method is used.

Assessment:
This happens because current page execution is stopped and execution is sent to the Application_EndRequest event in the application’s event pipeline.

The duty of Response.End method is to stop the page execution and raise the EndRequest event. Thus, the line of code that follows Response.End is not executed. Error occurs with Response.Redirect and Server.Transfer methods too because both the methods call Response.End internally.

When a call is made to the Abort method to destroy a thread, the CLR throws a ThreadAbortException. It is a special exception that can be caught, but will automatically be raised again at the end of the catch block. When this exception is re-raised, the CLR executes all the finally blocks before ending the thread.

Using Visual Studio debugger, we can see the internal message for the exception as “Thread was being aborted.”

Resolution:

  • Use a try-catch statement to catch the exception if needed
    try {
    }
    catch (System.Threading.ThreadAbortException ex){
    }
  • For Response.End : Invoke  HttpContext.Current.ApplicationInstance.CompleteRequest method instead of Response.End to bypass the code execution to the Application_EndRequest event
  • For Response.Redirect : Use an overload, Response.Redirect(String url, bool endResponse) that passes false for endResponse parameter to suppress the internal call to Response.End
    // code that follows Response.Redirect is executed
    Response.Redirect ("mynextpage.aspx", false);
  • For Server.Transfer : Use Server.Execute method to bypass abort. When Server.Execute is used, execution of code happens on the new page, post which the control returns to the initial page, just after where it was called.

Refer:
Microsoft Support Article – 312629
MSDN: HttpResponse.Redirect Method
MSDN: HttpResponse.End Method
MSDN: ThreadAbortException Class
MSDN: HttpServerUtility.Execute Method

Conclusion:
This is as designed. This exception has bad effect on Web application performance, which is why handling the scenario correctly is important.

 
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Posted by on January 27, 2013 in ASP.NET

 

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“Cannot evaluate expression because the code of the current method is optimized”

Problem description:
This is one the common error faced by a lot of Visual Studio users. Typically, they get the below error message during debugging:

“Cannot evaluate expression because the code of the current method is optimized.”

Assessment:
In .NET, “Function Evaluation (funceval)” is the ability of CLR to inject some arbitrary call while the debuggee is stopped somewhere. Funceval takes charge of the debugger’s chosen thread to execute requested method. Once funceval finishes, it fires a debug event. Technically, CLR have defined ways for debugger to issue a funceval.

CLR allows to initiate funceval only on those threads that are at GC safe point (i.e. when the  thread will not block GC) and Funceval Safe (FESafe) point (i.e. where CLR can actually do the hijack for the funceval.) together. Thus, possible scenarios for CLR, a thread must be:

1. stopped in managed code  (and at a GC safe point): This implies that we cannot do a funceval in native code. Since, native code is outside the CLR’s control, it is unable to setup the funceval.

2. stopped at a 1st chance or unhandled managed exception (and at a GC safe point): i.e at time of exception, to inspect as much as possible to determine why that exception occurred. (e.g: debugger may try to evaluate and see the Message property on raised exception.)

Overall, common ways to stop in managed code include stopping at a breakpoint, step, Debugger.Break call, intercepting an exception, or at a thread start. This helps in evaluating the method and expressions.

Refer: MSDN Blog: Rules of Funceval

Possible resolutions:
Based on the assessment, if thread is not at a FESafe and GCSafe points, CLR will not be able to hijack the thread to initiate funceval. Generally, following helps to make sure funceval initiates when expected:

Step #1:

Make sure that you are not trying to debug a “Release” build. Release is fully optimized and thus will lead to the error in discussion. By using the Standard toolbar or the Configuration Manager, you can switch between Debug & Release.

For more details about it: How to: Set Debug and Release Configurations

Step #2:

If you still get the error, Debug option might be set for optimization. Verify & Uncheck the “Optimize code” property under Project “Properties”:

  • Right click the Project
  • Select option “Properties”
  • Go to “Build” tab
  • Uncheck the checkbox “Optimize code”

Step #3:

If you still get the error, Debug Info mode might be incorrect. Verify & set it to “full” under “Advanced Build Settings”:

  • Right click the Project
  • Select option “Properties”
  • Go to “Build” tab
  • Click “Advanced” button
  • Set “Debug Info” as “full”

Step #4:

If you still face the issue, try the following:

  • Do a “Clean” & then a “Rebuild” of your solution file
  • While debugging:
    1. Go to modules window (VS Menu -> Debug -> Windows -> Modules)
    2. Find your assembly in the list of loaded modules.
    3. Check the Path listed against the loaded assembly is what you expect it to be
    4. Check the modified Timestamp of the file to confirm that the assembly was actually rebuilt
    5. Check whether or not the loaded module is optimised or not
  • Remote chance could be that your call stack is getting optimized because your method signature is too large (more than 256 argument bytes). Read more about it at this MSDN blog

Conclusion:
It’s not an error but an information based on certain settings and as designed based on how .NET runtime works.

 
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Posted by on January 26, 2013 in ASP.NET, Visual Studio

 

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“Invalid postback or callback argument”

Problem description:
This is one the common issues that a lot of ASP.NET beginners face, post and ask about. Typically, they post the error message as below and seek for resolution without sharing much about what they were trying to do.

[ArgumentException: Invalid postback or callback argument. Event validation is enabled using in configuration or <%@ Page EnableEventValidation=”true” %> in a page. For security purposes, this feature verifies that arguments to postback or callback events originate from the server control that originally rendered them. If the data is valid and expected, use the ClientScriptManager.RegisterForEventValidation method in order to register the postback or callback data for validation.]

Though, error stack trace itself suggests a quick resolution by setting eventvalidation off, it is not a recommended solution as it opens up a security hole. It is always good to know why it happened and how to solve/handle that root problem.

Assessment:
Event validation  is done to validate if the origin of the event is the related rendered control (and not some cross site script or so). Since control registers its events during rendering, events can be validated during postback or callback (via arguments of __doPostBack). This reduces the risk of unauthorized or malicious postback requests and callbacks.

Refer: MSDN: Page.EnableEventValidation Property

Based on above, possible scenarios that I have faced or heard that raises the issue in discussion are:
Case #1: If we have angular brackets in the request data, it looks like some script tag is being passed to server

Possible Solution:
HTML encode the angular brackets with help of JavaScript before submitting the form, i.e. replace “<” with “&lt;” and “>” with “&gt;”

function HTMLEncodeAngularBrackets(someString)
{
var modifiedString = someString.replace("<","&lt;");
modifiedString = modifiedString.replace(">","&gt;");
return modifiedString;
}

Case #2: If we write client script that changes a control in the client at run time, we might have a dangling event. An example could be having embedded controls where an inner control registers for postback but is hidden at runtime because of an operation done on outer control. This I read about on MSDN blog written by Carlo, when looking for same issue because of multiple form tags.

Possible Solution:
Manually register control for event validation within Render method of the page.

protected override void Render(HtmlTextWriter writer)
{
ClientScript.RegisterForEventValidation(myButton.UniqueID.ToString());
base.Render(writer);
}

As said, one of the other common scenario reported (which looks like falls in the this same category) is building a page where one form tag is embedded in another form tag that runs on server. Removing one of them corrects the flow and resolves the issue.

Case #3: If we re-define/instantiate controls or commands at runtime on every postback, respective/related events might go for a toss. A simple example could be of re-binding a datagrid on every pageload (including postbacks). Since, on rebind all the controls in grid will have a new ID, during an event triggered by datagrid control, on postback the control ID’s are changed and thus the event might not connect to correct control raising the issue.

Possible Solution:
This can be simply resolved by making sure that controls are not re-created on every postback (rebind here). Using Page property IsPostback can easily handle it. If one want to create control on every postback, then it is necessary to make sure that the ID’s are not changed.

protected void Page_Load(object sender, EventArgs e)
{
if(!Page.IsPostback)
{
// Create controls
// Bind Grid
}
}

Conclusion:
As said, easy/direct solution can be adding enableEventValidation=”false” in the Page directive or Web.config file but is not recommended. Based on the implementation and cause, find the root cause and apply the resolution accordingly.

 
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Posted by on January 24, 2013 in ASP.NET

 

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CodeProject MVP Status for 2013

CP MVP2013

Few days back, I got an email from Chris Maunder (Co-founder of www.codeproject.com) – I have received an MVP award for my contributions made through out the year. It is third time in a row that I have been awarded the same by CodeProject.

I love CodeProject and I enjoy participating in various conversations at forums there. Not just it, my CodeProject contributions have a big hand in getting recognized by Microsoft too. I would encourage anyone to have a go at contributing something to the community.

Thanks CodeProject team and other CP members who helped me throughout the year.

 
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Posted by on January 20, 2013 in CodeProject, General

 

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