Hidden Features of ASP.NET

Question

There are always features that would be useful in fringe scenarios, but for that very reason most people don't know them. I am asking for features that are not typically taught by the text books.

What are the ones that you know?

Answer 1

This seems like a huge, vague question... But I will throw in Reflection, as it has allowed me to do some incredibly powerful things like pluggable DALs and such.

Answer 2

HttpModules. The architecture is crazy elegant. Maybe not a hidden feature, but cool none the less.

Answer 3

HttpContext.Items as a request-level caching tool

Answer 4

HttpContext.IsCustomErrorEnabled is a cool feature.I've found it useful more than once. Here is a short post about it.

Answer 5

Two things stand out in my head:

1) You can turn Trace on and off from the code:

#ifdef DEBUG 
   if (Context.Request.QueryString["DoTrace"] == "true")
                {
                    Trace.IsEnabled = true;
                    Trace.Write("Application:TraceStarted");
                }
#endif

2) You can build multiple .aspx pages using only one shared "code-behind" file.

Build one class .cs file :

public class Class1:System.Web.UI.Page
    {
        public TextBox tbLogin;

        protected void Page_Load(object sender, EventArgs e)
        {

          if (tbLogin!=null)
            tbLogin.Text = "Hello World";
        }
    }

and then you can have any number of .aspx pages (after you delete .designer.cs and .cs code-behind that VS has generated) :

  <%@ Page Language="C#"  AutoEventWireup="true"  Inherits="Namespace.Class1" %>
     <form id="form1" runat="server">
     <div>
     <asp:TextBox  ID="tbLogin" runat="server"></asp: TextBox  >
     </div>
     </form>

You can have controls in the ASPX that do not appear in Class1, and vice-versa, but you need to remeber to check your controls for nulls.

Answer 6

You can use:

 Request.Params[Control.UniqueId]

To get the value of a control BEFORE viewstate is initialized (Control.Text etc will be empty at this point).

This is useful for code in Init.

Answer 7

• HttpContext.Current will always give you access to the current context's Request/Response/etc., even when you don't have access to the Page's properties (e.g., from a loosely-coupled helper class).

• You can continue executing code on the same page after redirecting the user to another one by calling Response.Redirect(url, false )

• You don't need .ASPX files if all you want is a compiled Page (or any IHttpHandler). Just set the path and HTTP methods to point to the class in the <httpHandlers> element in the web.config file.

• A Page object can be retrieved from an .ASPX file programmatically by calling PageParser.GetCompiledPageInstance(virtualPath,aspxFileName,Context)

Answer 8

throw new HttpException(404, "Article not found");

This will be caught by ASP.NET which will return the customErrors page. Learned about this one in a recent .NET Tip of the Day Post

Answer 9

By default, any content between tags for a custom control is added as a child control. This can be intercepted in an AddParsedSubObject() override for filtering or additional parsing (e.g., of text content in LiteralControls):

    protected override void AddParsedSubObject(object obj)
     { var literal = obj as LiteralControl;
       if (literal != null) Controls.Add(parseControl(literal.Text));
       else base.AddParsedSubObject(obj);
     }

...

   <uc:MyControl runat='server'>
     ...this text is parsed as a LiteralControl...
  </uc:MyControl>

Answer 10

If you place a file named *app_offline.htm* in the root of a web application directory, ASP.NET 2.0+ will shut-down the application and stop normal processing any new incoming requests for that application, showing only the contents of the app_offline.htm file for all new requests.

This is the quickest and easiest way to display your "Site Temporarily Unavailable" notice while re-deploying (or rolling back) changes to a Production server.

Also, as pointed out by marxidad, make sure you have at least 512 bytes of content within the file so IE6 will render it correctly.

Answer 11

ScottGu has a bunch of tricks at http://weblogs.asp.net/scottgu/archive/2006/04/03/441787.aspx

Answer 12

If you have ASP.NET generating an RSS feed, it will sometimes put an extra line at the top of the page. This won't validate with common RSS validators. You can work around it by putting the page directive <@Page> at the bottom of the page.

Answer 13

I thought it was neat when I dumped a xmlDocument() into a label and it displayed using it's xsl transforms.

Answer 14

Setting Server Control Properties Based on Target Browser and more. That one kinda took me by surprise.

Answer 15

Before ASP.NET v3.5 added routes you could create your own friendly URLs simply by writing an HTTPModule to and rewrite the request early in the page pipeline (like the BeginRequest event).

Urls like http://servername/page/Param1/SomeParams1/Param2/SomeParams2 would get mapped to another page like below (often using regular expressions).

HttpContext.RewritePath("PageHandler.aspx?Param1=SomeParms1&Param2=SomeParams2");

DotNetNuke has a really good HttpModule that does this for their friendly urls. Is still useful for machines where you can't deploy .NET v3.5.

Answer 16

Included in ASP.NET 3.5 SP1:

• customErrors now supports "redirectMode" attribute with a value of "ResponseRewrite". Shows error page without changing URL.
• The form tag now recognizes the action attribute. Great for when you're using URL rewriting

Answer 17

While testing, you can have emails sent to a folder on your computer instead of an SMTP server. Put this in your web.config:

<system.net>
    <mailSettings>
        <smtp deliveryMethod="SpecifiedPickupDirectory">
            <specifiedPickupDirectory pickupDirectoryLocation="c:\Temp\" />
        </smtp>
    </mailSettings>
</system.net>

Answer 18

Attach a class located in your App_Code folder to your Global Application Class file.

ASP.NET 2.0 - Global.asax - Code Behind file.

This works in Visual Studio 2008 as well.

Answer 19

You can find any control by using its UniqueID property:

Label label = (Label)Page.FindControl("UserControl1$Label1");

Answer 20

HttpContext.Current.IsDebuggingEnabled

This is great for determining which scripts to output (min or full versions) or anything else you might want in dev, but not live.

Answer 21

System.Web.VirtualPathUtility

Answer 22

You can use ASP.NET Comments within an .aspx page to comment out full parts of a page including server controls. And the contents that is commented out will never be sent to the client.

<%--
    <div>
        <asp:Button runat="server" id="btnOne"/>
    </div>
--%>

Answer 23

Valid syntax that VS chokes on:

<input type="checkbox" name="roles" value='<%# Eval("Name") %>' 
  <%# ((bool) Eval("InRole")) ? "checked" : "" %> 
  <%# ViewData.Model.IsInRole("Admin") ? "" : "disabled" %> />

Answer 24

I worked on a asp.net application which went through a security audit by a leading security company and I learned this easy trick to preventing a lesser known but important security vulnerability.

The below explanation is from: http://www.guidanceshare.com/wiki/ASP.NET_2.0_Security_Guidelines_-_Parameter_Manipulation#Consider_Using_Page.ViewStateUserKey_to_Counter_One-Click_Attacks

Consider using Page.ViewStateUserKey to counter one-click attacks. If you authenticate your callers and use ViewState, set the Page.ViewStateUserKey property in the Page_Init event handler to prevent one-click attacks.

void Page_Init (object sender, EventArgs e) {
  ViewStateUserKey = Session.SessionID;
}

Set the property to a value you know is unique to each user, such as a session ID, user name, or user identifier.

A one-click attack occurs when an attacker creates a Web page (.htm or .aspx) that contains a hidden form field named __VIEWSTATE that is already filled with ViewState data. The ViewState can be generated from a page that the attacker had previously created, such as a shopping cart page with 100 items. The attacker lures an unsuspecting user into browsing to the page, and then the attacker causes the page to be sent to the server where the ViewState is valid. The server has no way of knowing that the ViewState originated from the attacker. ViewState validation and HMACs do not counter this attack because the ViewState is valid and the page is executed under the security context of the user.

By setting the ViewStateUserKey property, when the attacker browses to a page to create the ViewState, the property is initialized to his or her name. When the legitimate user submits the page to the server, it is initialized with the attacker's name. As a result, the ViewState HMAC check fails and an exception is generated.

Answer 25

Usage of the ASHX file type:
If you want to just output some basic html or xml without going through the page event handlers then you can implement the HttpModule in a simple fashion

Name the page as SomeHandlerPage.ashx and just put the below code (just one line) in it

<%@ webhandler language="C#" class="MyNamespace.MyHandler" %>

Then the code file

using System;
using System.IO;
using System.Web;

namespace MyNamespace
{
    public class MyHandler: IHttpHandler
    {
     public void ProcessRequest (HttpContext context)
     {   
      context.Response.ContentType = "text/xml";
      string myString = SomeLibrary.SomeClass.SomeMethod();
      context.Response.Write(myString);
     }

     public bool IsReusable
     {
      get { return true; }
     }
    }
}

Answer 26

Enabling intellisense for MasterPages in the content pages
I am sure this is a very little known hack

Most of the time you have to use the findcontrol method and cast the controls in master page from the content pages when you want to use them, the MasterType directive will enable intellisense in visual studio once you to this

just add one more directive to the page

<%@ MasterType VirtualPath="~/Masters/MyMainMasterPage.master" %>

If you do not want to use the Virtual Path and use the class name instead then

<%@ MasterType TypeName="MyMainMasterPage" %>

Get the full article here

Answer 27

ClientScript property on Page object.

Answer 28

The Code Expression Builder (and others)

Sample markup:

Text = '<%$ Code: GetText() %>'
Text = '<%$ Code: MyStaticClass.MyStaticProperty %>'
Text = '<%$ Code: DateTime.Now.ToShortDateString() %>'
MaxLenth = '<%$ Code: 30 + 40 %>'

The real beauty of the code expression builder is that you can use databinding like expressions in non-databinding situations. You can also create other Expression Builders that perform other functions.

web.config:

<system.web>    
    <compilation debug="true">
        <expressionBuilders>
            <add expressionPrefix="Code" type="CodeExpressionBuilder" />

The cs class that makes it all happen:

[ExpressionPrefix("Code")]
public class CodeExpressionBuilder : ExpressionBuilder
{
    public override CodeExpression GetCodeExpression(
        BoundPropertyEntry entry,
        object parsedData,
        ExpressionBuilderContext context)
    {            
        return new CodeSnippetExpression(entry.Expression);
    }
}

Answer 29

System.Web.Hosting.HostingEnvironment.MapPath

Answer 30

Retail mode at the machine.config level:

<configuration>
  <system.web>
    <deployment retail="true"/>
  </system.web>
</configuration>

Overrides the web.config settings to enforce debug to false, turns custom errors on and disables tracing. No more forgetting to change attributes before publishing - just leave them all configured for development or test environments and update the production retail setting.

Answer 31

one feature came to my mind, sometimes you will need to hide some part of your page from the crowlers. you can do it with javascript or using this simple code:

if (Request.Browser.Crawler){
        HideArticleComments();

Answer 32

EnsureChildControls Method : It checks the child controls if they're initiated. If the child controls are not initiated it calls CreateChildControls method.

Answer 33

My team uses this a lot as a hack:

WebRequest myRequest = WebRequest.Create("http://www.google.com");
WebResponse myResponse = myRequest.GetResponse();
StreamReader sr = new StreamReader(myResponse.GetResponseStream());

// here's page's response loaded into a string for further use

String thisReturn = sr.ReadToEnd().Trim();

It loads a webpage's response as a string. You can send in post parameters too.

We use it in the place of ASCX/AJAX/WebServices when we need something cheap and fast. Basically, its a quick way to access web-available content across servers. In fact, we just dubbed it the "Redneck Web Service" yesterday.

Answer 34

Here's the best one. Add this to your web.config for MUCH faster compilation. This is post 3.5SP1 via this QFE.

<compilation optimizeCompilations="true">

Quick summary: we are introducing a new optimizeCompilations switch in ASP.NET that can greatly improve the compilation speed in some scenarios. There are some catches, so read on for more details. This switch is currently available as a QFE for 3.5SP1, and will be part of VS 2010.

The ASP.NET compilation system takes a very conservative approach which causes it to wipe out any previous work that it has done any time a ‘top level’ file changes. ‘Top level’ files include anything in bin and App_Code, as well as global.asax. While this works fine for small apps, it becomes nearly unusable for very large apps. E.g. a customer was running into a case where it was taking 10 minutes to refresh a page after making any change to a ‘bin’ assembly.

To ease the pain, we added an ‘optimized’ compilation mode which takes a much less conservative approach to recompilation.

Via here:

Answer 35

WebMethods.

You can using ASP.NET AJAX callbacks to web methods placed in ASPX pages. You can decorate a static method with the [WebMethod()] and [ScriptMethod()] attributes. For example:

[System.Web.Services.WebMethod()] 
[System.Web.Script.Services.ScriptMethod()] 
public static List<string> GetFruitBeginingWith(string letter)
{
 List<string> products = new List<string>() 
 { 
  "Apple", "Banana", "Blackberry", "Blueberries", "Orange", "Mango", "Melon", "Peach"
 };

 return products.Where(p => p.StartsWith(letter)).ToList();
}

Now, in your ASPX page you can do this:

<form id="form1" runat="server">
 <div>
  <asp:ScriptManager ID="ScriptManager1" runat="server" EnablePageMethods="true" />
  <input type="button" value="Get Fruit" onclick="GetFruit('B')" />
 </div>
</form>

And call your server side method via JavaScript using:

    <script type="text/javascript">
 function GetFruit(l)
 {
  PageMethods.GetFruitBeginingWith(l, OnGetFruitComplete);
 }

 function OnGetFruitComplete(result)
 {
  alert("You got fruit: " + result);
 }
</script>

Answer 36

One little known and rarely used feature of ASP.NET is:

Tag Mapping

It's rarely used because there's only a specific situation where you'd need it, but when you need it, it's so handy.

Some articles about this little know feature:

Tag Mapping in ASP.NET
Using Tag Mapping in ASP.NET 2.0

and from that last article:

Tag mapping allows you to swap compatible controls at compile time on every page in your web application. A useful example is if you have a stock ASP.NET control, such as a DropDownList, and you want to replace it with a customized control that is derived from DropDownList. This could be a control that has been customized to provide more optimized caching of lookup data. Instead of editing every web form and replacing the built in DropDownLists with your custom version, you can have ASP.NET in effect do it for you by modifying web.config:

<pages>
 <tagMapping>
   <clear />
   <add tagType="System.Web.UI.WebControls.DropDownList"
        mappedTagType="SmartDropDown"/>
  </tagMapping>
</pages>

Answer 37

Request.IsLocal Property :

It indicates whether current request is coming from Local Computer or not.

if( Request.IsLocal )
{
   LoadLocalAdminMailSettings();
}
else
{
   LoadServerAdminMailSettings();
}

Answer 38

DefaultButton property in Panels.

It sets default button for a particular panel.

Answer 39

MaintainScrollPositionOnPostback attribute in Page directive. It is used to maintain scroll position of aspx page across postbacks.

Answer 40

Check to see if the client is still connected, before starting a long-running task:

if (this.Response.IsClientConnected)
{
  // long-running task
}

Answer 41

Similarly to the optimizeCompilations=”true” solution, here another one to speed up the time you spend waiting in between builds (very good especially if you are working with a large project): create a ram-based drive (i.e. using RamDisk) and change your default “Temporary ASP.NET Files” to this memory-based drive.

The full details on how to do this is on my blog: http://www.wagnerdanda.me/2009/11/speeding-up-build-times-in-asp-net-with-ramdisk/

Basically you first and configure a RamDisk (again, in my blog there a link to a free ramdisk) and then you change your web.config according to this:

 <system.web>
 ....
     <compilation debug="true" tempDirectory="R:\ASP_NET_TempFiles\">
     ....
     </compilation>
 ....
 </system.web>

It greatly increase my development time, you just need invest in memory for you computer :)

Happy Programming!

Wagner Danda

Answer 42

Did you know it's possible to run ASP.Net outside of IIS or Visual Studio?

The whole runtime is packaged up and ready to be hosted in any process that wants to give it a try. Using ApplicationHost, HttpRuntime and HttpApplication classes, you too can grind up those .aspx pages and get shiny HTML output from them.

HostingClass host = ApplicationHost.CreateApplicationHost(typeof(HostingClass), 
                                            "/virtualpath", "physicalPath");
host.ProcessPage(urlToAspxFile); 

And your hosting class:

public class HostingClass : MarshalByRefObject
{
    public void ProcessPage(string url)
    {
        using (StreamWriter sw = new StreamWriter("C:\temp.html"))
        {
            SimpleWorkerRequest worker = new SimpleWorkerRequest(url, null, sw);
            HttpRuntime.ProcessRequest(worker);
        }
                    // Ta-dah!  C:\temp.html has some html for you.
    }
}

Answer 43

If you use web services instead WCF services, you can still use standard .Net membership to enforce authentication and login session behaviour on a set web services similarly to a how you would secure web site with membership forms authentication & without the need for a special session and/or soap headers implementations by simply calling System.Web.Security.FormsAuthentication.SetAuthCookie(userName, false) [after calling Membership.ValidateUser(userName, password) of course] to create cookie in the response as if the user has logged in via a web form. Then you can retrieve this authentication cookie with Response.Cookies[].Value and return it as a string to the user which can be used to authenticate the user in subsequent calls by re-creating the cookie in the Application_BeginRequest by extracting the
cookie method call param from the Request.InputStream and re-creating the auth cookie before the membership authenticates the request this way the membership provider gets tricked and will know the request is authenticated and enforce all its rules.

Sample web method signature to return this cookie to the user would be: string Login(userName,password)

Sample subsequent web method call would be: string DoSomething(string authcookie,string methodParam1,int methodParam2 etc,etc) where you need to extract authcookie(which is value obtained from Login method) param from the Request.InputStreamis

This also simulates a login session and calling FormsAuthentication.SignOut in a web method like this Logout(authcookie) would make the user need to sign in again.

Answer 44

Lots of people mentioned how to optimize your code when recompiling. Recently I discovered I can do most of my development (code-behind stuff) in the aspx page and skipping completely the build step. Just save the file and refresh your page. All you have to do is wrap your code in the following tag:

<script runat="server">

   Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
     Response.Write("Look Ma', I didn't even had to build!")
   End Sub

</script>

Once you are done, Just move all to the code-behind, build, test everything works and voila!

-D

Answer 45

After the website was published and deployed in the production server, If we need to do some changes on the server side button click event. We can override the existing click event by using the newkeyword in the aspx page itself.

Example

Code Behind Method

 Protected void button_click(sender object, e System.EventArgs) 
  {
     Response.Write("Look Ma', I Am code behind code!")  
  }

OverRided Method:

<script runat="server">   
   Protected void new button_click(sender object, e System.EventArgs) 
  {
     Response.Write("Look Ma', I am overrided method!")  
  }

</script

In this way we can easily fix the production server errors without redeployment.