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# Monday, March 10, 2014

In the article "Error handling in WCF based web applications" we've shown a custom error handler for RESTful service based on WCF. This time we shall do the same for Web API 2.1 service.

Web API 2.1 provides an elegant way to implementat custom error handlers/loggers, see the following article. Web API permits many error loggers followed by a single error handler for all uncaught exceptions. A default error handler knows to output an error both in XML and JSON formats depending on requested MIME type.

In our projects we use unique error reference IDs. This feature allows to an end-user to refer to any error that has happened during the application life time and pass such error ID to the technical support for further investigations. Thus, error details passed to the client-side contain an ErrorID field. An error logger generates ErrorID and passes it over to an error handler for serialization.

Let's look at our error handling implementation for a Web API application.

The first part is an implementation of IExceptionLogger interface. It assigns ErrorID and logs all errors:

/// Defines a global logger for unhandled exceptions.
public class GlobalExceptionLogger : ExceptionLogger
{
  /// Writes log record to the database synchronously.
  public override void Log(ExceptionLoggerContext context)
  {
    try
    {
      var request = context.Request;
      var exception = context.Exception;

      var id = LogError(
        request.RequestUri.ToString(),
        context.RequestContext == null ?
          null : context.RequestContext.Principal.Identity.Name,
        request.ToString(),
        exception.Message,
        exception.StackTrace);

      // associates retrieved error ID with the current exception
      exception.Data["NesterovskyBros:id"] = id;
    }
    catch
    {
      // logger shouldn't throw an exception!!!
    }
  }

  // in the real life this method may store all relevant info into a database.
  private long LogError(
    string address,
    string userid,
    string request,
    string message,
    string stackTrace)
  {
    ...
  }
}

The second part is the implementation of IExceptionHandler:

/// Defines a global handler for unhandled exceptions.
public class GlobalExceptionHandler : ExceptionHandler
{
  /// This core method should implement custom error handling, if any.
  /// It determines how an exception will be serialized for client-side processing.
  public override void Handle(ExceptionHandlerContext context)
  {
    var requestContext = context.RequestContext;
    var config = requestContext.Configuration;

    context.Result = new ErrorResult(
      context.Exception,
      requestContext == null ? false : requestContext.IncludeErrorDetail,
      config.Services.GetContentNegotiator(),
      context.Request,
      config.Formatters);
  }

  /// An implementation of IHttpActionResult interface.
  private class ErrorResult : ExceptionResult
  {
    public ErrorResult(
      Exception exception,
      bool includeErrorDetail,
      IContentNegotiator negotiator,
      HttpRequestMessage request,
      IEnumerable<MediaTypeFormatter> formatters) :
      base(exception, includeErrorDetail, negotiator, request, formatters)
    {
    }

    /// Creates an HttpResponseMessage instance asynchronously.
    /// This method determines how a HttpResponseMessage content will look like.
    public override Task<HttpResponseMessage> ExecuteAsync(CancellationToken cancellationToken)
    {
      var content = new HttpError(Exception, IncludeErrorDetail);
   
      // define an additional content field with name "ErrorID"
      content.Add("ErrorID", Exception.Data["NesterovskyBros:id"] as long?);

      var result =
        ContentNegotiator.Negotiate(typeof(HttpError), Request, Formatters);

      var message = new HttpResponseMessage
      {
        RequestMessage = Request,
        StatusCode = result == null ?
          HttpStatusCode.NotAcceptable : HttpStatusCode.InternalServerError
      };

      if (result != null)
      {
        try
        {
          // serializes the HttpError instance either to JSON or to XML
          // depend on requested by the client MIME type.
          message.Content = new ObjectContent<HttpError>(
            content,
            result.Formatter,
            result.MediaType);
        }
        catch
        {
          message.Dispose();

          throw;
        }
      }

      return Task.FromResult(message);
    }
  }
}

Last, but not least part of this solution is registration and configuration of the error logger/handler:

/// WebApi congiguation.
public static class WebApiConfig
{
  public static void Register(HttpConfiguration config)
  {
    ...

    // register the exception logger and handler
    config.Services.Add(typeof(IExceptionLogger), new GlobalExceptionLogger());
    config.Services.Replace(typeof(IExceptionHandler), new GlobalExceptionHandler());

    // set error detail policy according with value from Web.config
    var customErrors =
      (CustomErrorsSection)ConfigurationManager.GetSection("system.web/customErrors");

    if (customErrors != null)
    {
      switch (customErrors.Mode)
      {
        case CustomErrorsMode.RemoteOnly:
        {
         config.IncludeErrorDetailPolicy = IncludeErrorDetailPolicy.LocalOnly;

         break;
        }
        case CustomErrorsMode.On:
        {
          config.IncludeErrorDetailPolicy = IncludeErrorDetailPolicy.Never;

          break;
        }
        case CustomErrorsMode.Off:
        {
          config.IncludeErrorDetailPolicy = IncludeErrorDetailPolicy.Always;

          break;
        }
        default:
        {
          config.IncludeErrorDetailPolicy = IncludeErrorDetailPolicy.Default;

          break;
        }
      }
    }
  }
}

The client-side error handler remain almost untouched. The implementation details you may find in /Scripts/api/api.js and Scripts/controls/error.js files.

You may download the demo project here. Feel free to use this solution in your .NET projects.

Monday, March 10, 2014 10:56:24 AM UTC  #    Comments [0] -
.NET | ASP.NET | javascript | kendoui | Tips and tricks
# Monday, May 13, 2013

Although WCF REST service + JSON is outdated comparing to Web API, there are yet a lot of such solutions (and probably will appear new ones) that use such "old" technology.

One of the crucial points of any web application is an error handler that allows gracefully resolve server-side exceptions and routes them as JSON objects to the client for further processing. There are dozen approachesin Internet that solve this issue  (e.g. http://blog.manglar.com/how-to-provide-custom-json-exceptions-from-as-wcf-service/), but there is no one that demonstrates error handling ot the client-side. We realize that it's impossible to write something general that suits for every web application, but we'd like to show a client-side error handler that utilizes JSON and KendoUI.

On our opinion, the successfull error handler must display an understandable error message on one hand, and on the other hand it has to provide technical info for developers in order to investigate the exception reason (and to fix it, if need):

collapsed error dialog collapsed error dialog

You may download demo project here. It contains three crucial parts:

  • A server-side error handler that catches all exceptions and serializes them as JSON objects (see /Code/JsonErrorHandler.cs and /Code/JsonWebHttpBehaviour.cs).
  • An error dialog that's based on user-control defined in previous articles (see /scripts/controls/error.js, /scripts/controls/error.resources.js and /scripts/templates/error.tmpl.html).
  • A client-side error handler that displays errors in user-friendly's manner (see /scripts/api/api.js, method defaultErrorHandler()).

Of course this is only a draft solution, but it defines a direction for further customizations in your web applications.

Monday, May 13, 2013 11:09:02 PM UTC  #    Comments [0] -
.NET | ASP.NET | javascript | kendoui | Tips and tricks
# Sunday, January 6, 2013

Kendo UI Docs contains an article "How To: Load Templates from External Files", where authors review two way of dealing with Kendo UI templates.

While using Kendo UI we have found our own answer to: where will the Kendo UI templates be defined and maintained?

solution tree

In our .NET project we have decided to keep templates separately, and to store them under the "templates" folder. Those templates are in fact include html, head, and stylesheet links. This is to help us to present those tempates in the design view.

In our scripts folder, we have defined a small text transformation template: "templates.tt", which produces "templates.js" file. This template takes body contents of each "*.tmpl.html" file from "templates" folder and builds string of the form:

document.write('<script id="footer-template" type="text/x-kendo-template">...</script><script id="row-template" type="text/x-kendo-template">...</script>');

In our page that uses templates, we include "templates.js":

<!DOCTYPE html>
<html>
  <head>
    <script src="scripts/templates.js"></script>
    ...

Thus, we have:

  • clean separation of templates and page content;
  • automatically generated templates include file.

WebTemplates.zip contains a web project demonstrating our technique. "templates.tt" is text template transformation used in the project.

See also: Compile KendoUI templates.

Sunday, January 6, 2013 7:43:19 PM UTC  #    Comments [0] -
.NET | ASP.NET | javascript | Tips and tricks
# Monday, October 29, 2012

If you deal with web applications you probably have already dealt with export data to Excel. There are several options to prepare data for Excel:

  • generate CSV;
  • generate HTML that excel understands;
  • generate XML in Spreadsheet 2003 format;
  • generate data using Open XML SDK or some other 3rd party libraries;
  • generate data in XLSX format, according to Open XML specification.

You may find a good article with pros and cons of each solution here. We, in our turn, would like to share our experience in this field. Let's start from requirements:

  • Often we have to export huge data-sets.
  • We should be able to format, parametrize and to apply different styles to the exported data.
  • There are cases when exported data may contain more than one table per sheet or even more than one sheet.
  • Some exported data have to be illustrated with charts.

All these requirements led us to a solution based on XSLT processing of streamed data. The advantage of this solution is that the result is immediately forwarded to a client as fast as XSLT starts to generate output. Such approach is much productive than generating of XLSX using of Open XML SDK or any other third party library, since it avoids keeping a huge data-sets in memory on the server side.

Another advantage - is simple maintenance, as we achieve clear separation of data and presentation layers. On each request to change formatting or apply another style to a cell you just have to modify xslt file(s) that generate variable parts of XLSX.

As result, our clients get XLSX files according with Open XML specifications. The details of implementations of our solution see in our next posts.

Monday, October 29, 2012 3:34:38 PM UTC  #    Comments [0] -
.NET | ASP.NET | Thinking aloud | xslt
# Friday, August 3, 2012

Earlier we have shown how to build streaming xml reader from business data and have reminded about ForwardXPathNavigator which helps to create a streaming xslt transformation. Now we want to show how to stream content produced with xslt out of WCF service.

To achieve streaming in WCF one needs:

1. To configure service to use streaming. Description on how to do this can be found in the internet. See web.config of the sample Streaming.zip for the details.

2. Create a service with a method returning Stream:

[ServiceContract(Namespace = "http://www.nesterovsky-bros.com")]
[AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]
public class Service
{
  [OperationContract]
  [WebGet(RequestFormat = WebMessageFormat.Json)]
  public Stream GetPeopleHtml(int count, int seed)
  {
    ...
  }
}

2. Return a Stream from xsl transformation.

Unfortunately (we mentioned it already), XslCompiledTransform generates its output into XmlWriter (or into output Stream) rather than exposes result as XmlReader, while WCF gets input stream and passes it to a client.

We could generate xslt output into a file or a memory Stream and then return that content as input Stream, but this will defeat a goal of streaming, as client would have started to get data no earlier that the xslt completed its work. What we need instead is a pipe that form xslt output Stream to an input Stream returned from WCF.

.NET implements pipe streams, so our task is trivial. We have defined a utility method that creates an input Stream from a generator populating an output Stream:

public static Stream GetPipedStream(Action<Stream> generator)
{
  var output = new AnonymousPipeServerStream();
  var input = new AnonymousPipeClientStream(
    output.GetClientHandleAsString());

  Task.Factory.StartNew(
    () =>
    {
      using(output)
      {
        generator(output);
        output.WaitForPipeDrain();
      }
    },
    TaskCreationOptions.LongRunning);

  return input;
}

We wrapped xsl transformation as such a generator:

[OperationContract]
[WebGet(RequestFormat = WebMessageFormat.Json)]
public Stream GetPeopleHtml(int count, int seed)
{
  var context = WebOperationContext.Current;

  context.OutgoingResponse.ContentType = "text/html";
  context.OutgoingResponse.Headers["Content-Disposition"] =
    "attachment;filename=reports.html";

  var cache = HttpRuntime.Cache;
  var path = HttpContext.Current.Server.MapPath("~/People.xslt");
  var transform = cache[path] as XslCompiledTransform;

  if (transform == null)
  {
    transform = new XslCompiledTransform();
    transform.Load(path);
    cache.Insert(path, transform, new CacheDependency(path));
  }

  return Extensions.GetPipedStream(
    output =>
    {
      // We have a streamed business data.
      var people = Data.CreateRandomData(count, seed, 0, count);

      // We want to see it as streamed xml data.
      using(var stream =
        people.ToXmlStream("people", "http://www.nesterovsky-bros.com"))
      using(var reader = XmlReader.Create(stream))
      {
        // XPath forward navigator is used as an input source.
        transform.Transform(
          new ForwardXPathNavigator(reader),
          new XsltArgumentList(),
          output);
      }
    });
}

This way we have build a code that streams data directly from business data to a client in a form of report. A set of utility functions and classes helped us to overcome .NET's limitations and to build simple code that one can easily support.

The sources can be found at Streaming.zip.

Friday, August 3, 2012 10:32:49 PM UTC  #    Comments [0] -
.NET | ASP.NET | Thinking aloud | Tips and tricks | xslt
# Thursday, March 8, 2012

For a long time we were developing web applications with ASP.NET and JSF. At present we prefer rich clients and a server with page templates and RESTful web services.

This transition brings technical questions. Consider this one.

Browsers allow to store session state entirely on the client, so should we maintain a session on the server?

Since the server is just a set of web services, so we may supply all required arguments on each call.

At first glance we can assume that no session is required on the server. However, looking further we see that we should deal with data validation (security) on the server.

Think about a classic ASP.NET application, where a user can select a value from a dropdown. Either ASP.NET itself or your program (against a list from a session) verifies that the value received is valid for the user. That list of values and might be other parameters constitute a user profile, which we stored in session. The user profile played important role (often indirectly) in the validation of input data.

When the server is just a set of web services then we have to validate all parameters manually. There are two sources that we can rely to: (a) a session, (b) a user principal.

The case (a) is very similar to classic ASP.NET application except that with EnableEventValidation="true" runtime did it for us most of the time.
The case (b) requires reconstruction of the user profile for a user principal and then we proceed with validation of parameters.

We may cache user profile in session, in which case we reduce (b) to (a); on the other hand we may cache user profile in Cache, which is also similar to (a) but which might be lighter than (at least not heavier than) the solution with the session.

What we see is that the client session does not free us from server session (or its alternative).

Thursday, March 8, 2012 9:56:19 PM UTC  #    Comments [0] -
.NET | ASP.NET | Java | JSF and Facelets | Thinking aloud
# Friday, October 28, 2011

It has happened so, that we have never worked with jQuery, however were aware of it.

In early 2000 we have developed a web application that contained rich javascript APIs, including UI components. Later, we were actively practicing in ASP.NET, and later in JSF.

At present, looking at jQuery more closely we regret that we have failed to start using it earlier.

Separation of business logic and presentation is remarkable when one uses JSON web services. In fact server part can be seen as a set of web services representing a business logic and a set of resources: html, styles, scripts, others. Nor ASP.NET or JSF approach such a consistent separation.

The only trouble, in our opinion, is that jQuery has no standard data binding: a way to bind JSON data to (and from) html controls. The technique that will probably be standardized is called jQuery Templates or JsViews .

Unfortunatelly after reading about this binding API, and being in love with Xslt and XQuery we just want to cry. We don't know what would be the best solution for the task, but what we see looks uncomfortable to us.

Friday, October 28, 2011 10:59:23 PM UTC  #    Comments [0] -
ASP.NET | JSF and Facelets | Thinking aloud | Tips and tricks | xslt
# Sunday, August 28, 2011

AjaxControlToolkit has methods to access ViewState:

protected V GetPropertyValue<V>(string propertyName, V nullValue)
{
  if (this.ViewState[propertyName] == null)
  {
    return nullValue;
  }

  return (V) this.ViewState[propertyName];
}

protected void SetPropertyValue<V>(string propertyName, V value)
{
  this.ViewState[propertyName] = value;
}

...

public bool EnabledOnClient
{
  get { return base.GetPropertyValue("EnabledOnClient", true); }
  set { base.SetPropertyValue("EnabledOnClient", value); }
}

We find that code unnecessary complex and nonoptimal. Our code to access ViewState looks like this:

public bool EnabledOnClient
{
  get { return ViewState["EnabledOnClient"] as bool? ?? true); }
  set { ViewState["EnabledOnClient"] = value; }
}

Sunday, August 28, 2011 7:35:13 PM UTC  #    Comments [0] -
ASP.NET | Tips and tricks
# Tuesday, May 3, 2011

Already for a couple of days we're trying to create a UserControl containing a TabContainer. To achieve the goal we have created a page with a ToolkitScriptManager and a user control itself.

Page:

<form runat="server">
  <ajax:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server"/>
  <uc1:WebUserControl1 ID="WebUserControl11" runat="server" />
</form>

User control:

<%@ Control Language="C#" %>
<%@ Register
  Assembly="AjaxControlToolkit"
  Namespace="AjaxControlToolkit"
  TagPrefix="ajax" %>

<ajax:TabContainer ID="Tab" runat="server" Width="100%">
  <ajax:TabPanel runat="server" HeaderText="Tab1" ID="Tab1">
    <ContentTemplate>Panel 1</ContentTemplate>
  </ajax:TabPanel>
</ajax:TabContainer>

What could be more simple?

But no, there is a problem. At run-time control works perfectly, but at the designer it shows an error instead of a normal design view:

Error Rendering Control - TabContainer1
An unhandled exception has occurred.
Could not find any resources appropriate for the specified culture or the neutral culture. Make sure "AjaxControlToolkit.Properties.Resources.NET4.resources" was correctly embedded or linked into assembly "AjaxControlToolkit" at compile time, or that all the satellite assemblies required are loadable and fully signed.

That's a stupid error, which says nothing about the real problem reason. We had to attach a debugger to a Visual Studio just to realize what the problem is.

So, the error occurs at the following code of AjaxControlToolkit.ScriptControlBase:

private void EnsureScriptManager()
{
  if (this._scriptManager == null)
  {
    this._scriptManager = ScriptManager.GetCurrent(this.Page);

    if (this._scriptManager == null)
    {
      throw new HttpException(Resources.E_NoScriptManager);
    }
  }
}

Originally, the problem is due to the fact that ScriptManager is not found, and code wants to report an HttpException, but fun is that we recieve a different exception, which is releted to a missing resouce text for a message Resources.E_NoScriptManager. It turns out that E_NoScriptManager text is found neither in primary no in resource assemblies.

As for original problem, it's hard to say about reason of why ScriptManager is not available at design time. We, however, observed that a ScriptManager registers itself for a ScriptManager.GetCurrent() method at run-time only:

protected internal override void OnInit(EventArgs e)
{
...
  if (!base.DesignMode)
  {
...
    iPage.Items[typeof(ScriptManager)] = this;
...
  }
}

So, it's not clear what they (toolkit's developers) expected to get at design time.

These observations leave uneasiness regarding the quality of the library.

Tuesday, May 3, 2011 7:45:51 PM UTC  #    Comments [8] -
ASP.NET | Thinking aloud
# Sunday, July 11, 2010

It does not matter that DataBindExtender looks not usual in the ASP.NET. It turns to be so handy that built-in data binding is not considered to be an option.

After a short try, you uderstand that people tried very hard and have invented many controls and methods like ObjectDataSource, FormView, Eval(), and Bind() with outcome, which is very specific and limited.

In contrast DataBindExtender performs:

  • Two or one way data binding of any business data property to any control property;
  • Converts value before it's passed to the control, or into the business data;
  • Validates the value.

See an example:

<asp:TextBox id=Field8 EnableViewState="false" runat="server"></asp:TextBox>
<bphx:DataBindExtender runat='server'
  EnableViewState='false'
  TargetControlID='Field8'
  ControlProperty='Text'
  DataSource='<%# Import.ClearingMemberFirm %>'
  DataMember='Id'
  Converter='<%# Converters.AsString("XXXXX", false) %>'
  Validator='<%# (extender, value) => Functions.CheckID(value as string) %>'/>

Here, we beside a regualar two way data binding of a property Import.ClearingMemberFirm.Id to a property Field8.Text, format (parse) Converters.AsString("XXXXX", false), and finally validate an input value with a lambda function (extender, value) => Functions.CheckID(value as string).

DataBindExtender works also well in template controls like asp:Repeater, asp:GridView, and so on. Having your business data available, you may reduce a size of the ViewState with EnableViewState='false'. This way DataBindExtender approaches page development to a pattern called MVC.

Recently, we have found that it's also useful to have a way to run a javascript during the page load (e.g. you want to attach some client side event, or register a component). DataBindExtender provides this with OnClientInit property, which is a javascript to run on a client, where this refers to a DOM element:

... OnClientInit='$addHandler(this, "change", function() { handleEvent(event, "Field8"); } );'/>

allows us to attach onchange javascript event to the asp:TextBox.

So, meantime we're very satisfied with what we can achieve with DataBindExtender. It's more than JSF allows, and much more stronger and neater to what ASP.NET has provided.

The sources can be found at DataBindExtender.cs

Sunday, July 11, 2010 7:07:03 AM UTC  #    Comments [4] -
ASP.NET | Thinking aloud | Tips and tricks
# Tuesday, June 22, 2010

Recently we were raising a question about serialization of ASPX output in xslt.

The question went like this:

What's the recommended way of ASPX page generation?
E.g.:

------------------------
 <%@ Page AutoEventWireup="true"
   CodeBehind="CurMainMenuP.aspx.cs"
   EnableSessionState="True"
   Inherits="Currency.CurMainMenuP"
   Language="C#"
   MaintainScrollPositionOnPostback="True"
   MasterPageFile="Screen.Master" %>

<asp:Content ID="Content1" runat="server" ContentPlaceHolderID="Title">CUR_MAIN_MENU_P</asp:Content>

<asp:Content ID="Content2" runat="server" ContentPlaceHolderID="Content">
  <span id="id1222146581" runat="server"
    class="inputField system UpperCase" enableviewstate="false">
    <%# Dialog.Global.TranCode %>
  </span>
  ...
------------------------

Notice aspx page directives, data binding expessions, and prefixed tag names without namespace declarations.

There was a whole range of expected answers. We, however, looked whether somebody have already dealed with the task and has a ready solution at hands.

In general it seems that xslt community is very angry about ASPX: both format and technology. Well, put this aside.

The task of producing ASPX, which is almost xml, is not solvable when you're staying with pure xml serializer. Xslt's xsl:character-map does not work at all. In fact it looks as a childish attempt to address the problem, as it does not support character escapes but only grabs characters and substitutes them with strings.

We have decided to create ASPX serializer API producing required output text. This way you use <xsl:output method="text"/> to generate ASPX pages.

With this goal in mind we have defined a little xml schema to describe ASPX irregularities in xml form. These are:

  • <xs:element name="declared-prefix"> - to describe known prefixes, which should not be declared;
  • <xs:element name="directive"> - to describe directives like <%@ Page %>;
  • <xs:element name="content"> - a transparent content wrapper;
  • <xs:element name="entity"> - to issue xml entity;
  • <xs:element name="expression"> - to describe aspx expression like <%# Eval("A") %>;
  • <xs:element name="attribute"> - to describe an attribute of the parent element.

This approach greately simplified for us an ASPX generation process.

The API includes:

Tuesday, June 22, 2010 10:25:41 AM UTC  #    Comments [0] -
Announce | ASP.NET | Thinking aloud | Tips and tricks | xslt
# Tuesday, June 15, 2010

In previous posts we were crying about problems with JSF to ASP.NET migration. Let's point to another one.

Consider that you have an input field, whose value should be validated:

<input type="text" runat="server" ID="id1222146409" maxlength="4"/>
<bphx:DataBindExtender runat="server"
  TargetControlID="id1222146409" ControlProperty="Value"
  DataSource="<%# Import.AaControlAttributes %>"
  DataMember="UserEnteredTrancode"/>

Here we have an input control, whose value is bound to Import.AaControlAttributes.UserEnteredTrancode property. But what is missed is a value validation. Somewhere we have a function that could answer the question whether the value is valid. It should be called like this: Functions.IsTransactionCodeValid(value).

Staying within standard components we can use a custom validator on the page:

<asp:CustomValidator runat="server"
  ControlToValidate="id1222146409"
  OnServerValidate="ValidateTransaction"
  ErrorMessage="Invalid transaction code."/>

and add the following code-behind:

protected void ValidateTransaction(object source, ServerValidateEventArgs args)
{
  args.IsValid = Functions.IsTransactionCodeValid(args.Value);
}

This approach works, however it pollutes the code-behind with many very similar methods. The problem is that the validation rules in most cases are not property of page but one of data model. That's why page validation methods just forward check to somewhere.

While thinking on how to simplify the code we have came up with more conscious and short way to express validators, namely using lambda functions. To that end we have introduced a Validator property of type ValueValidator over DataBindExtender. Where

/// <summary>A delegate to validate values.</summary>
/// <param name="extender">An extender instance.</param>
/// <param name="value">A value to validate.</param>
/// <returns>true for valid value, and false otherwise.</returns>
public delegate bool ValueValidator(DataBindExtender extender, object value);

/// <summary>An optional data member validator.</summary>
public virtual ValueValidator Validator { get; set; }

With this new property the page markup looks like this:

<input type="text" runat="server" ID="id1222146409" maxlength="4"/>
<bphx:DataBindExtender runat="server"
  TargetControlID="id1222146409" ControlProperty="Value"
  DataSource="<%# Import.AaControlAttributes %>"
  DataMember="UserEnteredTrancode"
  Validator='<%# (extender, value) => Functions.IsTransactionCodeValid(value as string) %>'
  ErrorMessage="Invalid transaction code."/>

This is almost like an event handler, however it allowed us to call data model validation logic without unnecessary code-behind.

The updated DataBindExtender can be found at DataBindExtender.cs.

Tuesday, June 15, 2010 6:36:44 AM UTC  #    Comments [0] -
ASP.NET | Thinking aloud | Tips and tricks
# Thursday, June 10, 2010

Being well behind of the latest news and traps of the ASP.NET, we're readily falling on each problem. :-)

This time it's a script injection during data binding.

In JSF there is a component to output data called h:outputText. Its use is like this:

<span jsfc="h:outputText" value="#{myBean.myProperty}"/>

The output is a span element with data bound value embeded into content. The natural alternative in ASP.NET seems to be an asp:Label control:

<asp:Label runat="server" Text="<%# Eval("MyProperty") %>"/>

This almost works except that the h:outputText escapes data (you may override this and specify attribute escape="false"), and asp:Label never escapes the data.

This looks as a very serious omission in ASP.NET (in fact very close to a security hole). What are chances that when you're creating a new page, which uses data binding, you will not forget to fix code that wizard created for you and to change it to:

<asp:Label runat="server" Text="<%# Server.HtmlEncode(Eval("MyProperty")) %>"/>

Eh? Think what will happen if MyProperty will return a text that looks like a script (e.g.: <script>alert(1)</script>), while you just wanted to output a label?

To address the issue we've also introduced a property Escape into DataBindExtender. So at present we have a code like this:

<asp:Label runat="server" ID="MyLabel"/>
<bphx:DataBindExtender runat="server" TargetControlID="MyLabel"
  ControlProperty="Text" ReadOnly="true" Escape="true"
  DataSource="<%# MyBean %>" DataMember="MyProperty"/>

See also: A DataBindExtender, Experience of JSF to ASP.NET migration

Thursday, June 10, 2010 1:06:19 PM UTC  #    Comments [0] -
ASP.NET | Thinking aloud | Tips and tricks
# Saturday, June 5, 2010

After struggling with ASP.NET data binding we found no other way but to introduce our little extender control to address the issue.

We were trying to be minimalistic and to introduce two way data binding and to support data conversion. This way extender control (called DataBindExtender) have following page syntax:

<asp:TextBox id=TextBox1 runat="server"></asp:TextBox>
<cc1:DataBindExtender runat="server"
  DataSource="<%# Data %>"
  DataMember="ID"
  TargetControlID="TextBox1"
  ControlProperty="Text" />

Two way data binding is provided with DataSource object (notice data binding over this property) and a DataMember property from the one side, and TargetControlID and ControlProperty from the other side. DataBindExtender supports Converter property of type TypeConverter to support custom converters.

DataBindExtender is based on AjaxControlToolkit.ExtenderControlBase class and implements System.Web.UI.IValidator. ExtenderControlBase makes implementation of extenders extremely easy, while IValidator plugs natuarally into page validation (Validate method, Validators collections, ValidationSummary control).

The good point about extenders is that they are not visible in designer, while it exposes properties in extended control itself. The disadvantage is that it requires Ajax Control Toolkit, and also ScriptManager component of the page.

To simplify the use DataBindExtender gets data from control and puts the value into data source in Validate method, and puts data into control in OnPreRender method; thus no specific action is required to perform data binding.

Source for the DataBindExtender is DataBindExtender.cs.

Saturday, June 5, 2010 11:22:03 AM UTC  #    Comments [0] -
ASP.NET | Thinking aloud | Tips and tricks
# Saturday, May 29, 2010

We used to think that ASP.NET is a way too powerful than JSF. It might be still true, but not when you are accustomed to JSF and spoiled with its code practice...

Looking at both technologies from a greater distance, we now realize that they give almost the same level of comfort during development, but they are different. You can feel this after you were working for some time with one technology and now are to implement similar solution in opposite one. That is where we have found ourselves at present.

The funny thing is that we did expect some problems but in a different place. Indeed, both ASP.NET and JSF are means to define a page layout and to map input and output of business data. While with the presentation (controls, their compositions, masters, styles and so on) you can find more or less equal analogies, the differences of implementation of data binding is a kind of a pain.

We have found that data binding in ASP.NET is somewhat awkward. Its Eval and Bind is bearable in simple cases but almost unusable when you business data is less trivial, or if you have to apply custom data formatting.

In JSF, with its Expression Language, we can perform two way data binding for rather complex properties like ${data.items[index + 5].property}, or to create property adapters ${my:asSomething(data.bean, "property").Value}, or add standard or custom property converters. In contrast data binding in ASP.NET is limited to simple property path (no expressions are supported), neither custom formatters are supported (try to format number as a telephone number).

Things work well when you're designing ASP.NET application from scratch, as you naturally avoid pitfalls, however when you got existing business logic and need to expose it to the web, you have no other way but to write a lot of code behind just to smooth out the problems that ASP.NET exhibits.

Another solution would be to design something like extender control that would attach more proper data binding and formatting facilities to control properties. That would allow to make page definitions in more declarative way, like what we have now in JSF.

Saturday, May 29, 2010 2:16:05 PM UTC  #    Comments [0] -
ASP.NET | JSF and Facelets | Thinking aloud
# Sunday, May 23, 2010

While porting a solution from JSF to ASP.NET we have seen an issue with synchronization of access to a data stored in a session from multiple requests.

Consider a case when you store a business object in a session.

Going through the request lifecycle we observe that this business object may be accessed at different stages: data binding, postback event handler, security filters, other.

Usually this business object is mutable and does not assume concurent access. Browsers, however, may easily issue multiple requests to the same session at the same time. In fact, such behaviour, is not even an exception, as browsers nowadays are often sending concurrent requests.

In the JSF we're using a sync object, which is part of business object itself; lock it and unlock at the begin and at the end of a request correspondingly. This works perfectly as JSF guarantees that:

  • lock is released after it's acquired (we use request scope bean with @PostConstruct and @PreDestroy annotations to lock and unlock);
  • both lock and unlock take place in the same thread.

ASP.NET, in contrast, tries to be more asynchronous, and allows for different stages of request to take place in different threads. This could be seen indirectly in the documentation, which does not give any commitments in this regards, and with code inspection where you can see that request can begin in one thread, and a next stage can be queued for the execution into the other thread.

In addition, ASP.NET does not guarantee that if BeginRequest has been executed then EndRequest will also run.

The conclusion is that we should not use locks to synchronize access to the same session object, but rather try to invent other means to avoid data races.

Update msdn states:

Concurrent Requests and Session State

Access to ASP.NET session state is exclusive per session, which means that if two different users make concurrent requests, access to each separate session is granted concurrently. However, if two concurrent requests are made for the same session (by using the same SessionID value), the first request gets exclusive access to the session information. The second request executes only after the first request is finished. (The second session can also get access if the exclusive lock on the information is freed because the first request exceeds the lock time-out.)

This means that the required synchronization is already built into ASP.NET. That's good.

Sunday, May 23, 2010 12:22:35 PM UTC  #    Comments [0] -
ASP.NET | Thinking aloud
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