Migration of MSAjax wiki from Google Code; closes #26

MSAjax.md

@@ -0,0 +1,59 @@
+---
+title: Using ELMAH with ASP.NET AJAX
+layout: page
+permalink: a/msajax/
+article: true
+---
+
+This article describes the `MsAjaxDeltaErrorLogModule` that was introduced with [ELMAH 1.0 BETA 3](https://bitbucket.org/project-elmah/main/downloads/ELMAH-1.0-BETA3-bin.zip).
+
+## The Problem
+
+The first incarnation of [ASP.NET AJAX](http://www.asp.net/ajax/) written against v2.0 of the .NET Framework (v1.0.61025.0) is unfortunately not 100% compatible with ELMAH. When an unhandled exception occurs during a [Partial PostBack][partial-postback], the original code catches the exception, and unceremoniously calls `Response.End()`, preventing ELMAH from capturing it. This code can be seen from [Reflector](http://www.aisto.com/roeder/dotnet/) by looking in `System.Web.Extensions.dll` at the `System.Web.UI.PageRequestManager.OnPageError` method:
+
+{% highlight c# %}
+private void OnPageError(object sender, EventArgs e)
+{
+    ...
+    if (flag)
+    {
+        ...
+        this._owner.IPage.Response.End();
+    }
+}
+{% endhighlight %}
+
+## The Solution
+
+To overcome this you can add the `MsAjaxDeltaErrorLogModule` to your application as follows:
+
+{% highlight xml %}
+<httpModules>
+    <!-- 
+        This entry is only to be used if you are using ASP.NET AJAX v1.0.x.x
+        and want to capture errors during Partial Postbacks. It is not required 
+        for the version of ASP.NET AJAX that shipped with .NET Framework v3.5!
+    -->
+    <add name="MsAjaxDeltaErrorLog" type="Elmah.MsAjaxDeltaErrorLogModule, Elmah"/>
+</httpModules>
+{% endhighlight %}
+
+This module checks to see if the current request is a [Partial PostBack][partial-postback], and if so sets up an alternative handler to capture any unhandled exception.
+
+## Potential Issues
+
+Unfortunately, the `MsAjaxDeltaErrorLogModule` is not a perfect solution. Due to the way it works, there is an extremely small possibility that errors will get logged _twice_ within ELMAH. For this to occur, the following must happen:
+
+  1. The unhandled exception must occur during a [Partial PostBack][partial-postback]
+  1. It must occur _before_ the `OnInit` method of `System.Web.UI.PageRequestManager` is called.
+
+This will mean that both the standard ELMAH modules and the `MsAjaxDeltaErrorModule` will capture the exception.
+
+In reality, the chances of this happening are going to be fairly slender.
+
+## .NET Framework 3.5
+
+An updated version of Microsoft Ajax was shipped with version 3.5 of the .NET Framework. This new version changed the way that unexpected errors in [Partial PostBacks][partial-postback] were handled, removing the call to `Response.End`. Therefore, if you continue to use the `MsAjaxDeltaErrorLogModule` with this version of the framework, you will see all unhandled errors in [Partial PostBacks][partial-postback] being logged _twice_ in ELMAH.
+
+
+  [partial-postback]: http://www.asp.net/AJAX/Documentation/Live/overview/PartialPageRenderingOverview.aspx