R – ASP.NET Post Application_Error Event

asp.nethttphttp-headershttpmodulepage-lifecycle

I'm trying to find an event that will fire immediately after all Application_Error event handlers so that I can modify the response sent (messing with the status code and 'location' headers and creating a new body specifically) using a custom HttpModule.

I've tried hooking into Application_EndRequest (as I've read that this is the only handler that is guaranteed to fire) but this is too late in the request processing to modify the response headers and I get a HttpException:

Server cannot append header after HTTP headers have been sent.

Best Answer

I think ASP.Net launches a new response after ApplicationError, just for a different error-handling page. Try tacking into this response.

Update OK: Here's how to do it!

I wanted to make sure we can both attach an error handler and that we can gracefully handle the event in a page, so we can't make the last server error null, and we also want to be able to add headers!

In web.config, add:

<customErrors redirectMode="ResponseRewrite" mode="On">
    <error statusCode="500" redirect="~/ErrorHandler.aspx"/>
</customErrors>

Or whatever error specifically you're looking to catch. And under httpModules:

<add name="ErrorModule" type="TestErrorLifecycle.ErrorModule, TestErrorLifecycle"/>

The code to log the error is in this module:

public class ErrorModule : IHttpModule
{
    private volatile object locker = new object();

    public void Init(HttpApplication context)
    {
        context.Error += context_Error;
    }

    void context_Error(object sender, EventArgs e)
    {
        var app = sender as HttpApplication;

        lock (locker)
        {
            assertLogDirectory(app.Server);

            using (var fs = File.AppendText(app.Server.MapPath("~/logs/errorlog.txt")))
            {
                var lastException = app.Server.GetLastError();
                if (lastException == null) throw new ApplicationException("Not expected...");
                fs.WriteLine(lastException.Message);
            }
        }

        // we could also do a Request.Redirect() here...
    }

    private void assertLogDirectory(HttpServerUtility server)
    {
        var logdir = server.MapPath("~/logs/");
        if (!Directory.Exists(logdir))
            Directory.CreateDirectory(logdir);
    }

    public void Dispose()
    {
    }
}

I just wrote it to the filesystem. You could use kernel file transactions or an exclusive lock or whatever, but because I know I will only write to this file from here, I chose a simple private semaphore.

I added this in Default.aspx.cs just to test:

protected void Page_Load(object sender, EventArgs e)
{
    throw new ApplicationException("wtf, something wrong??!");
}

Created file ErrorHandler.aspx with this in it (some parts omitted for shortness):

<body>
    <h2>Error handler</h2>
    <form id="form1" runat="server">
        <p>Last error: <asp:Label ID="lblError" runat="server" /></p>
    </form>
</body>

And here's the page-class:

public partial class ErrorHandler : Page
{
    public ErrorHandler()
    {
        Load += ErrorHandler_Load;
    }

    private void ErrorHandler_Load(object sender, EventArgs e)
    {
        Response.AddHeader("X-TESTING", "Yes it works...");
        lblError.Text = Server.GetLastError() == null ? "noo, why is it null?!?" : Server.GetLastError().Message;
    }
}

Finally, I get these response headers at the ErrorHandler.aspx:

Server: ASP.NET Development Server/9.0.0.0
Date: Mon, 23 Feb 2009 00:37:45 GMT
X-AspNet-Version: 2.0.50727
X-TESTING: Yes it works...
Cache-Control: private
Content-Type: text/html; charset=utf-8
Content-Length: 725
Connection: Close
200 OK

If you try to add to Response.Headers-collection directly you need to run IIS in pipeline mode: http://forums.asp.net/p/1253457/2323117.aspx

I hope this helps! Cheers Henrik

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