Logging of errors and tracing bugs is a very important part of development, so we’ve put together a script which records all relevant information at the point of the error, and logs it to our Error Tracking System, as well as sends out an email to the site administrator with the error.
First, we want to record the website address where this error occurred, so we get this in 2 parts from

ErrorPath = "http://" + Request.ServerVariables["HTTP_HOST"] + Request.Path;

RawUrl = Request.RawUrl;

Next, we get the last exception from the server stack

Exception ErrorInfo = Server.GetLastError().GetBaseException();
if (ErrorInfo != null)
{
    strError += "Error Message: " + ErrorInfo.Message +
        "nError Source: " + ErrorInfo.Source +
        "nError Target Site: " + ErrorInfo.TargetSite;
}

The Error Type and Source

ErrorType = Server.GetLastError().GetType().ToString();

if (Server.GetLastError().GetType() == typeof(HttpUnhandledException))
{
    ErrorSource = (Server.GetLastError() as HttpUnhandledException).GetHtmlErrorMessage();
}

The QueryString data, Post data and Session variables

for (int i = 0; i < Context.Request.QueryString.Count; i++)
{
    QueryStringData += Context.Request.QueryString.Keys[i] + ":tt" + Context.Request.QueryString[i] + "n";
}

for (int i = 0; i < Context.Request.Form.Count; i++)
{
    PostData += Context.Request.Form.Keys[i] + ":tt" + Context.Request.Form[i] + "n";
}

for (int i = 0; i < Context.Session.Count; i++)
{
    SessionData += Context.Session.Keys[i] + ":tt" + Context.Session[i] + "n";
}

If the user is authenticated, we can get his username

if (User.Identity.IsAuthenticated)
{
    UserAccountName = User.Identity.Name;
}

Next, we record the Stack Trace

StackTrace = Server.GetLastError().StackTrace;

Last, the Server Variables

for (int i = 0; i < Context.Request.ServerVariables.Count; i++)
{
    ServerVariables += Context.Request.ServerVariables.Keys[i] + ":tt" + Context.Request.ServerVariables[i] + "n";
}

And now we have all the information we need to later retrace and duplicate the error to allow us to fix it.
Our Error Tracking System, once released will allow you to group the errors by Url and Error Type, and view all this information in a very helpful manner.

Finally, we send an email to the developer with all the information we’ve gathered

System.Net.Mail.MailMessage m = new System.Net.Mail.MailMessage();
m.To.Add(System.Configuration.ConfigurationManager.AppSettings["DebugEmail"]);
m.Subject = "Error occurred on " + Request.ServerVariables["HTTP_HOST"];
m.IsBodyHtml = false;
m.Body = strError;
System.Net.Mail.SmtpClient c = new System.Net.Mail.SmtpClient();
try
{
    c.Send(m);
}
catch { }

To make our life easier when deploying this script to many websites, we store a few variables in web.config under appSettings

And finally, we set our mail server

Complete code to be placed in your global.asax

void Application_Error(object sender, EventArgs e)
{
    ErrorTracking.TrackingInfo info = new ErrorTracking.TrackingInfo();
    info.ErrorPath = "http://" + Request.ServerVariables["HTTP_HOST"] + Request.Path;
    info.RawUrl = Request.RawUrl;
    string strError = "Error in: http://" + Request.ServerVariables["HTTP_HOST"] + Request.Path +
    "nUrl: " + Request.RawUrl + "nn";
    try
    {
        // Get the exception object for the last error message that occured.
        Exception ErrorInfo = Server.GetLastError().GetBaseException();
        if (ErrorInfo != null)
        {
            strError += "Error Message: " + ErrorInfo.Message +
                "nError Source: " + ErrorInfo.Source +
                "nError Target Site: " + ErrorInfo.TargetSite;
            info.ErrorMessage = ErrorInfo.Message;
        }
        strError += "Error Type: " + Server.GetLastError().GetType().ToString();
        info.ErrorType = Server.GetLastError().GetType().ToString();
        if (Server.GetLastError().GetType() == typeof(HttpUnhandledException))
        {
            strError += "Source: " + (Server.GetLastError() as HttpUnhandledException).GetHtmlErrorMessage();
            info.ErrorSource = (Server.GetLastError() as HttpUnhandledException).GetHtmlErrorMessage();
        }
        info.QueryStringData = "";
        strError += "nnQueryString Data:n-----------------n";
        // Gathering QueryString information
        for (int i = 0; i < Context.Request.QueryString.Count; i++)
        {
            strError += Context.Request.QueryString.Keys[i] + ":tt" + Context.Request.QueryString[i] + "n";
            info.QueryStringData += Context.Request.QueryString.Keys[i] + ":tt" + Context.Request.QueryString[i] + "n";
        }
        strError += "nPost Data:n----------n";
        info.PostData = "";
        // Gathering Post Data information
        for (int i = 0; i < Context.Request.Form.Count; i++)
        {
            strError += Context.Request.Form.Keys[i] + ":tt" + Context.Request.Form[i] + "n";
            info.PostData += Context.Request.Form.Keys[i] + ":tt" + Context.Request.Form[i] + "n";
        }
        strError += "nSession Data:n----------n";
        // Gathering Session information
        for (int i = 0; i < Context.Session.Count; i++)
        {
            strError += Context.Session.Keys[i] + ":tt" + Context.Session[i] + "n";
        }
        strError += "n";
        if (User.Identity.IsAuthenticated)
        {
            strError += "User:tt" + User.Identity.Name + "nn";
            info.UserAccountName = User.Identity.Name;
        }
        info.StackTrace = Server.GetLastError().StackTrace;
        strError += "Exception Stack Trace:n----------------------n" + Server.GetLastError().StackTrace +
            "nnServer Variables:n-----------------n";
        // Gathering Server Variables information
        info.ServerVariables = "";
        for (int i = 0; i < Context.Request.ServerVariables.Count; i++)
        {
            strError += Context.Request.ServerVariables.Keys[i] + ":tt" + Context.Request.ServerVariables[i] + "n";
            info.ServerVariables += Context.Request.ServerVariables.Keys[i] + ":tt" + Context.Request.ServerVariables[i] + "n";
        }
        strError += "n";
    }
    catch (Exception s) { strError += s.Message; }
    info.CompleteSource = strError;
    info.SiteKey = Guid.Parse(System.Configuration.ConfigurationManager.AppSettings["ErrorTrackingKey"]);
    info.IsRead = false;
    info.IsResolved = false;
    info.DateLogged = DateTime.Now;
    info.ErrorCause = ErrorTracking.ErrorCauses.Unknown;
    info.UserAccountID = Guid.Empty;
    info.ErrorSource = "";
    info.UserAccountName = "";
    ErrorTracking.ErrorTrackingServiceClient client = new ErrorTracking.ErrorTrackingServiceClient();
    client.SubmitError(info);
    // Send email to developer
    System.Net.Mail.MailMessage m = new System.Net.Mail.MailMessage();
    m.To.Add(System.Configuration.ConfigurationManager.AppSettings["DebugEmail"]);
    m.Subject = "Error occurred on " + Request.ServerVariables["HTTP_HOST"];
    m.IsBodyHtml = false;
    m.Body = strError;
    System.Net.Mail.SmtpClient c = new System.Net.Mail.SmtpClient();
    try
    {
        c.Send(m);
    }
    catch { }
}