How to create dynamic custom error page depending on status code?

I'm trying to create dynamic custom web-form error page where content changes according to the status code. I have configured httpErrors in web.config like this:

<httpErrors errorMode="Custom" defaultPath="\errorpages\ErrorPage.aspx" defaultResponseMode="ExecuteURL" existingResponse="Auto" allowAbsolutePathsWhenDelegated="true" >
      <remove statusCode="503" subStatusCode="-1" />
      <remove statusCode="500" subStatusCode="-1" />
      <remove statusCode="401" subStatusCode="-1" />
      <remove statusCode="404" subStatusCode="-1" />
      <error statusCode="503" responseMode="ExecuteURL" path="/errorpages/ErrorPage.aspx" />
      <error statusCode="500" responseMode="ExecuteURL" path="/errorpages/ErrorPage.aspx" />
      <error statusCode="401" responseMode="ExecuteURL" path="/errorpages/ErrorPage.aspx" />
      <error statusCode="404" responseMode="ExecuteURL" path="/errorpages/ErrorPage.aspx" />
    </httpErrors>

QUESTION: How can I get the status code in code behind file before the page renders so it changes the content depending on the status code?

I wish to have 1 file for all the errors and not 4 different files.

I tried to use Server.GetLastError(), but it's always null whether I put it in Global.asax Page_Error(object sender, EventArgs e) method or code behind file.

Solution

You could add a QueryString with the status code and handle that on the error page to show the correct contents. So first add the code as Querystring

<error statusCode="404" path="/errorpages/ErrorPage.aspx?code=404" />

Then the ErrorPage.aspx page you can do something like this

<%@ Page Language="C#" %>

<script runat="server">

    int error = 0;
    string errorname = "";

    protected void Page_Load(object sender, EventArgs e)
    {
        //check if there is a code
        if (Request.QueryString["code"] != null)
        {
            //is the code a correct number
            int.TryParse(Request.QueryString["code"], out error);
        }

        if (error == 403)
        {
            errorname = error + " Forbidden";
        }
        else if (error == 404)
        {
            errorname = error + " Not Found";
        }
        else if (error == 500)
        {
            errorname = error + " Server Error";
        }
        else
        {
            errorname = "Unhandled Error";
            error = 500;
        }
    }

</script>

<html>
<head>
    <title>MySiteName - <%= errorname %></title>
</head>
<body>
    <center>
        <a href="/">
            <img src="/images/<%= error %>.png" border="0" vspace="50">
        </a>
    </center>
</body>
</html>