根据 Alexander 的回答做了一些研究，我也在我的 AspExceptionHandler 中实现了 IHttpHandler。
我的班级现在看起来像：
public class AspExceptionHandler : IHttpModule, IHttpHandler
{
public void Dispose() { }
public void Init(HttpApplication context)
{
context.Error += new EventHandler(ErrorHandler);
}
private void ErrorHandler(object sender, EventArgs e)
{
HttpApplication application = (HttpApplication)sender;
try
{
// Gather information
Exception currentException = application.Server.GetLastError(); ;
String errorPage = "http://companywebsite.be/error.aspx";
HttpException httpException = currentException as HttpException;
if (httpException == null || httpException.GetHttpCode() != 404)
{
application.Server.Transfer(errorPage, true);
}
//The error is a 404
else
{
// Continue
application.Server.ClearError();
String shouldMail404 = true;
//Try and redirect to the proper page.
String requestedFile = application.Request.Url.AbsolutePath.Trim('/').Split('/').Last();
// Redirect if required
String redirectURL = getRedirectURL(requestedFile.Trim('/'));
if (!String.IsNullOrEmpty(redirectURL))
{
//Redirect to the proper URL
}
//If we can't redirect properly, we set the statusCode to 404.
else
{
//Report the 404
}
}
}
catch (Exception ex)
{
ExceptionCatcher.FillWebException(HttpContext.Current, ref ex);
ExceptionCatcher.CatchException(ex);
}
}
public bool IsReusable
{
get { return true; }
}
public void ProcessRequest(HttpContext context)
{
if (!File.Exists(context.Request.PhysicalPath))
{
throw new HttpException(404, String.Format("The file {0} does not exist", context.Request.PhysicalPath));
}
else
{
context.Response.TransmitFile(context.Request.PhysicalPath);
}
}
}
在 ProcessRequest 方法（IHttpHandler 要求）中，我检查文件是否存在。
如果它不存在，我会抛出一个被我的类的 HttpModule 部分捕获的 HttpException。
我的 web.config 中的 system.webServer 节点现在如下所示：
<modules runAllManagedModulesForAllRequests="true">
<add name="AspExceptionHandler" type="Company.Exceptions.AspExceptionHandler, Company.Exceptions" preCondition="managedHandler" />
</modules>
<handlers>
<add name="AspExceptionHandler" type="Company.Exceptions.AspExceptionHandler, Company.Exceptions" verb="*" path="*.html" />
</handlers>
我在这篇文章中找到了答案：HttpHandler fire only if file doesn't exist