如何精准捕获404与500错误？求代码具体放置位置

Hey there! Let’s get your 404 and 500 error handling sorted out using your preferred tools—global.asax and web.config—since you’re on IIS 8 which plays nicely with these approaches. I’ll break down exactly where to put the code and why your previous attempts might have failed.

Web.Config Solution (IIS 8+) #

This is your simplest option, but the key is placing the code in the correct hierarchy within your root web.config (the one in your project’s root folder, not a subfolder). Here’s the full setup:

1. <system.web> Section (Handles ASP.NET Dynamic Errors) #

Add this directly inside the root <configuration> node (alongside sections like <appSettings> or <connectionStrings>):

<system.web>
  <customErrors mode="On" defaultRedirect="~/Error/500" redirectMode="ResponseRewrite">
    <error statusCode="404" redirect="~/Error/404" />
    <error statusCode="500" redirect="~/Error/500" />
  </customErrors>
</system.web>

• mode="On": Ensures errors are redirected even when testing locally (if you used RemoteOnly, you wouldn’t see errors on your dev machine)
• redirectMode="ResponseRewrite": Keeps the original error URL in the browser bar (better for SEO and user experience than a full redirect)

2. <system.webServer> Section (Handles Static File Errors) #

This catches missing static files (like a non-existent .html or .css) that the <system.web> section won’t pick up. Add this also inside the root <configuration> node:

<system.webServer>
  <httpErrors errorMode="Custom" existingResponse="Replace">
    <remove statusCode="404" subStatusCode="-1" />
    <remove statusCode="500" subStatusCode="-1" />
    <error statusCode="404" path="/Error/404" responseMode="ExecuteURL" />
    <error statusCode="500" path="/Error/500" responseMode="ExecuteURL" />
  </httpErrors>
</system.webServer>

• existingResponse="Replace": Tells IIS to override any default error pages
• responseMode="ExecuteURL": Runs your custom error page without changing the URL

Required Error Controller & Routes #

You’ll need an ErrorController to serve the error views, plus route mappings. Here’s the controller code:

public class ErrorController : Controller
{
    public ActionResult NotFound()
    {
        Response.StatusCode = 404;
        return View(); // Make sure you have a NotFound.cshtml view
    }

    public ActionResult ServerError()
    {
        Response.StatusCode = 500;
        return View(); // Make sure you have a ServerError.cshtml view
    }
}

And add these routes to your RouteConfig.cs:

routes.MapRoute(
    name: "Error404",
    url: "Error/404",
    defaults: new { controller = "Error", action = "NotFound" }
);

routes.MapRoute(
    name: "Error500",
    url: "Error/500",
    defaults: new { controller = "Error", action = "ServerError" }
);

Global.asax Solution #

If you’d rather handle errors via code, here’s how to fix your global.asax.cs setup to catch both 404 and 500 errors:

1. Application_Error (Catches Unhandled Exceptions = 500 Errors) #

This method fires whenever an unhandled exception occurs. Make sure you include Server.ClearError() to stop IIS from showing its default page:

protected void Application_Error(object sender, EventArgs e)
{
    var exception = Server.GetLastError();
    var httpException = exception as HttpException;

    // Clear the error so IIS doesn't override your custom page
    Server.ClearError();

    var routeData = new RouteData();
    routeData.Values["controller"] = "Error";

    if (httpException != null)
    {
        switch (httpException.GetHttpCode())
        {
            case 404:
                routeData.Values["action"] = "NotFound";
                Response.StatusCode = 404;
                break;
            case 500:
                routeData.Values["action"] = "ServerError";
                Response.StatusCode = 500;
                break;
            default:
                routeData.Values["action"] = "ServerError";
                Response.StatusCode = 500;
                break;
        }
    }
    else
    {
        // Non-HTTP exceptions (like null references) count as 500 errors
        routeData.Values["action"] = "ServerError";
        Response.StatusCode = 500;
    }

    // Execute the error controller action
    var errorController = new ErrorController();
    errorController.Execute(new RequestContext(new HttpContextWrapper(Context), routeData));
}

2. Application_EndRequest (Catches 404s Missed by Application_Error) #

Some 404s (like invalid routes that never hit a controller) won’t trigger Application_Error, so add this to catch them:

protected void Application_EndRequest()
{
    if (Response.StatusCode == 404 && !Response.IsRequestBeingRedirected)
    {
        Server.ClearError();
        Response.RedirectToRoute("Error404");
    }
}

Note: This relies on the Error404 route we defined earlier in RouteConfig.cs.

Why Your Previous Attempts Failed #

• Web.Config: You probably placed the code in the wrong section (not directly under <configuration>) or used mode="RemoteOnly" which hides errors locally.
• Global.asax: You might have forgotten to call Server.ClearError() (which lets IIS show its default page instead of yours) or didn’t set the Response.StatusCode correctly.

内容的提问来源于stack exchange，提问作者Bradley William Elko