如何让404错误调用自定义404处理方法？

Alright, let's walk through getting your custom 404 error handler up and running properly in ASP.NET. You've already found the two main configuration paths, so let's break them down, fill in the gaps, and cover the key details you need to avoid headaches.

1. 传统ASP.NET框架（system.web节点） #

This works best for classic ASP.NET or when using IIS in classic mode. Here's the complete configuration you started with, plus context:

<system.web>
  <customErrors mode="On" defaultRedirect="~/home">
    <error redirect="~/home/page_not_found" statusCode="404" />
  </customErrors>
</system.web>

• mode="On": Forces custom errors to be shown to all users. For development, use RemoteOnly instead—this lets you see detailed error messages locally while showing custom pages to remote visitors.
• defaultRedirect="~/home": The fallback page for any unhandled errors (like 500s) that don't have a specific rule.
• <error redirect="~/home/page_not_found" statusCode="404" />: Maps 404 errors directly to your HomeController's Page_NotFound action method. Just make sure that action exists and returns the custom view you've built.

2. IIS 7+集成模式（system.webServer节点） #

If you're running on IIS 7 or later (most modern deployments), this configuration is more reliable because it works at the server level. Here's the completed version of what you started:

<system.webServer>
  <httpErrors errorMode="Custom" existingResponse="Replace">
    <remove statusCode="404" subStatusCode="-1" />
    <remove statusCode="500" subStatusCode="-1" />
    <!-- Option 1: Use a static HTML file -->
    <error statusCode="404" path="/Error404.html" responseMode="File" />
    <!-- Option 2: Use an MVC controller action (uncomment if needed) -->
    <!-- <error statusCode="404" path="/home/page_not_found" responseMode="ExecuteURL" /> -->
  </httpErrors>
</system.webServer>

• errorMode="Custom": Disables IIS's default error pages entirely.
• existingResponse="Replace": Ensures your custom response replaces any existing error output (prevents conflicts with customErrors settings).
• <remove> tags: Clears IIS's default 404/500 configurations so your custom rules take precedence.
• responseMode: Choose based on your needs:
  • File: Serves a static HTML file directly (fast, no server-side processing).
  • ExecuteURL: Runs the specified URL (like an MVC controller action) and returns its output (great for dynamic 404 pages).
  • Redirect: Sends a 302 redirect to the error page (not recommended for 404s, since it changes the URL in the browser and can hurt SEO).

• MVC Routing Check: If you're using ASP.NET MVC, make sure your route configuration doesn't catch all unknown URLs (like a wildcard route) — that can bypass the 404 error handling. If you have a wildcard route, add a constraint or handle 404s in Global.asax's Application_Error event.
• Don't Mix Configs: If you set up both system.web and system.webServer settings, IIS in integrated mode will prioritize the system.webServer rules. Pick one based on your environment.
• Test Properly: Local testing might still show detailed errors even with mode="On" — try deploying to a staging server or use RemoteOnly and access the site from another machine to verify your custom 404 works.

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