Many developers encounter this frustrating scenario: your ASP.NET MVC3 application runs perfectly in Visual Studio's debugger, but when deployed to IIS 7.0, all static files (CSS, JavaScript, images) mysteriously return 404 errors. Let's dive deep into solving this common IIS configuration issue.

The primary culprit is usually IIS's Static Content feature being disabled. IIS 7.0 doesn't serve static files by default unless specifically configured to do so. This differs from the development server in Visual Studio which handles static files automatically.

First, check if the Static Content feature is installed:

%windir%\system32\inetsrv\appcmd list config /section:staticContent

If you get an error or missing mime types, you'll need to install the feature.

For Windows Vista/Server 2008:

1. Open Server Manager
2. Navigate to Roles → Web Server (IIS) → Add Role Services
3. Check "Static Content" under Common HTTP Features
4. Complete the installation wizard

If you can't modify server settings, add this to your web.config:

<system.webServer>
    <staticContent>
        <mimeMap fileExtension=".css" mimeType="text/css" />
        <mimeMap fileExtension=".js" mimeType="application/javascript" />
    </staticContent>
</system.webServer>

Verify that the StaticFile handler is properly mapped:

<system.webServer>
    <handlers>
        <add name="StaticFile" path="*" verb="*" 
            type="System.Web.StaticFileHandler" />
    </handlers>
</system.webServer>

Ensure the IIS_IUSRS group has read permissions on:

• Your application folder
• The specific static files
• The containing directories

Create a test.html in your root directory with basic content. If this works but CSS/JS still fails, it confirms a mime type or handler issue rather than permissions.

For projects where you can't modify server configuration, consider storing static files in a virtual directory:

// In IIS Manager
1. Right-click your site → Add Virtual Directory
2. Alias: "Content"
3. Physical path: (your static files location)
4. Set proper permissions

Enable detailed logging to pinpoint the exact failure point:

1. In IIS Manager, select your site
2. Open "Failed Request Tracing"
3. Create a new rule for 404 errors
4. Reproduce the issue and check the logs

After implementing fixes:

1. Clear browser cache (Ctrl+F5)
2. Check IIS logs (%SystemDrive%\inetpub\logs\LogFiles)
3. Verify file paths are case-sensitive (especially on deployed servers)

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When deploying an ASP.NET MVC3 application to IIS 7 on Windows Vista, you might encounter a frustrating scenario where your pages load but all CSS and JavaScript files return 404 errors. This typically happens despite everything working perfectly in Visual Studio's debug environment.

Several factors could contribute to this issue:

1. Missing Static Content feature in IIS
2. Incorrect MIME type configuration
3. Authorization rules blocking access
4. Handler Mappings interfering with static files

First, check if the Static Content feature is installed:

1. Open "Turn Windows features on or off"
2. Navigate to: Internet Information Services > World Wide Web Services > Common HTTP Features
3. Ensure "Static Content" is checked

If Static Content is installed but files still aren't served, try this web.config addition:

<system.webServer>
  <handlers>
    <remove name="StaticFile" />
    <add name="StaticFile" path="*" verb="*" modules="StaticFileModule" 
         resourceType="Either" requireAccess="Read" />
  </handlers>
</system.webServer>

Ensure the IIS_IUSRS group has read access to your static files:

1. Right-click your site's folder
2. Properties > Security > Edit > Add
3. Enter "IIS_IUSRS" and grant Read permissions

Some CSS/JS files might need explicit MIME types. Add these to web.config:

<system.webServer>
  <staticContent>
    <mimeMap fileExtension=".css" mimeType="text/css" />
    <mimeMap fileExtension=".js" mimeType="application/javascript" />
  </staticContent>
</system.webServer>

If your static files are in a virtual directory, ensure it's properly configured in IIS Manager with the same permissions as the main application.

Enable tracing to get detailed error information:

1. In IIS Manager, select your site
2. Open "Failed Request Tracing Rules"
3. Create a new rule for 404 errors
4. Examine the trace log for clues