How to show a custom 404 page in ASP.NET without redirect?

Question

When a request is 404 in ASP.NET on IIS 7 i want a custom error page to be displayed. The URL in the address bar should not change, so no redirect. How can i do this?

Answer 1

As a general ASP.NET solution, in the customErrors section in the web.config, add the redirectMode="ResponseRewrite" attribute.

<customErrors mode="On" redirectMode="ResponseRewrite">
  <error statusCode="404" redirect="/404.aspx" />
</customErrors>

Note: this internally uses a Server.Transfer() so the redirect must be an actual file on the webserver. It can't be an MVC route.

Answer 2

I use an http module to handle this. It works for other types of errors, not just 404s, and allows you to continue using the custom errors web.config section to configure which page is displayed.

public class CustomErrorsTransferModule : IHttpModule
{
    public void Init(HttpApplication context)
    {
        context.Error += Application_Error;
    }

    public void Dispose()  {  }

    private void Application_Error(object sender, EventArgs e)
    {
        var error = Server.GetLastError();
        var httpException = error as HttpException;
        if (httpException == null)
            return;

        var section = ConfigurationManager.GetSection("system.web/customErrors") as CustomErrorsSection;
        if (section == null)
            return;

        if (!AreCustomErrorsEnabledForCurrentRequest(section))
            return;

        var statusCode = httpException.GetHttpCode();
        var customError = section.Errors[statusCode.ToString()];

        Response.Clear();
        Response.StatusCode = statusCode;

        if (customError != null)
            Server.Transfer(customError.Redirect);
        else if (!string.IsNullOrEmpty(section.DefaultRedirect))
            Server.Transfer(section.DefaultRedirect);
    }

    private bool AreCustomErrorsEnabledForCurrentRequest(CustomErrorsSection section)
    {
        return section.Mode == CustomErrorsMode.On ||
               (section.Mode == CustomErrorsMode.RemoteOnly && !Context.Request.IsLocal);
    }

    private HttpResponse Response
    {
        get { return Context.Response; }
    }

    private HttpServerUtility Server
    {
        get { return Context.Server; }
    }

    private HttpContext Context
    {
        get { return HttpContext.Current; }
    }
}

enable in your web.config in the same way as any other module

<httpModules>
     ...
     <add name="CustomErrorsTransferModule" type="WebSite.CustomErrorsTransferModule, WebSite" />
     ...
</httpModules>