ASP.NET MVC Custom Error Handling Application_Error Global.asax?

Instead of creating a new route for that, you could just redirect to your controller/action and pass the information via querystring. For instance:

protected void Application_Error(object sender, EventArgs e) {
  Exception exception = Server.GetLastError();
  Response.Clear();

  HttpException httpException = exception as HttpException;

  if (httpException != null) {
    string action;

    switch (httpException.GetHttpCode()) {
      case 404:
        // page not found
        action = "HttpError404";
        break;
      case 500:
        // server error
        action = "HttpError500";
        break;
      default:
        action = "General";
        break;
      }

      // clear error on server
      Server.ClearError();

      Response.Redirect(String.Format("~/Error/{0}/?message={1}", action, exception.Message));
    }

Then your controller will receive whatever you want:

// GET: /Error/HttpError404
public ActionResult HttpError404(string message) {
   return View("SomeView", message);
}

There are some tradeoffs with your approach. Be very very careful with looping in this kind of error handling. Other thing is that since you are going through the asp.net pipeline to handle a 404, you will create a session object for all those hits. This can be an issue (performance) for heavily used systems.

To answer the initial question "how to properly pass routedata to error controller?":

IController errorController = new ErrorController();
errorController.Execute(new RequestContext(new HttpContextWrapper(Context), routeData));

Then in your ErrorController class, implement a function like this:

[AcceptVerbs(HttpVerbs.Get)]
public ViewResult Error(Exception exception)
{
    return View("Error", exception);
}

This pushes the exception into the View. The view page should be declared as follows:

<%@ Page Language="C#" Inherits="System.Web.Mvc.ViewPage<System.Exception>" %>

And the code to display the error:

<% if(Model != null) { %>  <p><b>Detailed error:</b><br />  <span class="error"><%= Helpers.General.GetErrorMessage((Exception)Model, false) %></span></p> <% } %>

Here is the function that gathers the all exception messages from the exception tree:

    public static string GetErrorMessage(Exception ex, bool includeStackTrace)
    {
        StringBuilder msg = new StringBuilder();
        BuildErrorMessage(ex, ref msg);
        if (includeStackTrace)
        {
            msg.Append("\n");
            msg.Append(ex.StackTrace);
        }
        return msg.ToString();
    }

    private static void BuildErrorMessage(Exception ex, ref StringBuilder msg)
    {
        if (ex != null)
        {
            msg.Append(ex.Message);
            msg.Append("\n");
            if (ex.InnerException != null)
            {
                BuildErrorMessage(ex.InnerException, ref msg);
            }
        }
    }

I struggled with the idea of centralizing a global error handling routine in an MVC app before. I have a post on the ASP.NET forums.

It basically handles all your application errors in the global.asax without the need for an error controller, decorating with the [HandlerError] attribute, or fiddling with the customErrors node in the web.config.

Perhaps a better way of handling errors in MVC is to apply the HandleError attribute to your controller or action and update the Shared/Error.aspx file to do what you want. The Model object on that page includes an Exception property as well as ControllerName and ActionName.

This may not be the best way for MVC ( https://stackoverflow.com/a/9461386/5869805 )

Below is how you render a view in Application_Error and write it to http response. You do not need to use redirect. This will prevent a second request to server, so the link in browser's address bar will stay same. This may be good or bad, it depends on what you want.

Global.asax.cs

protected void Application_Error()
{
    var exception = Server.GetLastError();
    // TODO do whatever you want with exception, such as logging, set errorMessage, etc.
    var errorMessage = "SOME FRIENDLY MESSAGE";

    // TODO: UPDATE BELOW FOUR PARAMETERS ACCORDING TO YOUR ERROR HANDLING ACTION
    var errorArea = "AREA";
    var errorController = "CONTROLLER";
    var errorAction = "ACTION";
    var pathToViewFile = $"~/Areas/{errorArea}/Views/{errorController}/{errorAction}.cshtml"; // THIS SHOULD BE THE PATH IN FILESYSTEM RELATIVE TO WHERE YOUR CSPROJ FILE IS!

    var requestControllerName = Convert.ToString(HttpContext.Current.Request.RequestContext?.RouteData?.Values["controller"]);
    var requestActionName = Convert.ToString(HttpContext.Current.Request.RequestContext?.RouteData?.Values["action"]);

    var controller = new BaseController(); // REPLACE THIS WITH YOUR BASE CONTROLLER CLASS
    var routeData = new RouteData { DataTokens = { { "area", errorArea } }, Values = { { "controller", errorController }, {"action", errorAction} } };
    var controllerContext = new ControllerContext(new HttpContextWrapper(HttpContext.Current), routeData, controller);
    controller.ControllerContext = controllerContext;

    var sw = new StringWriter();
    var razorView = new RazorView(controller.ControllerContext, pathToViewFile, "", false, null);
    var model = new ViewDataDictionary(new HandleErrorInfo(exception, requestControllerName, requestActionName));
    var viewContext = new ViewContext(controller.ControllerContext, razorView, model, new TempDataDictionary(), sw);
    viewContext.ViewBag.ErrorMessage = errorMessage;
    //TODO: add to ViewBag what you need
    razorView.Render(viewContext, sw);
    HttpContext.Current.Response.Write(sw);
    Server.ClearError();
    HttpContext.Current.Response.End(); // No more processing needed (ex: by default controller/action routing), flush the response out and raise EndRequest event.
}

View

@model HandleErrorInfo
@{
    ViewBag.Title = "Error";
    // TODO: SET YOUR LAYOUT
}
<div class="">
    ViewBag.ErrorMessage
</div>
@if(Model != null && HttpContext.Current.IsDebuggingEnabled)
{
    <div class="" style="background:khaki">
        <p>
            <b>Exception:</b> @Model.Exception.Message <br/>
            <b>Controller:</b> @Model.ControllerName <br/>
            <b>Action:</b> @Model.ActionName <br/>
        </p>
        <div>
            <pre>
                @Model.Exception.StackTrace
            </pre>
        </div>
    </div>
}