Develop Reference

Error 520 on asp.net web app in production

Our asp.net web app is returning Error 520 on a page in production though no error in development.
This particular error skips our custom error page and present a different error page with Cloudflare.
As in this image
When you inspect the page, you see
"Error: Promised response from onMessage listener went out of scope"
in console
No error is logged by our logging tools, and we cannot tell why it's skipping our custom error page.
Any direction as to how to solve this?

Override the default error page 500.30 - ASP.NET Core app failed to start

I am attempting to create a custom error page display to run in my app when there is an issue with the database connection string.
When I alter the string to something invalid, I receive the error mentioned in the title above.
Is there any way to override this page and show a more informative one that would tell me that my DB connection string is wrong?
There is a InvalidOperationException that is thrown in the Startup.cs file, but I'm unsure on how to extract this from the startup file and use it, when my app fails to start in the first place.
Is this possible to do?

You can disable the default error page by using the disableStartUpErrorPage="true" setting in web.config for the IIS hosting module. This will just fallback to another custom error page of your choosing served by IIS, rather than allowing you to show a dynamic custom page.
Documentation

Angular app in UWP project does not load second page

I have an Angular project that I am trying to incorporate into a Universal Windows Platform (UWP) application. I have tested the Angular code in my web browser, and all works as expected there. When I try to run the Angular application, that is housed with-in the UWP app bundle, I am unable to navigate to pages.
Technical Stack
I have an Angular web application that I have tested in a web browser. it works fine. I run the following command to build the application.
ng build --base-href=/Assets/Web
I then take the built files, and copy them into my UWP project. The folder structure is below.
-- Root
---- Assets
------ Web
-------- assets
-------- index.html
-------- main.js
-------- main.js.map
-------- ect...
Basically, the Angular project is put into the /Assets/Web folder within my UWP project.
Then in my MainPage.xaml file I have a WebView with the following code.
<WebView
Name="webView"
Source="ms-appx-web:///Assets/Web/index.html"
NavigationStarting="WebView_NavigationStarting"></WebView>
Problem
When I run my UWP app I see the initial landing page of my Angular project and all images/styles load correctly. When I click on to the next page in my Angular app, via a this.router.navigate([RouterLinks.secondPage]) command then the screen just turns white. I see the following in my JavaScript console.
HTML1300: Navigation occurred.
secondPage
HTML1527: DOCTYPE expected. Consider adding a valid HTML5 doctype: "!DOCTYPE html>".
secondPage (1,1)
It looks like the routing started to the correct page, but it seems that the rendering never completed.
On initial app load I see a bunch of errors in my VS Output window.
'Identity.exe' (Script): Loaded 'Script Code (WebView/3.0)'.
SourceMap ms-appx-web://d4abdafc-2387-4a97-9c8e-b370601c7288/Assets/Web/runtime.js.map read failed: The URI prefix is not recognized..SourceMap ms-appx-web://d4abdafc-2387-4a97-9c8e-b370601c7288/Assets/Web/polyfills.js.map read failed: The URI prefix is not recognized..Exception was thrown at line 1145, column 35 in ms-appx-web://d4abdafc-2387-4a97-9c8e-b370601c7288/Assets/Web/polyfills.js
0x800a139e - JavaScript runtime error: 2
SourceMap ms-appx-web://d4abdafc-2387-4a97-9c8e-b370601c7288/Assets/Web/styles.js.map read failed: The URI prefix is not recognized..SourceMap ms-appx-web://d4abdafc-2387-4a97-9c8e-b370601c7288/Assets/Web/scripts.js.map read failed: The URI prefix is not recognized..Exception was thrown at line 1323, column 21 in ms-appx-web://d4abdafc-2387-4a97-9c8e-b370601c7288/Assets/Web/scripts.js
0x800a139e - JavaScript runtime error: SyntaxError
Exception was thrown at line 1341, column 21 in ms-appx-web://d4abdafc-2387-4a97-9c8e-b370601c7288/Assets/Web/scripts.js
0x800a139e - JavaScript runtime error: SyntaxError
Exception was thrown at line 10757, column 21 in ms-appx-web://d4abdafc-2387-4a97-9c8e-b370601c7288/Assets/Web/scripts.js
0x800a138f - JavaScript runtime error: Array.prototype.filter: 'this' is null or undefined
Exception was thrown at line 10757, column 21 in ms-appx-web://d4abdafc-2387-4a97-9c8e-b370601c7288/Assets/Web/scripts.js
0x800a138f - JavaScript runtime error: Array.prototype.map: 'this' is null or undefined
Then after the router navigation I see the following errors in the Output window.
Exception was thrown at line 67843, column 4 in ms-appx-web://d4abdafc-2387-4a97-9c8e-b370601c7288/Assets/Web/vendor.js
0x800a139e - JavaScript runtime error: SyntaxError
Exception was thrown at line 63600, column 13 in ms-appx-web://d4abdafc-2387-4a97-9c8e-b370601c7288/Assets/Web/vendor.js
0x800a139e - JavaScript runtime error: [object Object]
Exception was thrown at line 63658, column 9 in ms-appx-web://d4abdafc-2387-4a97-9c8e-b370601c7288/Assets/Web/vendor.js
0x800a139e - JavaScript runtime error: [object Object]
This is when I see the blank screen.
I know this issue has something to do with these errors, but not sure how to fix them. I even tried to put the entire Angular project straight into the root directory of my UWP project, and still had the same issue.
One thing to mention is I did have this as a UWP Web Context application, and during that time everything worked correctly. Unfortunately, I had to move away from that template since I had a need to integrate API calls using the native layer. Right now this is a simple blank UWP project.
Does anyone have any idea what I could be doing wrong?
Thanks!

Turns out the issue was the submit input type was causing the form to submit and would actually load the URL from the router.navigate command. It is a very strange issue, and seems to only occur in the Edge browser. All other browsers the submit event was prevented internally by Angular.
I fixed the issue by passing the event into the click function via the HTML template, and called the preventDefault() function on the event within my custom handler. This stopped the browser from submitting the form and thus sending the browser to a different URL that does not exist. See example below.
In form.component.html
<input id="submit" role="button" (click)="myEventHandler($event);" value="Submit" type="submit" />
In form.component.ts
myEventHandler = function (e) {
if (e) {
e.preventDefault();
}
this.router.navigate(["nextPage"]);
}

Custom error pages

I have configured the HttpErrors section in my web.config.
My custom error page is shown, but not when an exception occurs outside a web request (eg: Application startup, module initializing...), I get the following error:
The page cannot be displayed because an internal server error has occurred.
How can I make sure that my custom error page is ALWAYS shown?

You can't. The customer error pages are being configured at the start of the ASP.NET application, which fails in your case. There is no ASP.NET pipeline to handle the error pages.
Usually there is not much to do, besides making a proxy that filters out HTTP 500 error messages and replaces it with another message.

Does ASP.NET Custom Errors override the actual HTTP response code?

If I were to specify a page for redirect using the ASP.NET Custom Errors feature, would my application still spit out the HTTP status code for that particular error?
For example if had a line in my web.config that had all Internal Server Errors redirect to Errors/500.aspx and then I encountered a 500 error, I would then be redirected to my custom Errors/500.aspx page. Will my application still respond with a HTTP 500 at any point in this exchange?
I'm just curious...

No, because your browser would interpret that 500 response as an error and show you it's error. Instead, it probably responds with one of the redirect status codes to send you to the defined error page.