An example of a fault tolerant API call from C# to an Angular front-end in Angular 8

ang_service.ts

//------------------------------------------------------------------------
//Angular Service (calling C#) and handling responses
//retrieves a list of Card, or an empty object and throws an error message 
//in console if the result isnt a list of Card objects
//------------------------------------------------------------------------

//GET REQUEST
getAllCards(): Observable<Card[]> {
    return this.http.get<Card[]>('/api/GetAllCards', {})
        .pipe(catchError(this.handleError<Card[]>('getAllCards')));
}

/*
//EXAMPLE POST REQUEST
getAllCards(): Observable<Card[]> {
    return this.http.post<Card[]>('/api/GetAllCards', {}) //note .get changed to .post
        .pipe(catchError(this.handleError<Card[]>('getAllCards',Card[]))); //note the Card[] param passed
}
*/

//handleError method in the Ang Service
  private handleError<T>(operation = 'operation', result?: T) {
      return (error: any): Observable<T> => {
          let res = "Service error: " + `${operation} failed\n`
          if (error instanceof HttpErrorResponse)
              res += 'Msg: '+error.message + '<br>Status: ' + error.status + '<br>Detail: ' + error.error + '<br>Text: ' + error.statusText + '<br>Object: ' + JSON.stringify(result)
          
          if (result != null) //aka is a POST request, pass obj data too
              res+=' Error with http request when passing object with a POST request: '+JSON.stringify(res))
          else
              res+=' for a GET request';
      
          console.log(res)
          // Let the app keep running by returning an empty result.
          return of(result as T);
      };
  }

c#_controller.cs

[Route("api")]
[ApiController] //so we dont have to call model.isValid any more. returns 400 status code if model invalid
public class MyController : Controller{
    //------------------------------------------------------------------------
    //C# API call. returns a List of objects or an HttpErrorResponse
    //------------------------------------------------------------------------

    [Route("GetAllCards"),HttpGet] //Change GET to POST if needed
    public ActionResult<IEnumerable<Card>> getCards()
    {
        try
        {
            return _mc.Cards.ToList(); //code to return a list of objects using EF Core
        }catch(Exception e)
        {
            //note that if you are returning a StatusCode the second param MUST be 
            //an object to be handled by the Angular catchError correctly. Thus the cast to Json
            return StatusCode(500, Json(e.Message)); 
        }
    }
}