Web API Global Exception Handler using an ExceptionFilterAttribute filter

Understanding what it means handling exceptions

Exceptions inside your code typically occur in the following scenarios:

1. Your code throws a specific exception with an explicit human-readable message that you would want to display the user to understand went wrong.
2. Your code threw an unexpected exception that you did not handle inside a try/catch function.

I think it is a code idea to handle this somewhere to return a proper error code to the user with a user-friendly friendly. Also a good opportunity to obscure sensitive data like a database error with details about your database tables.

Enter the ExceptionFilterAttribute

Creating the OnExceptionAttribute extending the ExceptionFilterAttribue

To get start I am going to create a file called OnExceptionAttribute. I place this file inside the App_Start folder. Here is a stubbed out file that will be filled in shortly.

using System.ComponentModel.DataAnnotations;
using System.Net;
using System.Net.Http;
using System.Web.Http.Filters;
namespace Api
{
    public class OnExceptionAttribute : ExceptionFilterAttribute
    {
        public override void OnException(HttpActionExecutedContext context)
        {
        }
    }
}

The class OnExceptionAttribute extends the built-in ExceptionFilterAttribute class. The next part is I create a function called OnException that overrides the main function in the ExceptionFilterAttribute. This will be expanded shortly.

Before this code will be executed I know to update the previously file App_Start/FilterConfig.cs. The new filter will be added to the HttpFilterCollection. Here is an updated file:

using System;
using System.Configuration;
using System.Web.Mvc;
namespace Api
{
    public class FilterConfig
    {
        public static void RegisterGlobalFilters(GlobalFilterCollection filters)
        {
        }
        /// 

        /// These filters are for WebApi controllers
        /// 
        /// 
        public static void RegisterGlobalFilters(System.Web.Http.Filters.HttpFilterCollection filters)
        {
            filters.Add(new OnExceptionAttribute());
        }
    }
}

Now when any exception occurs in our Web API our OnExceptionAttribute will be executed.

Let's now expand the OnExceptionAttribute function OnException to start handling a few exceptions and returning friendly HTTP Status Codes and response messages.

using System.ComponentModel.DataAnnotations;
using System.Net;
using System.Net.Http;
using System.Web.Http.Filters;
namespace Api
{
    public class OnExceptionAttribute : ExceptionFilterAttribute
    {
        public override void OnException(HttpActionExecutedContext context)
        {
            var exceptionType = context.Exception.GetType().ToString();
            var exception = context.Exception.InnerException ?? context.Exception;
            HttpResponseMessage httpResponseMessage;
            switch (exceptionType)
            {
                case "System.Data.DuplicateNameException":
                    httpResponseMessage = CreateHttpResponseMessage(context.Request, 
						HttpStatusCode.BadRequest, context.Exception);
                    break;
                case "System.Data.ObjectNotFoundException":
                case "System.Data.Entity.Core.ObjectNotFoundException":
                    httpResponseMessage = CreateHttpResponseMessage(context.Request, 
						HttpStatusCode.NotFound, context.Exception);
                    break;
                case "System.UnauthorizedAccessException":
					httpResponseMessage = CreateHttpResponseMessage(context.Request, 
						HttpStatusCode.Forbidden, context.Exception);
                    break;
                case "System.ComponentModel.DataAnnotations.ValidationException":
					httpResponseMessage = CreateHttpResponseMessage(context.Request, 
						HttpStatusCode.BadRequest, context.Exception as ValidationException);
                    break;
                case "System.Data.Entity.Infrastructure.DbUpdateException":
					httpResponseMessage = CreateHttpResponseMessage(context.Request, 
						HttpStatusCode.InternalServerError, "An error occurred, please try again.");
                    break;
                case "System.Data.Entity.Core.OptimisticConcurrencyException":
                case "System.Data.OptimisticConcurrencyException":
                    httpResponseMessage = CreateHttpResponseMessage(context.Request, 
						HttpStatusCode.InternalServerError, "An error occurred, please try again. If error persists contact support.");
                    break;
                default:
                    httpResponseMessage = CreateHttpResponseMessage(context.Request,
                                HttpStatusCode.InternalServerError, context.Exception);                    
                    break;
            }
            context.Response = httpResponseMessage;
        }
		public HttpResponseMessage CreateHttpResponseMessage(HttpRequestMessage request, HttpStatusCode statusCode, string error)
        {
            return request.CreateResponse(statusCode, error);
        }
    }
}

There is a lot of code in the above example, but here are the key things happening:

1. Get the exception type.
2. Get the exception; attempts to ensure the InnerException is used.
3. Create a variable to store the HttpResponseMessage that will be set at the end of the function.
4. A big switch statement to handle the various exception types differently.
5. A case statement for a variety of different exception types.
6. Based on the exception type set different HttpStatusCode. E.g. HttpStatusCode.NotFound when a particular object was not found while searching a database.
7. Call a private function CreateHttpResponseMessage that returns a HttpResponseMessage
8. Set the Response on the HttpActionExecutedContext

In my example a return a mix of HttpStatusCode to help developers how to handle a different response status code. I like to use the following statements to help you define what status code to send back:

• 4xx - You effed up. Fix your request and try again.
• 5xx - We effed up. Perhaps try again later or Google the error for help.

That's it.

Published on Apr 19, 2019

Tags: Uncategorized | ASP.NET MVC and Web API Tutorial | web api | exceptionattributefilter