Custom exceptions - Am I doing it right?

Question

I am currently writing a .net REST web api, and am using custom exceptions so that I can send back detailed information to the client as well as the correct http response to stay compliant with REST.. but it seems like I am explicitly throwing a lot of exceptions.

For example: I have an API for uploading image files. These image files have to be a certain file format and under a certain size. If my code finds that an image is the wrong file format, I throw a new custom exception like:

throw new FileTypeException();

Or, if the file is too big, meaning the file.Size() is greater than an amount I have set in the web.config, I throw a different custom exception like:

throw new FileSizeException();

I then catch these exceptions in my Post() method and return a 400 BadRequest and a reason.

So, my code is not actually throwing an error, its just the request does not meet specifications. Is this bad practice?

Answer 1

This might be more appropriate as a Programming question rather than Stack Overflow.

Throwing custom exceptions can be very useful in the right scenario, and a lot of unnecessary work and complexity in the wrong scenario. Usually the sanity check I use, is different handling for each distinct exception type.

For example, your FileTypeException and FileSizeException - is there a different catch for each type? Or are you just returning a different message?

If it's the latter, try to consolidate where you can or use the framework provided exceptions. If it's the former, then you're using custom exceptions right.

Answer 2

An exception filter works great for these cases. In your WebApiConfig, register an exception filter

config.Filters.Add(new CustomExceptionFilterAttribute());

Code for a basic filter:

//needed for CreateErrorResponse(statusCode, message)
using System.Net.Http

public class CustomExceptionFilterAttribute : ExceptionFilterAttribute
{
    public override void OnException(HttpActionExecutedContext context)
    {
        //you can get the underlying exception here
        var type = context.Exception.GetType();

        //I like to map exception types to status codes
        var statusCode = GetHttpStatusCode(type);

        //respond with a proper status code and the message
        context.Response = context.Request.CreateErrorResponse(statusCode, context.Exception.Message);
    }
}

Now in any controller code you can throw and it will hit your filter

public IHttpActionResult Get(int id)
{
    throw new FileSizeException();
}