Exception handling in Web API

Question

In my Web API project, I created sub projects (class libraries) where I handle actual data handling operations. My backend database is DocumentDB.

My question is how do I tell my Web API action methods of any errors I may encounter within data methods in my class libraries? Once my Web API method knows about the error, I can just return Http status 500 or something like that but I'm not sure what I should have in the catch part (see below) and how I can notify the calling Web API method of the error encountered?

--- Web API Method ---

public async Task<IHttpActionResult> DoSomething(Employee emp)
{
   var employeeRecord = await MyClassLibrary.DoSomethingWithEmployee(emp);

   // Here, I want to check for errors
}

--- Class Library Code ---

public static async Task<Employee> DoSomethingWithEmployee(Employee emp)
{
   try
   {
      // Logic here to call DocumentDB and create employee document
   }
   catch
   {
      // This is where I catch the error but how do I notify the calling Web API method that there was an error?
   }
}

You can let the exception just bubble up. Note, your code should try to avoid exceptions if at all possible. Exceptions should be for truly exceptional things such as the database being unavailable. — mason
– mason, Commented Nov 25, 2014 at 21:53
@sam I will not worry about checking database exception or class library exception inside DoSomething. Ideally, you do not want to handle each and every exception inside action method. Instead, let GlobalExceptionHandler handle those error. You can see that kind of approach in open source MVC projects. (See my answer below) — Win
– Win, Commented Nov 25, 2014 at 22:31

Win · Accepted Answer · 2017-10-02 13:43:13Z

ASP.NET Web API 2.1 have framework support for global handling of unhandled exceptions.

It allows use to customize the HTTP response that is sent when an unhandled application exception occurs.

So, do not catch exception in Class Library. If you are required to log exception in Class Library, then re-throw those exception to Presentation.

WebApiConfig

public static class WebApiConfig
{
    public static void Register(HttpConfiguration config)
    {
        // ...

        config.Services.Replace(typeof (IExceptionHandler), 
            new GlobalExceptionHandler());
    }
}

GlobalExceptionHandler

public class GlobalExceptionHandler : ExceptionHandler
{
    public override void Handle(ExceptionHandlerContext context)
    {
        var exception = context.Exception;

        var httpException = exception as HttpException;
        if (httpException != null)
        {
            context.Result = new CustomErrorResult(context.Request,
                (HttpStatusCode) httpException.GetHttpCode(), 
                 httpException.Message);
            return;
        }

        // Return HttpStatusCode for other types of exception.

        context.Result = new CustomErrorResult(context.Request, 
            HttpStatusCode.InternalServerError,
            exception.Message);
    }
}

CustomErrorResult

public class CustomErrorResult : IHttpActionResult
{
    private readonly string _errorMessage;
    private readonly HttpRequestMessage _requestMessage;
    private readonly HttpStatusCode _statusCode;

    public CustomErrorResult(HttpRequestMessage requestMessage, 
       HttpStatusCode statusCode, string errorMessage)
    {
        _requestMessage = requestMessage;
        _statusCode = statusCode;
        _errorMessage = errorMessage;
    }

    public Task<HttpResponseMessage> ExecuteAsync(
       CancellationToken cancellationToken)
    {
        return Task.FromResult(_requestMessage.CreateErrorResponse(
            _statusCode, _errorMessage));
    }
}

Credit to ASP.NET Web API 2: Building a REST Service from Start to Finish, and source code.

Haitham Shaddad · Accepted Answer · 2016-12-21 08:04:45Z

The error handling depends on your logic and how your API respond to its consumers.

Basically, you have to use HTTP Status Codes according to the type of error.

In your data access and business layer methods, you can depend on the return type. For example, in all methods that queries the database, if the object is not there, you can return NULL, and in your web API, if the method returns NULL, then simply return NotFound() which will respond to the client with a 404.

As for the exceptions:

You can use Error Codes in your business and data access layer and check for these codes in your web API actions. Then return a suitable status code accordingly. Ex: return a status code of 500 if there has been a connection issue to the database, or return a 400 (Bad Request) if the user didn't provide all required action parameters in the correct format.

In case of any other exception that you didn't catch, you can go with the global exception handler described by @Win

Piou · Accepted Answer · 2016-12-21 07:29:11Z

0

If you want to intercept and log the error in your console application but still forward the error to the caller, just use throw; at the end of your catch statement in your console application.

It will rethrow the same exception to the caller, so your application can be aware of the exception in the "callee" as well as in the "caller".

answered Dec 21, 2016 at 7:29

Piou

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