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?
   }
}

Answer 1

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.