Return custom error objects in Web API

These answers are way more complicated than they need to be.

public static class WebApiConfig
{
    public static void Register(HttpConfiguration config)
    {
        config.Filters.Add(new HandleApiExceptionAttribute());
        // ...
    }
}

public class HandleApiExceptionAttribute : ExceptionFilterAttribute
{
    public override void OnException(HttpActionExecutedContext context)
    {
        var request = context.ActionContext.Request;

        var response = new
        {
             //Properties go here...
        };

        context.Response = request.CreateResponse(HttpStatusCode.BadRequest, response);
    }
}

That’s all you need. It’s also nice and easy to unit test:

[Test]
public async void OnException_ShouldBuildProperErrorResponse()
{
    var expected = new 
    {
         //Properties go here...
    };

    //Setup
    var target = new HandleApiExceptionAttribute()

    var contextMock = BuildContextMock();

    //Act
    target.OnException(contextMock);

    dynamic actual = await contextMock.Response.Content.ReadAsAsync<ExpandoObject>();

    Assert.AreEqual(expected.Aproperty, actual.Aproperty);
}

private HttpActionExecutedContext BuildContextMock()
{
    var requestMock = new HttpRequestMessage();
    requestMock.Properties.Add(HttpPropertyKeys.HttpConfigurationKey, new HttpConfiguration());

    return new HttpActionExecutedContext()
    {
        ActionContext = new HttpActionContext
        {
            ControllerContext = new HttpControllerContext
            {
                Request = requestMock
            }

        },
        Exception = new Exception()
    };
}