What happens if a Web API controller throws an uncaught exception? By default, most exceptions are translated into an HTTP response with status code 500, Internal Server Error.
The **HttpResponseException** type is a special case. This exception returns any HTTP status code that you specify in the exception constructor. For example, the following method returns 404, Not Found, if the *id* parameter is not valid.
You can customize how Web API handles exceptions by writing an *exception filter*. An exception filter is executed when a controller method throws any unhandled exception that is *not* an **HttpResponseException** exception. The **HttpResponseException** type is a special case, because it is designed specifically for returning an HTTP response.
Exception filters implement the **System.Web.Http.Filters.IExceptionFilter** interface. The simplest way to write an exception filter is to derive from the **System.Web.Http.Filters.ExceptionFilterAttribute** class and override the **OnException** method.
> [!NOTE]
> Exception filters in ASP.NET Web API are similar to those in ASP.NET MVC. However, they are declared in a separate namespace and function separately. In particular, the **HandleErrorAttribute** class used in MVC does not handle exceptions thrown by Web API controllers.
Here is a filter that converts **NotImplementedException** exceptions into HTTP status code 501, Not Implemented:
To apply the filter globally to all Web API controllers, add an instance of the filter to the **GlobalConfiguration.Configuration.Filters** collection. Exeption filters in this collection apply to any Web API controller action.
If you use the "ASP.NET MVC 4 Web Application" project template to create your project, put your Web API configuration code inside the `WebApiConfig` class, which is located in the App\_Start folder:
The **HttpError** object provides a consistent way to return error information in the response body. The following example shows how to return HTTP status code 404 (Not Found) with an **HttpError** in the response body.
**CreateErrorResponse** is an extension method defined in the **System.Net.Http.HttpRequestMessageExtensions** class. Internally, **CreateErrorResponse** creates an **HttpError** instance and then creates an **HttpResponseMessage** that contains the **HttpError**.
In this example, if the method is successful, it returns the product in the HTTP response. But if the requested product is not found, the HTTP response contains an **HttpError** in the request body. The response might look like the following:
Notice that the **HttpError** was serialized to JSON in this example. One advantage of using **HttpError** is that it goes through the same [content-negotiation](../formats-and-model-binding/content-negotiation.md) and serialization process as any other strongly-typed model.
### HttpError and Model Validation
For model validation, you can pass the model state to **CreateErrorResponse**, to include the validation errors in the response:
For more information about model validation, see [Model Validation in ASP.NET Web API](../formats-and-model-binding/model-validation-in-aspnet-web-api.md).
### Using HttpError with HttpResponseException
The previous examples return an **HttpResponseMessage** message from the controller action, but you can also use **HttpResponseException** to return an **HttpError**. This lets you return a strongly-typed model in the normal success case, while still returning **HttpError** if there is an error:
