There are several built-in error responses. The body of each error response complies with RFC 7807: Problem Details.

In earlier versions, the error responses bodies complied with the Microsoft REST Guidelines error response format, which is itself the error response format used by the OData protocol (see OData JSON Format §21.1). There wasn't a broad standard at that time, which made any common error response format sensible.

Each problem detail also contains a code extension to retain a level of backward compatibility for clients that may have relied on that value. If you need to retain the old functionality, refer to backward compatibility below.

All versioned services require that an API version be specified. When a client makes a request without providing an API version, then the server will respond with a bad request. This behavior is typically not exhibited when the API is version-neutral or the AssumeDefaultVersionWhenUnspecified option is configured to true.

When a client requested API version does not match any of the available controllers or their actions, then the server will respond with a problem. If the ReportApiVersions option is true, then the supported versions will be returned to the client in the api-supported-versions HTTP header.

_{1: Defined by ApiVersioningOptions.UnsupportedApiVersionStatusCode
2: The value is always 404 when versioning by URL segment}

When a client makes a request with an API version, but the value is malformed or cannot be parsed, then the server will respond with a bad request. This typically occurs where the value contains incomplete version components or the date-only form is invalid (ex: 2016-02-30).

When a client requests a specific API version, the specified API version must be unambiguous to the server. A client is allowed to specify an API version more than once, but if the values are not identical, then the server will respond with a bad request.

GET /resource?api-version=1.0 HTTP/1.1
host: localhost
api-version: 1.0

Figure 1: Multiple, unambiguous API versions requested

GET /resource?api-version=1.0 HTTP/1.1
host: localhost
api-version: 2.0

Figure 2: Ambiguous API versions requested between in query string and headers

GET /resource?api-version=1.0&api-version=2.0 HTTP/1.1
host: localhost

Figure 3: Ambiguous API versions requested in the query string

GET /resource HTTP/1.1
host: localhost
api-version: 1.0
api-version: 2.0

Figure 4: Ambiguous API versions requested in the headers

Error responses can be customized or extended in a variety of ways.

RFC 7807 was ratified after active development on ASP.NET Web API ceased. There are no out-of-the-box services provided. API Versioning provides a backport of the ProblemDetails type as well as the IProblemDetailsFactory. The default implementation can be replaced by implementing IProblemDetailsFactory and exposing it as a resolvable service via HttpConfiguration.DependencyResolver.

Applies to .NET 7+

In ASP.NET Core, you must opt into using problem details via:

services.AddProblemDetails();

If problem details are not added, clients will receive an error response which only has the HTTP status code. You might choose this approach if you don't want a response body or your error responses do not comply with RFC 7807.

To modify the way a problem is written to clients, you can implement and register a your own IProblemDetailsWriter implementation. Each registered implementation is injected into the IProblemDetailsService. The first matching writer is used to write the response body. For more information see the ASP.NET Core Problem Details documentation.

Applies to .NET 6

The IProblemDetailsService was not added to support ProblemDetails in Minimal APIs and MVC Core until .NET 7. In API Versioning 6.x, the IProblemDetailsFactory interface was used to bridge this gap. Contrary to the opt-in behavior of AddProblemDetails(), a default implementation of IProblemDetailsFactory is automatically registered for Minimal APIs. If MVC Core is added, then a decorated adapter is automatically provided over ProblemDetailsFactory. You have the choice of replacing the entire IProblemDetailsFactory service or the MVC Core specific ProblemDetailsFactory.

The IProblemDetailsFactory interface was completely removed in .NET 7+ because it is no longer used in any way.

While it is possible to customize error responses and retain the previous Error Object format, there is considerable work required to enable this behavior and may block adoption of new library versions. Additional extensions have been added to retain backward compatibility or continue to use Error Objects if you so desire.

Applies to 7.1.0+

ASP.NET Web API does not provide an out-of-the-box dependency injection container; however, the following extension method will wire up the necessary changes without having to add one of your own.

configuration.ConvertProblemDetailsToErrorObject();

Using Error Object responses is as simple as registering the ErrorObjectWriter to emit them. The critical part of each setup is the order in which the writer is registered. If the writer is not registered in the correct order, it will not be selected. Each configuration must occur before AddApiVersioning().

The default implementation of the ErrorObjectWriter only writes Error Objects for API versioning related errors. The default ASP.NET Core behavior provided by AddProblemDetails() is used for writing other types of errors. If you want to use Error Objects for other error responses, you can extend ErrorObjectWriter and override which types of Problem Details it should match - perhaps all of them.

Applies to 8.1.0+

AddErrorObjects() adds the default behavior; however, you can register a custom ErrorObjectWriter via AddErrorObject<TWriter>(). Both methods allow a custom Action<JsonOptions> setup and will configure the default behavior if not otherwise specified.

builder.Services.AddProblemDetails().AddErrorObjects();
builder.Services.ApiVersioning();

builder.Services.AddControllers();
builder.Services.AddErrorObjects().AddProblemDetails();
builder.Services.ApiVersioning().AddMvc();

Applies to 7.1.0+

builder.Services.AddProblemDetails();
builder.Services.TryAddEnumerable( ServiceDescriptor.Singleton<IProblemDetailsWriter, ErrorObjectWriter>() );
builder.Services.ApiVersioning();

builder.Services.AddControllers();
builder.Services.TryAddEnumerable( ServiceDescriptor.Singleton<IProblemDetailsWriter, ErrorObjectWriter>() );
builder.Services.AddProblemDetails();
builder.Services.ApiVersioning().AddMvc();

Applies to .NET 6 and 6.5.0+

Since IProblemDetailsService did not exist in .NET 6, you must instead replace IProblemDetailsFactory with the ErrorObjectFactory service. The configuration process and order are the same regardless of whether you are using Minimal APIs or controllers. The replaced service should occur before AddApiVersioning().

builder.Services.AddSingleton<IProblemDetailsFactory, ErrorObjectFactory>();
builder.Services.ApiVersioning();