The IApiVersionReader interface defines the behavior of how an API version is read in its raw, unparsed form from the current HTTP request. There are multiple methods for reading an API version provided out-of-the-box or you can implement your own. The default, configured API version reader is a composed instance
QueryStringApiVersionReader and UrlSegmentApiVersionReader.

public interface IApiVersionReader
{
    IReadOnlyList<string> Read( HttpRequestMessage request );
}

public interface IApiVersionReader
{
    IReadOnlyList<string> Read( HttpRequest request );
}

The QueryStringApiVersionReader reads the requested API version from the requested query string. The default query string parameter name is api-version. The constructor for this class accepts the name of a query string parameter so that an alternate query string parameter can be used.

// svc?api-version=2.0
AddApiVersioning( options => options.ApiVersionReader = new QueryStringApiVersionReader() );

// svc?v=2.0
AddApiVersioning( options => options.ApiVersionReader = new QueryStringApiVersionReader( "v" ) );

The HeaderApiVersionReader reads the requested API version from a HTTP request header. There is no default or standard HTTP header. You must define which HTTP header name or names contain the API version information. This method of API versioning does not conform to the Microsoft REST Guidelines.

AddApiVersioning( options => options.ApiVersionReader = new HeaderApiVersionReader( "api-version" ) );

The MediaTypeApiVersionReader reads the requested API version from a HTTP media type request header. The supported headers are Content-Type and Accept. If both headers are present, then Content-Type is preferred. If the Accept header specifies qualities, then the API version associated with the highest quality is selected. This behavior is independent of media type negotiation. The default media type parameter is "v", but you may specify an alternate name. This method of API versioning does not conform to the Microsoft REST Guidelines; however, it is generally accepted as a fully REST-compliant method of versioning.

// Content-Type: application/json;v=2.0
AddApiVersioning( options => options.ApiVersionReader = new MediaTypeApiVersionReader() );

// Content-Type: application/json;version=2.0
AddApiVersioning( options => options.ApiVersionReader = new MediaTypeApiVersionReader( "version" ) );

The MediaTypeApiVersionReaderBuilder is also available with additional features that allow:

• Define multiple media type parameters
• Mutually include specific media types
• Mutually exclude specific media types
• Match media types by template
• Match media types by pattern
• Disambiguate between multiple API versions

// Accept: application/json;v=2.0
AddApiVersioning(
    options =>
    {
        var builder = new MediaTypeApiVersionReaderBuilder();

        options.ApiVersionReader = builder.Parameter( "v" )
                                          .Include( "application/json" )
                                          .Build();
    } );

// Accept: application/vnd.my.company.v1+json
AddApiVersioning(
    options =>
    {
        var builder = new MediaTypeApiVersionReaderBuilder();

        options.ApiVersionReader = builder.Template( "application/vnd.my.company.v{version}+json" )
                                          .Build();
    } );

Multiple IApiVersionReader implementations can be combined using composition instead of inheritance. For convenience, you can use ApiVersionReader.Combine to compose multiple API version reading styles.

AddApiVersioning(
    options => options.ApiVersionReader = ApiVersionReader.Combine(
        new QueryStringApiVersionReader(),
        new HeaderApiVersionReader() { HeaderNames = { "x-ms-api-version" } } ) );