API version conventions allow you to specify API version information for your services without having to use .NET attributes. There are a number of reasons why you might choose this option. The most common reasons are:

• Centralized management and application of all service API versions
• Apply API versions to services defined by controllers in external .NET assemblies
• Dynamically apply API versions from external sources; for example, from configuration

Consider the following controller:

[RoutePrefix( "my" )]
public class MyController : ApiController
{
    [Route]
    public IHttpActionResult Get() => Ok();
}

[ApiController]
[Route( "[controller]" )]
public class MyController : ControllerBase
{
    [HttpGet]
    public IActionResult Get() => Ok();
}

Instead of applying the ApiVersionAttribute to the controller, we can instead choose to define a convention in the API versioning options.

configuration.AddApiVersioning( options =>
{
    options.Conventions.Controller<MyController>().HasApiVersion( 1.0 );
} );

services.AddApiVersioning()
        .AddMvc( options =>
        {
            options.Conventions.Controller<MyController>().HasApiVersion( 1.0 );
        } );

All of the semantics that can be expressed with .NET attributes can be defined using conventions. Consider what version 2.0 of the previous controller with interleaved API versions might look like:

[RoutePrefix( "my" )]
public class MyController : ApiController
{
    [Route]
    public IHttpActionResult Get() => Ok();

    [Route]
    public IHttpActionResult GetV2() => Ok();

    [Route( "{id:int}" )]
    public IHttpActionResult GetV2( int id ) => Ok();
}

[ApiController]
[Route( "[controller]" )]
public class MyController : ControllerBase
{
    [HttpGet]
    public IActionResult Get() => Ok();

    [HttpGet]
    public IActionResult GetV2() => Ok();

    [HttpGet( "{id:int}" )]
    public IActionResult GetV2( int id ) => Ok();
}

The API version conventions might then be defined as:

options.Conventions.Controller<MyController>()
                   .HasDeprecatedApiVersion( 1.0 )
                   .HasApiVersion( 2.0 )
                   .Action( c => c.GetV2() ).MapToApiVersion( 2.0 )
                   .Action( c => c.GetV2( default ) ).MapToApiVersion( 2.0 );

If you use API version conventions and .NET attributes, then the constructed ApiVersionModel for the corresponding controller will be an aggregated union of the two sets of information.

You can also define custom conventions via the IControllerConvention interface and add them to the builder:

public interface IControllerConvention
{
    bool Apply( IControllerConventionBuilder controller,
                HttpControllerDescriptor controllerDescriptor );
}

public interface IControllerConvention
{
    bool Apply( IControllerConventionBuilder controller, ControllerModel controllerModel );
}

Custom conventions are added to the convention builder through the API versioning options:

options.Conventions.Add( new MyCustomConvention() );

This built-in convention allows you to version your controllers by the namespace they reside in when applied.

options.Conventions.Add( new VersionByNamespaceConvention() );

The defined namespace name must conform to the API version format so that it can be parsed. The language-neutral syntax in BNF is:

'v' | 'V' : [<year> : ['_'] : <month> : ['_'] : <day>] : [<major['_' : minor]>] : [<status>]

The . character is considered a namespace delimiter in many programming languages. This character must be changed to _ so that newly added files have the correct format. In addition, most languages do not allow the name of a namespace to start with a number. Since a leading character is required, the first character must be v or V. There is no requirement as to where the API version must appear in the namespace.

By default, API versions derived from a namespace will be considered supported. If the controller is decorated with the ObsoleteAttribute, then the API version inferred from the containing namespace will be considered deprecated.

Examples

• Contoso.Api.v1.Controllers → 1.0
• Contoso.Api.v1_1.Controllers → 1.1
• Contoso.Api.v0_9_Beta.Controllers → 0.9-Beta
• Contoso.Api.v20180401.Controllers → 2018-04-01
• Contoso.Api.v2018_04_01.Controllers → 2018-04-01
• Contoso.Api.v2018_04_01_Beta.Controllers → 2018-04-01-Beta
• Contoso.Api.v2018_04_01_1_0_Beta.Controllers → 2018-04-01.1.0-Beta

Contoso
└ Api
  ├─ v1
  │  └ Controllers
  ├─ v2
  │  └ Controllers
  └─ v2_5
     └ Controllers

Figure 1: Sample folder layout with numeric API versions

Contoso
└ Api
  ├─ v2018_07_01
  │  └ Controllers
  ├─ v2018_08_01
  │  └ Controllers
  └─ v2018_09_01
     └ Controllers

Figure 2: Sample folder layout with date API versions