AspNetCore.Docs/aspnetcore/migration/webapi.md

title	author	description	ms.author	ms.date	uid
Migrate from ASP.NET Web API to ASP.NET Core	ardalis	Learn how to migrate a Web API implementation from ASP.NET Web API to ASP.NET Core MVC.	riande	05/10/2018	migration/webapi

Migrate from ASP.NET Web API to ASP.NET Core

By Steve Smith and Scott Addie

Web APIs are HTTP services that reach a broad range of clients, including browsers and mobile devices. ASP.NET Core MVC includes support for building Web APIs providing a single, consistent way of building web applications. In this article, we demonstrate the steps required to migrate a Web API implementation from ASP.NET Web API to ASP.NET Core MVC.

View or download sample code (how to download)

Review ASP.NET Web API project

This article uses the sample project, ProductsApp, created in the article Getting Started with ASP.NET Web API 2 as its starting point. In that project, a simple ASP.NET Web API project is configured as follows.

In Global.asax.cs, a call is made to WebApiConfig.Register:

[!code-csharp]

WebApiConfig is defined in App_Start, and has just one static Register method:

[!code-csharp]

This class configures attribute routing, although it's not actually being used in the project. It also configures the routing table, which is used by ASP.NET Web API. In this case, ASP.NET Web API will expect URLs to match the format /api/{controller}/{id}, with {id} being optional.

The ProductsApp project includes just one simple controller, which inherits from ApiController and exposes two methods:

[!code-csharp]

Finally, the model, Product, used by the ProductsApp, is a simple class:

[!code-csharp]

Now that we have a simple project from which to start, we can demonstrate how to migrate this Web API project to ASP.NET Core MVC.

Create the Destination Project

Using Visual Studio, create a new, empty solution, and name it WebAPIMigration. Add the existing ProductsApp project to it, then, add a new ASP.NET Core Web Application Project to the solution. Name the new project ProductsCore.

Next, choose the Web API project template. We will migrate the ProductsApp contents to this new project.

Delete the Project_Readme.html file from the new project. Your solution should now look like this:

Migrate Configuration

ASP.NET Core no longer uses Global.asax, web.config, or App_Start folders. Instead, all startup tasks are done in Startup.cs in the root of the project (see Application Startup). In ASP.NET Core MVC, attribute-based routing is now included by default when UseMvc() is called; and, this is the recommended approach for configuring Web API routes (and is how the Web API starter project handles routing).

[!code-csharp]

Assuming you want to use attribute routing in your project going forward, no additional configuration is needed. Simply apply the attributes as needed to your controllers and actions, as is done in the sample ValuesController class that's included in the Web API starter project:

[!code-csharp]

Note the presence of [controller] on line 8. Attribute-based routing now supports certain tokens, such as [controller] and [action]. These tokens are replaced at runtime with the name of the controller or action, respectively, to which the attribute has been applied. This serves to reduce the number of magic strings in the project, and it ensures the routes will be kept synchronized with their corresponding controllers and actions when automatic rename refactorings are applied.

To migrate the Products API controller, we must first copy ProductsController to the new project. Then simply include the route attribute on the controller:

[Route("api/[controller]")]

You also need to add the [HttpGet] attribute to the two methods, since they both should be called via HTTP Get. Include the expectation of an "id" parameter in the attribute for GetProduct():

// /api/products
[HttpGet]
...

// /api/products/1
[HttpGet("{id}")]

At this point, routing is configured correctly; however, we can't yet test it. Additional changes must be made before ProductsController will compile.

Migrate Models and Controllers

The last step in the migration process for this simple Web API project is to copy over the Controllers and any Models they use. In this case, simply copy Controllers/ProductsController.cs from the original project to the new one. Then, copy the entire Models folder from the original project to the new one. Adjust the namespaces to match the new project name (ProductsCore). At this point, you can build the application, and you will find a number of compilation errors. These should generally fall into the following categories:

• ApiController does not exist

• System.Web.Http namespace does not exist

• IHttpActionResult does not exist

Fortunately, these are all very easy to correct:

• Change ApiController to Controller (you may need to add using Microsoft.AspNetCore.Mvc)

• Delete any using statement referring to System.Web.Http

• Change any method returning IHttpActionResult to return a IActionResult

Once these changes have been made and unused using statements removed, the migrated ProductsController class looks like this:

[!code-csharp]

You should now be able to run the migrated project and browse to /api/products; and, you should see the full list of 3 products. Browse to /api/products/1 and you should see the first product.

ASP.NET 4.x Web API 2 compatibility shim

A useful tool when migrating ASP.NET Web API projects to ASP.NET Core is the Microsoft.AspNetCore.Mvc.WebApiCompatShim library. The compatibility shim extends ASP.NET Core to allow a number of different Web API 2 conventions to be used. The sample ported previously in this document is basic enough that the compatibility shim was not necessary. For larger projects, using the compatibility shim can be useful for temporarily bridging the API gap between ASP.NET Core and ASP.NET Web API 2.

The Web API compatibility shim is meant to be used as a temporary measure to facilitate migrating large Web API projects to ASP.NET Core. Over time, projects should be updated to use ASP.NET Core patterns instead of relying on the compatibility shim.

Compatibility features included in Microsoft.AspNetCore.Mvc.WebApiCompatShim include:

• Adds an ApiController type so that controllers' base types don't need to be updated.
• Enables Web API-style model binding. ASP.NET Core MVC model binding functions similarly to MVC 5, by default. The compatibility shim changes model binding to be more similar to Web API 2 model binding conventions. For example, complex types are automatically bound from the request body.
• Extends model binding so that controller actions can take parameters of type HttpRequestMessage.
• Adds message formatters allowing actions to return results of type HttpResponseMessage.
• Adds additional response methods that Web API 2 actions may have used to serve responses:
  • HttpResponseMessage generators:
    • CreateResponse<T>
    • CreateErrorResponse
  • Action result methods:
    • BadRequestErrorMessageResult
    • ExceptionResult
    • InternalServerErrorResult
    • InvalidModelStateResult
    • NegotiatedContentResult
    • ResponseMessageResult
• Adds an instance of IContentNegotiator to the app's DI container and makes content negotiation-related types from Microsoft.AspNet.WebApi.Client available. This includes types like DefaultContentNegotiator, MediaTypeFormatter, etc.

To use the compatibility shim, you need to:

• Install the Microsoft.AspNetCore.Mvc.WebApiCompatShim NuGet package.
• Register the compatibility shim's services with the app's DI container by calling services.AddMvc().AddWebApiConventions() in the app's Startup.ConfigureServices method.
• Define Web API-specific routes using MapWebApiRoute on the IRouteBuilder in the app's IApplicationBuilder.UseMvc call.

Summary

Migrating a simple ASP.NET Web API project to ASP.NET Core MVC is fairly straightforward, thanks to the built-in support for Web APIs in ASP.NET Core MVC. The main pieces every ASP.NET Web API project will need to migrate are routes, controllers, and models, along with updates to the types used by controllers and actions.