[View or download sample code](https://github.com/aspnet/Docs/tree/master/aspnetcore/web-api/define-controller/samples) ([how to download](xref:tutorials/index#how-to-download-a-sample))
This document explains how to build a web API in ASP.NET Core and when it's most appropriate to use each feature.
## Derive class from ControllerBase
Inherit from the [ControllerBase](/dotnet/api/microsoft.aspnetcore.mvc.controllerbase) class in a controller that's intended to serve as a web API. For example:
The `ControllerBase` class provides access to numerous properties and methods. In the preceding example, some such methods include [BadRequest](/dotnet/api/microsoft.aspnetcore.mvc.controllerbase.badrequest) and [CreatedAtAction](/dotnet/api/microsoft.aspnetcore.mvc.controllerbase.createdataction). These methods are invoked within action methods to return HTTP 400 and 201 status codes, respectively. The [ModelState](/dotnet/api/microsoft.aspnetcore.mvc.controllerbase.modelstate) property, also provided by `ControllerBase`, is accessed to perform request model validation.
ASP.NET Core 2.1 introduces the [[ApiController]](/dotnet/api/microsoft.aspnetcore.mvc.apicontrollerattribute) attribute to denote a web API controller class. For example:
This attribute is commonly coupled with `ControllerBase` to gain access to useful methods and properties. `ControllerBase` provides access to methods such as [NotFound](/dotnet/api/microsoft.aspnetcore.mvc.controllerbase.notfound) and [File](/dotnet/api/microsoft.aspnetcore.mvc.controllerbase.file).
Another approach is to create a custom base controller class annotated with the `[ApiController]` attribute:
|**[[FromRoute]](/dotnet/api/microsoft.aspnetcore.mvc.fromrouteattribute)** | Route data from the current request |
|**[[FromServices]](xref:mvc/controllers/dependency-injection#action-injection-with-fromservices)** | The request service injected as an action parameter |
> Do **not** use `[FromRoute]` when values might contain `%2f` (that is `/`) because `%2f` won't be unescaped to `/`. Use `[FromQuery]` if the value might contain `%2f`.
Without the `[ApiController]` attribute, binding source attributes are explicitly defined. In the following example, the `[FromQuery]` attribute indicates that the `discontinuedOnly` parameter value is provided in the request URL's query string:
Inference rules are applied for the default data sources of action parameters. These rules configure the binding sources you're otherwise likely to manually apply to the action parameters. The binding source attributes behave as follows:
* **[FromBody]** is inferred for complex type parameters. An exception to this rule is any complex, built-in type with a special meaning, such as [IFormCollection](/dotnet/api/microsoft.aspnetcore.http.iformcollection) and [CancellationToken](/dotnet/api/system.threading.cancellationtoken). The binding source inference code ignores those special types. When an action has more than one parameter explicitly specified (via `[FromBody]`) or inferred as bound from the request body, an exception is thrown. For example, the following action signatures cause an exception:
* **[FromForm]** is inferred for action parameters of type [IFormFile](/dotnet/api/microsoft.aspnetcore.http.iformfile) and [IFormFileCollection](/dotnet/api/microsoft.aspnetcore.http.iformfilecollection). It's not inferred for any simple or user-defined types.
* **[FromRoute]** is inferred for any action parameter name matching a parameter in the route template. When multiple routes match an action parameter, any route value is considered `[FromRoute]`.
* **[FromQuery]** is inferred for any other action parameters.
The default inference rules are disabled with the following code in *Startup.ConfigureServices*:
When an action parameter is annotated with the [[FromForm]](/dotnet/api/microsoft.aspnetcore.mvc.fromformattribute) attribute, the `multipart/form-data` request content type is inferred.
The default behavior is disabled with the following code in *Startup.ConfigureServices*:
Actions are inaccessible via [conventional routes](xref:mvc/controllers/routing#conventional-routing) defined in [UseMvc](/dotnet/api/microsoft.aspnetcore.builder.mvcapplicationbuilderextensions.usemvc#Microsoft_AspNetCore_Builder_MvcApplicationBuilderExtensions_UseMvc_Microsoft_AspNetCore_Builder_IApplicationBuilder_System_Action_Microsoft_AspNetCore_Routing_IRouteBuilder__) or by [UseMvcWithDefaultRoute](/dotnet/api/microsoft.aspnetcore.builder.mvcapplicationbuilderextensions.usemvcwithdefaultroute#Microsoft_AspNetCore_Builder_MvcApplicationBuilderExtensions_UseMvcWithDefaultRoute_Microsoft_AspNetCore_Builder_IApplicationBuilder_) in *Startup.Configure*.
