Building an API with .NET is a straightforward process that can be accomplished with a few simple steps. In this blog, we will go over how to create a basic .NET API using ASP.NET Core, which is the most popular and recommended framework for building web APIs on .NET.

• Step 1: Install the .NET SDK
  To get started, you will need to have the .NET SDK (Software Development Kit) installed on your machine. You can download the SDK from the official .NET website. Once the SDK is installed, you should have access to the dotnet command-line tool, which you can use to create, build, and run .NET applications.

• Step 2: Create a new API project
  To create a new API project, open the command prompt and navigate to the directory where you want to create your project. Then run the following command:
  

dotnet new webapi -n MyApi

This command will create a new API project called "MyApi" using the webapi template. The template will create the basic structure of an API, including a startup file, a controller, and a launchSettings.json file.

• Step 3: Add a new controller A controller is responsible for handling incoming HTTP requests and returning the appropriate response. To add a new controller, create a new file in the Controllers folder called "ValuesController.cs" and add the following code:

[ApiController]
[Route("[controller]")]
public class ValuesController : ControllerBase
{
    // GET api/values
    [HttpGet]
    public ActionResult<IEnumerable<string>> Get()
    {
        return new string[] { "value1", "value2" };
    }
}

This code defines a new controller called "ValuesController" that handles GET requests to the "api/values" endpoint. The controller returns an array of strings as the response.

• Step 4: Build and run the API To build and run the API, navigate to the project directory in the command prompt and run the following command:

dotnet run

This command will build and run the API, and you should see the message "Now listening on: http://localhost:5000" in the command prompt.

• Step 5: Test the API To test the API, open a web browser and navigate to "http://localhost:5000/api/values". You should see the array of strings returned by the controller.

Congratulations, you have now created a basic .NET API! Of course, this is just the beginning, and you can add more functionality and features to your API as needed. Some popular libraries and frameworks that you can use to extend your API include:

Entity Framework Core for database access
AutoMapper for mapping between models and entities
Swagger for generating API documentation
MediatR for handling CQRS pattern
This is just a basic example to help you get started with .NET API development. With .NET's rich set of tools and libraries, the possibilities for building robust and scalable APIs are endless.