To create a Web API DLL in ASP.NET Core, you need to follow these steps:

Step 1: Set up a new ASP.NET Core Web API project.

  • Open Visual Studio or your preferred code editor.
  • Click on “Create a new project” and select “ASP.NET Core Web Application”.
  • Choose the API template and click “Next”.
  • Provide a project name and location, and click “Create”.

Step 2: Define your API endpoints and business logic.

  • In the newly created project, you’ll find a file called “Startup.cs”. Open it.
  • Inside the ConfigureServices method, configure any services required by your API, such as database connections or third-party integrations.
  • Inside the Configure method, define your API endpoints using the app.UseEndpoints method. For example:
app.UseEndpoints(endpoints =>
{
    endpoints.MapControllers();
});
  • Create a new folder called “Controllers” and add a new class inside it, e.g., “SampleController.cs”.
  • In the controller class, define your API endpoints using attributes like [HttpGet] or [HttpPost]. Here’s an example:
[ApiController]
[Route("api/[controller]")]
public class SampleController : ControllerBase
{
    [HttpGet]
    public IActionResult Get()
    {
        // Business logic to fetch data
        return Ok("Hello, World!");
    }
}

Step 3: Build the project.

  • Save your changes and build the project to ensure there are no compilation errors.

Step 4: Publish the project as a DLL.

  • Right-click on the project in Visual Studio’s Solution Explorer and select “Publish”.
  • Choose the desired target location and click “Publish”.

After completing these steps, you will have a DLL file that contains your ASP.NET Core Web API. This DLL can be referenced and used in other projects or hosted on a web server to expose your API endpoints.