ASMX Web Serviecs offer a straightforward approach for creating SOAP-based services in the .NET Framework. A service is defined by a class inheriting from WebService and methods decorated with the [WebMethod] attribute.

// ASMX Service Implementation
using System.Web.Services;

[WebService(Namespace = "http://yournamespace.org/")]
public class CalculationService : WebService
{
    [WebMethod]
    public decimal SumValues(decimal valueA, decimal valueB)
    {
        return valueA + valueB;
    }

    [WebMethod]
    public string CreateWelcomeMessage(string userName)
    {
        return $"Welcome, {userName}!";
    }
}

Clients can add a service reference to this endpoint to generate a proxy, allowing remotee invocation of the SumValues and CreateWelcomeMessage methods via SOAP.

For modern development, ASP.NET Core Web APIs provide a RESTful alternative using controller classes. These utilize attribute routing and return structured HTTP rseponses.

// ASP.NET Core Web API Controller
using Microsoft.AspNetCore.Mvc;

[ApiController]
[Route("api/[controller]")]
public class ApiCalculatorController : ControllerBase
{
    [HttpGet("calculate-sum/{operand1}/{operand2}")]
    public ActionResult<decimal> GetSum(int operand1, int operand2)
    {
        return operand1 + operand2;
    }

    [HttpGet("welcome-message")]
    public ActionResult<string> GetMessage([FromQuery] string visitor)
    {
        if (string.IsNullOrWhiteSpace(visitor))
            return BadRequest("Visitor name must be provided.");

        return $"Greetings, {visitor}!";
    }
}

In this REST API, the GetSum action is accessible via a GET request to a URL like /api/ApiCalculator/calculate-sum/10/20. The GetMessage action uses a query string parameter, e.g., /api/ApiCalculator/welcome-message?visitor=Alice. The controller methods leverage ActionResult<T> to return HTTP status codes along with the response data.