public class GetClientController : Controller
    {
        LoadClient loadClient = new LoadClient();

        [HttpGet]
        public IEnumerable<Cliente> GetLiberaItens(double cpf)
        {
            return loadClient.GetCliente().AsEnumerable().ToList();
        }
    }

using System.Net.Http;
using System.Web.Http;

[Route("api/getcliente/{cpf}")]
        public List<Cliente> GetCliente(Int64 cpf)
        {
            List<Cliente> cliente = new List<Cliente>();

            cliente.Add(new Cliente(123456, "p@vb.com", "986754007", "CB", "EML"));
            cliente.Add(new Cliente(908734, "x@cb.com", "988877731", "CB", "SMS"));
            cliente.Add(new Cliente(674300, "p@pf.com", "965241131", "PF", "EML"));
            cliente.Add(new Cliente(101654, "p@ex.com", "987450101", "EX", "EML"));
            cliente.Add(new Cliente(501274, "p@pa.com", "986754144", "PA", "SMS"));

            var lista = cliente
                .Where(cp => cp.Cpf == cpf).ToList();

            return lista;
        }

public class GetClientController : Controller
    {
        LoadClient loadClient = new LoadClient();

        [HttpGet]
        [Route("api/getcliente/{cpf}")]
        public IEnumerable<Cliente> GetCliente(Int64 cpf)
        {
            return loadClient.GetCliente(cpf).AsEnumerable().ToList();
        }
    }