IM_NEW/User.WebApi/Controllers/Auth/RefreshRequest.cs

using FluentValidation;

namespace IdentityService.WebApi.Controllers.Auth
{
    public class RefreshRequest
    {
        public string RefreshToken { get; set; }
    }

    public class RefreshTokenValidator : AbstractValidator<RefreshRequest>
    {
        public RefreshTokenValidator()
        {
            RuleFor(r => r.RefreshToken)
                .NotEmpty()
                .NotNull()
                ;
        }
    }
}