avaloniaProjForMe/kuzminIVProjectWorkOnDemo/Models/UserPartialClass.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace kuzminIVProjectWorkOnDemo.Models
{
    public partial class User
    {
        public string FIO { get => Fname + " " + Sname + " " + Mname; }
    }
}