WPF_Practice/PacticeSolution/MVVMSample/Model/ViewModelMain.cs

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

namespace MVVMSample.Model
{
    internal class ViewModelMain : NotifierMain
    {
        private string _firstName;
        private string _lastName;
        private string _fullName;
        private string _birthday;

        private bool _isBirthdayError = false;

        public string FirstName 
        { 
            get { return _firstName; }
            set
            {
                _firstName = value;
                _fullName = $"{_lastName} {_firstName}";
                NotifyChanged("FirstName", "FullName");
            }
        }

        public string LastName
        {
            get { return _lastName; }
            set
            {
                _lastName = value;
                _fullName = $"{_lastName} {_firstName}";
                NotifyChanged("LastName", "FullName");
            }
        }

        public string FullName { get { return _fullName; } }

        public string Birthday
        {
            get { return _birthday; }
            set
            {
                if (Regex.IsMatch(value, "^[0-9]*$"))
                {
                    _birthday = value;
                    _isBirthdayError = false;
                }
                else
                {
                    _birthday = value;
                    _isBirthdayError = true;
                }
                NotifyChanged("Birthday", "BirthdayError", "BirthdayErrorMessage");
            }
        }

        public bool BirthdayError { get { return _isBirthdayError; } }

        public string BirthdayErrorMessage
        {
            get
            {
                if (!_isBirthdayError)
                    return $"Birthday: {_birthday}";
                else
                    return $"Please input the number value ({Birthday})";
            }
        }
    }
}