Model View ViewModel in WPF

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;
using System.Windows.Input;

namespace Assignments {
    //:::: My View Model Class ::::
    public class UserViewModel: INotifyPropertyChanged,
    IDataErrorInfo {
        public ObservableCollection < Model > _UsersList;
        //Constructor for View Model Class
        public UserViewModel() {
            _UsersList = new ObservableCollection < Model > ();
        }

        // Get Set Properties for Member Variables
        //Name
        private string name;
        public string Name {
            get {
                return name;
            }
            set {
                name = value;
                NotifyPropertyChanged("Name");
            }
        }
        //Gender
        private string gender;
        public string Gender {
            get {
                return gender;
            }
            set {
                gender = value;
                NotifyPropertyChanged("Gender");
            }
        }
        //SapiD
        private string sapid;
        public string SapId {
            get {
                return sapid;
            }
            set {
                sapid = value;
                NotifyPropertyChanged("SapId");
            }
        }
        //mOBILE nUMBER
        private string num;
        public string Num {
            get {
                return num;
            }
            set {
                num = value;
                NotifyPropertyChanged("Num");
            }
        }
        //Date of Birth
        private DateTime ? dob;
        public DateTime ? Dob {
            get {
                return dob;
            }
            set {
                dob = value;
                NotifyPropertyChanged("Dob");
            }
        }

        //:::: Command Bindings ::::
        //:::: For Submit Button we had call MyAction() Function ::::
        private ICommand _clickCommand;
        public ICommand ClickCommand {
            get {
                if (_clickCommand == null) {
                    _clickCommand = new CommandHandler(() = > MyAction(), true);
                }
                return _clickCommand;
            }
        }

        public void MyAction() {
            // Write My Actions Commands Here
            //Binding text Box property from UserViewModel & Model Classes
            _UsersList.Add(new Model() {
                SapId = SapId, Name = Name, Gender = Gender, Dob = Dob, Num = Num
            });

            Name = string.Empty;
            SapId = string.Empty;
            //Dob = DateTime.Now;
            Gender = string.Empty;
            Num = string.Empty;
        }

        //:::: For Clear Button Event we had call MyClear() function ::::
        private ICommand _clickClear;
        public ICommand ClickClear {
            get {
                if (_clickClear == null) {
                    _clickClear = new CommandHandler(() = > MyClear(), true);
                }
                return _clickClear;
            }
        }

        public void MyClear() {
            // Write here Commands for Clear all Text boxes
            Name = string.Empty;
            SapId = string.Empty;
            Gender = string.Empty;
            Num = string.Empty;
        }

        // INotify PropertyChange Command
        public event PropertyChangedEventHandler PropertyChanged;
        private void NotifyPropertyChanged(String info) {
            if (PropertyChanged != null) {
                PropertyChanged(this, new PropertyChangedEventArgs(info));
            }
        }
        //For Displaying Errors as a String
        public string Error {
            get {
                return this[string.Empty];
            }
        }
        // String Errors Checking for TextBoxes
        public string this[string columnName] {
            get {
                string result = null;
                // Conditions for the TextBox Validations with Model Name Property
                if (columnName == "Dob") {
                    //For Empty
                    if (Dob == null) {
                        result = "Please enter DOB";
                        return result.ToString();
                    }
                    //For Not selecting Present Date
                    if (Dob == System.DateTime.Now) {
                        result = "Please enter valid DOB";
                        return result;
                    }
                }
                if (columnName == "Gender") {
                    //For Empty
                    if (this.Gender == null) result = "Please enter your Gender";
                    return result;
                }
                if (columnName == "SapId") {
                    //For Empty
                    if (this.SapId == null) {
                        result = "Please enter sapid";
                        return result;
                    }
                }
                if (columnName == "Name") {
                    //For Empty
                    if (Name == null) {
                        result = "Please enter your Name";
                        return result;
                    }
                    // For 50 Chars
                    if (Name.Length > 50) {
                        result = "Name can not be longer than 50 chars";
                        return result;
                    }
                    //For Special Characters only
                    string st = @
                    "!|@|#|\$|%|\?|\>|\<|\*";
                    if (Regex.IsMatch(Name, st)) {
                        result = "Special chars not allowed";
                        return result;
                    }
                    //For Characters only
                    string str = @
                    "^[0-9]+$";
                    if (Regex.IsMatch(Name, str)) {
                        result = "Only chars are allowed";
                        return result;
                    }
                }
                if (columnName == "Num") {
                    //For Empty
                    if (this.Num == null) {
                        result = "Please enter Mobile Number";
                        return result;
                    }
                    // Not MAx than 10 digit
                    if (this.Num.Length > 10) return "Number cannot be more than 10 digit";
                    //Only Number
                    string str = @
                    "^[0A-Za-z]+$";
                    if (Regex.IsMatch(Num, str)) {
                        result = "Only Numbers are allowed";
                        return result;
                    }
                }
                return result;
            }
        }
    } //End of UserView Class

    //******** Icommand Interface Functions Starts Here **********
    public class CommandHandler: ICommand {
        private Action _action;
        private bool _canExecute;
        public CommandHandler(Action action, bool canExecute) {
            _action = action;
            _canExecute = canExecute;
        }
        public bool CanExecute(object parameter) {
            return _canExecute;
        }
        public event EventHandler CanExecuteChanged;
        public void Execute(object parameter) {
            _action();
        }
    }

    // **************************************************************************

    //:::: My Model Class ::::

    public class Model {
        public string name;
        public string gender;
        public DateTime ? dob;
        public String num;
        public string sapid;

        // ### GEt SEt PRoperties for Member Variables ###

        public string Name {
            get {
                return name;
            }
            set {
                name = value;
            }
        }

        public string Gender {
            get {
                return gender;
            }
            set {
                gender = value;
            }
        }
        public DateTime ? Dob {
            get {
                return dob;
            }
            set {
                dob = value;
            }
        }
        public String Num {
            get {
                return num;
            }
            set {
                num = value;
            }
        }
        public string SapId {
            get {
                return sapid;
            }
            set {
                sapid = value;
            }
        }


    } // End Of Model Class
} // End Of NAmeSpace