Как получить доступ к значению текстового поля в viewmodel в шаблоне MVVM? Мой код выглядит следующим образом

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Я новичок в MVVM, моя проблема в том, что я не могу получить доступ к значению текстового поля MainWindow(View) в ViewModel.

Что я уже пробовал:

Это свойства модели

<pre> public class ModelEntity:INotifyPropertyChanged
    {
        public ModelEntity()
        { }
        private string _firstname;
        private string _lastName;
        private string _address;
        private string _contact;
        private string _dedscription;

        public string FirstName { get { return _firstname; } set { _firstname = value;OnPropertyChanged("FirstName"); } }
        public string LastName { get { return _lastName; } set { _lastName = value; OnPropertyChanged("LastName"); } }
        public string Address { get { return _address; } set { _address = value; OnPropertyChanged("Address"); } }
        public string Contact { get { return _contact; } set { _contact = value; OnPropertyChanged("Contact"); } }
        public string Description { get { return _dedscription; } set { _dedscription = value; OnPropertyChanged("Description"); } }

        public event PropertyChangedEventHandler PropertyChanged;
        public void OnPropertyChanged(string PropertyName)
        {
            if(PropertyChanged!=null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(PropertyName));
            }
        }
    }

Ниже приведен код ViewModel

public  class ViewModel
 {
     MyDemoServiceReference.ModelEntity objentity;
     public ObservableCollection<Model.ModelEntity> obsColl { get; set; }
     MyDemoServiceReference.MyDemoServiceClient objservice = new MyDemoServiceReference.MyDemoServiceClient();
     public ModelEntity objmodel { get; set; }
     public ViewModel()
     {
        // obsColl = new ObservableCollection<ModelEntity>();
         // objentity = new MyDemoServiceReference.ModelEntity();
         objmodel = new ModelEntity();
         AddUser = new RelayCommand(SaveData);
     }
     public RelayCommand AddUser { get; set; }
     public RelayCommand UpdateUser { get; set; }



     void SaveData(object parameter)
     {

         // ModelEntity objmodel = new ModelEntity();
         // MyDemoServiceReference.ModelEntity objentity = new MyDemoServiceReference.ModelEntity();

         objentity.FirstName = objmodel.FirstName;
         if (objservice.SaveData(objentity))
             MessageBox.Show("Data Inserted");
         else
             MessageBox.Show("Data Not Inserted");

     }
 }

Ниже приведен код Xmal

<Window x:Class="WPFDemo.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:Model="clr-namespace:WPFDemo.Model"
        xmlns:viewmodel="clr-namespace:WPFDemo.ViewModel"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
 
    <Border Background="Chocolate" BorderThickness="1">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition/>
            <RowDefinition/>
            <RowDefinition/>
            <RowDefinition/>
            <RowDefinition/>
            <RowDefinition/>
            </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition/>
            <ColumnDefinition/>
        </Grid.ColumnDefinitions>
            <TextBox Grid.Row="0" Height="50" Width="150" Text="{Binding ModelEntity.FirstName ,Mode=TwoWay}"  Grid.Column="1"></TextBox>
            <TextBox Grid.Row="1" Height="50" Width="150" Text="{Binding ModelEntity.LastName,Mode=TwoWay}" Grid.Column="1"></TextBox>
            <TextBox Grid.Row="2" Height="50" Width="150" Text="{Binding ModelEntity.Address,Mode=TwoWay}" Grid.Column="1"></TextBox>
            <TextBox Grid.Row="3" Height="50" Width="150" Text="{Binding ModelEntity.Contact,Mode=TwoWay}" Grid.Column="1"></TextBox>
            <TextBox Grid.Row="4" Height="50" Width="150" Text="{Binding ModelEntity.Description,Mode=TwoWay}" Grid.Column="1"></TextBox>
            <Label Grid.Row="0" Height="50" Width="150" Content="FirstName" Grid.Column="0"></Label>
            <Label Grid.Row="1" Height="50" Width="150" Content="LastName" Grid.Column="0"></Label>
            <Label Grid.Row="2"  Height="50" Width="150" Content="Address" Grid.Column="0"></Label>
            <Label Grid.Row="3" Height="50" Width="150" Content="Contact" Grid.Column="0"></Label>
            <Label Grid.Row="4"  Height="50" Width="150" Content="Description" Grid.Column="0"></Label>
            <Button Content="Save" Grid.Row="5" Grid.Column="2" Command="{Binding Path=AddUser}"  Height="50" Width="100"></Button>
        </Grid>
   </Border>
</Window>

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