Update View from View Model

MainWindow.xaml

<Window x:Class="_0message.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">

    <Grid>
        <TextBox HorizontalAlignment="Left" Margin="12,266,0,0" Name="tbSendMessageText" VerticalAlignment="Top" Width="392" Height="auto" AcceptsReturn="False" Text="{Binding SendText, Mode=TwoWay}" />
        <TextBlock Height="248" HorizontalAlignment="Left" Margin="12,12,0,0" Name="tbReceiveMessage" VerticalAlignment="Top" Width="479" Text="{Binding ReceivedText, Mode=TwoWay}" />
        <Button Content="Send" Height="23" HorizontalAlignment="Left" Margin="416,266,0,0" Name="bSend" VerticalAlignment="Top" Width="75" Command="{Binding Send}" />
    </Grid>
</Window>

MainWindowViewModel.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.ComponentModel;
using System.Windows.Input;

namespace _0message
{
    public class MainWindowViewModel : INotifyPropertyChanged
    {

        public event PropertyChangedEventHandler PropertyChanged;


        public void SendMessage()
        {
            _0Logic logic = new _0Logic();
            logic.Send(SendText);
        }
       
        protected void OnNotifyPropertyChanged(string property)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(property));
            }
        }

        private ICommand _sendCommand;
        public ICommand Send
        {
            get
            {
                if (_sendCommand == null)
                {
                    _sendCommand = new Command(() => SendMessage());
                }
                return _sendCommand;
            }
        }

        private string _sendtext = String.Empty;
        public string SendText
        {
            get
            {
                return _sendtext;
            }
            set
            {
                if (_sendtext != value)
                {
                    _sendtext = value;
                    OnNotifyPropertyChanged("SendText");
                }
            }
        }
    }

    public class Command : ICommand
    {
        private Action _action = null;

        public Command(Action action)
        {
            _action = action;
        }

        public bool CanExecute(object parameter)
        {
            return true;
        }

        public event EventHandler CanExecuteChanged;

        public void Execute(object parameter)
        {
            _action();
        }
    }
}