silverlight validations not working

<TextBox Grid.Column="1" Height="23" HorizontalAlignment="Left" Name="textBox1" VerticalAlignment="Top" Width="203" Margin="0,5,0,0"
                 Text="{Binding UsrNm, Mode=TwoWay, ValidatesOnExceptions=True}" />


        <TextBox Grid.Column="1" Grid.Row="1" Height="23" HorizontalAlignment="Left" Name="textBox2" VerticalAlignment="Top" Width="203" Margin="0,5,0,0" 
                 Text="{Binding UsrPwd, Mode=TwoWay, ValidatesOnExceptions=True}" />
        <Button Content="LogIn" Grid.Column="1" Grid.Row="2" Height="39" HorizontalAlignment="Left" Margin="112,27,0,0" Name="button1" VerticalAlignment="Top" Width="91" Click="button1_Click" />

public MainPage()
        {
            InitializeComponent();
            LoginInfo lgin = new LoginInfo();
            LayoutRoot.DataContext = lgin;
        }

        private void button1_Click(object sender, RoutedEventArgs e)
        {
            foreach (var ele in LayoutRoot.Children)
            {
                //In our case check for text box
                if (ele is TextBox)
                {
                    //manually update source
                    (ele as TextBox).GetBindingExpression(TextBox.TextProperty).UpdateSource();
                }

            }
        }

//and i took a class as logininfo.cs

 public class LoginInfo
    {
        private string _usrnm;
        private string _usrpwd;

        public string UsrNm 
        {
            get { return _usrnm; }
            set { _usrnm = value; } 
        }
        public string UsrPwd
        {
            get { return _usrpwd; }
            set { _usrpwd = value; } 
        }
    }