Switching to MVVM

<Window x:Class="Step8.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:Step8"
        Title="MainWindow" Height="400" Width="600">
    <Window.Resources>
        <Style TargetType="{x:Type TextBox}">
            <Style.Triggers>
                <Trigger Property="Validation.HasError" Value="true">
                    <Setter Property="ToolTip"
        Value="{Binding RelativeSource={x:Static RelativeSource.Self},
                        Path=(Validation.Errors)[0].ErrorContent}"/>
                </Trigger>
            </Style.Triggers>
        </Style>
    </Window.Resources>    
    <Grid Background="LemonChiffon">
        <Grid.RowDefinitions>
            <RowDefinition Height="2*"/>
            <RowDefinition Height="2*"/>
            <RowDefinition/>
        </Grid.RowDefinitions>
        <Grid Grid.Row="0">
            <Grid.ColumnDefinitions>
                <ColumnDefinition/>
                <ColumnDefinition/>
            </Grid.ColumnDefinitions>
            <Grid.RowDefinitions>
                <RowDefinition/>
                <RowDefinition/>
                <RowDefinition/>
                <RowDefinition/>
            </Grid.RowDefinitions>
            <TextBlock Grid.Column="0" Grid.ColumnSpan="2" Grid.Row="0" FontSize="18" FontWeight="Bold"
                       HorizontalAlignment="Center" VerticalAlignment="Center">Loan Amortization</TextBlock>
            <TextBlock Grid.Column="0" Grid.Row="1" VerticalAlignment="Center" Margin="5">Principle Amount</TextBlock>
            <TextBlock Grid.Column="0" Grid.Row="2" VerticalAlignment="Center" Margin="5">Interest Rate</TextBlock>
            <TextBlock Grid.Column="0" Grid.Row="3" VerticalAlignment="Center" Margin="5">Duration</TextBlock>
            <TextBox Grid.Column="1" Grid.Row="1" Margin="5" VerticalAlignment="Center">
                <TextBox.Text>
                    <Binding Path="Principle">
                        <Binding.ValidationRules>
                            <local:MyValidation/>
                        </Binding.ValidationRules>
                    </Binding>
                </TextBox.Text>
            </TextBox>
            <TextBox Grid.Column="1" Grid.Row="2" Margin="5" VerticalAlignment="Center">
                <TextBox.Text>
                    <Binding Path="InterestRate">
                        <Binding.ValidationRules>
                            <local:MyValidation/>
                        </Binding.ValidationRules>
                    </Binding>
                </TextBox.Text>
            </TextBox>
            <TextBox Grid.Column="1" Grid.Row="3" Margin="5" VerticalAlignment="Center">
                <TextBox.Text>
                    <Binding Path="Duration">
                        <Binding.ValidationRules>
                            <local:MyValidation/>
                        </Binding.ValidationRules>
                    </Binding>
                </TextBox.Text>
            </TextBox>
        </Grid>
        <DataGrid Grid.Row="1" Name="lstAmortization" Margin="5">
        </DataGrid>
        <Grid Grid.Row="2">
            <Grid.ColumnDefinitions>
                <ColumnDefinition/>
                <ColumnDefinition/>
            </Grid.ColumnDefinitions>
            <Button Grid.Column="0" Name="btnCalculate" Width="75" Height="45" Command="{Binding CalculateAmortizationCommand}" >Calculate</Button>
            <Button Grid.Column="1" Name="btnExit" Width="75" Height="45" Command="{Binding ExitCommand}" >Exit</Button>
        </Grid>
    </Grid>
</Window>