How to bind multiple textboxes with a single textblock in wpf

If you need to update the sum on TextBox lost focus event you can use classic events. This is the XAML (I used just four TextBoxes, but it is easy to extend):

<StackPanel>
    <TextBox Name="txt01" Margin="3" HorizontalAlignment="Stretch" LostFocus="txt_LostFocus" TextChanged="txt_TextChanged" />
    <TextBox Name="txt02" Margin="3" HorizontalAlignment="Stretch" LostFocus="txt_LostFocus" TextChanged="txt_TextChanged" />
    <TextBox Name="txt03" Margin="3" HorizontalAlignment="Stretch" LostFocus="txt_LostFocus" TextChanged="txt_TextChanged" />
    <TextBox Name="txt04" Margin="3" HorizontalAlignment="Stretch" LostFocus="txt_LostFocus" TextChanged="txt_TextChanged" />
    <TextBlock Name="sum" Margin="3,10,3,3" />
</StackPanel>

In the code you have the event handlers:

private void txt_LostFocus(object sender, RoutedEventArgs e)
{
    int value1;
    int value2;
    TextBox textBox = (TextBox)sender;

    if (textBox.Tag is bool && (bool)textBox.Tag)
    {

        if (Int32.TryParse(textBox.Text, out value1))
        {
            if (String.IsNullOrEmpty(sum.Text))
            {
                sum.Text = textBox.Text;
            }
            else
            {
                Int32.TryParse(sum.Text, out value2);
                sum.Text = Convert.ToString(value1 + value2);
            }
        }

        textBox.Tag = false;
    }
}

private void txt_TextChanged(object sender, TextChangedEventArgs e)
{
    TextBox textBox = (TextBox)sender;
    textBox.Tag = true;
}

On the other side, if you can give up the "LostFocus" requirement, you can use MultiBinding (in this case it works just in "PropertyChanged mode", since TextBoxes are now the sources):

<StackPanel>
    <TextBox Name="txt01" Margin="3" HorizontalAlignment="Stretch" />
    <TextBox Name="txt02" Margin="3" HorizontalAlignment="Stretch" />
    <TextBox Name="txt03" Margin="3" HorizontalAlignment="Stretch" />
    <TextBox Name="txt04" Margin="3" HorizontalAlignment="Stretch" />
    <TextBlock Name="sum" Margin="3,10,3,3">
        <TextBlock.Text>
            <MultiBinding Converter="{StaticResource AddValueConverter}" Mode="OneWay">
                <MultiBinding.Bindings>
                    <Binding ElementName="txt01" Path="Text" />
                    <Binding ElementName="txt02" Path="Text" />
                    <Binding ElementName="txt03" Path="Text" />
                    <Binding ElementName="txt04" Path="Text" />
                </MultiBinding.Bindings>
            </MultiBinding>
        </TextBlock.Text>
    </TextBlock>
</StackPanel>

You just need to write a simple converter:

public class AddValueConverter : IMultiValueConverter
{

    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        int sum = 0;
        int result;

        foreach(object value in values)
        {
            if (Int32.TryParse(System.Convert.ToString(value), out result))
            {
                sum += result;
            }
        }

        return System.Convert.ToString(sum);
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
    {
        throw new NotSupportedException();
    }
}