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<Window x:Class="WPF_MultiBinding.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="CodeArsenal.net"
        Height="158" Width="245"
        xmlns:local="clr-namespace:WPF_MultiBinding">
    <Window.Resources>
        <local:MultiValueConverter x:Key="MultiValueConverter" />
    </Window.Resources>
    
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="10" />
            <ColumnDefinition Width="65" />
            <ColumnDefinition Width="Auto" />
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="10" />
            <RowDefinition Height="Auto" />
        </Grid.RowDefinitions>

        <TextBox Name="textBoxOne" Grid.Row="0" Height="20" MaxLength="8" Grid.Column="1"
                 Text="{Binding TextOne, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
        
        <TextBox Name="textBoxTwo" Grid.Row="1" Height="20" MaxLength="8" Grid.Column="1" 
                 Text="{Binding TextTwo, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
        
        <TextBox Name="textBoxThree" Grid.Row="2" Height="20" MaxLength="8" Grid.Column="1"
                 Text="{Binding TextThree, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>

        <TextBlock Name="textBlockOutput" Grid.Row="4" Grid.Column="2">
            <TextBlock.Text>
                <MultiBinding Converter="{StaticResource MultiValueConverter}">
                    <Binding Path="TextOne" />
                    <Binding Path="TextTwo" />
                    <Binding Path="TextThree" />
                </MultiBinding>
            </TextBlock.Text>
        </TextBlock>
    </Grid>
</Window>

public class ViewModel : INotifyPropertyChanged
{
    public ViewModel() { }

    public event PropertyChangedEventHandler PropertyChanged;
    private string _textOne;
    public string TextOne 
    {
        get { return _textOne; } 
        set
        {
            _textOne = value;
            NotifyPropertyChanged("TextOne");
        } 
    }

    private string _textTwo;
    public string TextTwo
    {
        get { return _textTwo; }
        set
        {
            _textTwo = value;
            NotifyPropertyChanged("TextTwo");
        } 
    }

    private string _textThree;
    public string TextThree
    {
        get { return _textThree; }
        set
        {
            _textThree = value;
            NotifyPropertyChanged("TextThree");
        } 
    }

    public void NotifyPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

public class MultiValueConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        string one = values[0] as string;
        string two = values[1] as string;
        string three = values[2] as string;
        if(!string.IsNullOrEmpty(one) && !string.IsNullOrEmpty(two) && !string.IsNullOrEmpty(three))
        {
            return one + two + three;
        }
        return null;
    }

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