XAML bind command parameter to DataGrid column name

Question

I am using Galasoft MVVM Light. I have a DataGrid with four columns with a gross amount, a VAT-rate, VAT-amount and net amount. When the user changes one of the values then I need to recalculate the other three. I have a relay command attached to the CellEditEnding event of the DataGrid to activate code in the ViewModel that does the calculation. But I need to know which value has been changed in order to do the calculation correctly. How do I specify in XAML which column/cell has triggered the event, ie how to bind the command parameter to the name of the triggering column/cell?

<DataGrid
     ....>
<i:Interaction.Triggers>
    <i:EventTrigger EventName="CellEditEnding">
        <mvvm:EventToCommand Command="{Binding  CalculateAusgabe_Command, 
                    Mode=OneWay, 
                    UpdateSourceTrigger=PropertyChanged, 
                    diag:PresentationTraceSources.TraceLevel=High}"
              CommandParameter="{Binding RelativeSource={RelativeSource AncestorType={x:Type DataGridTextColumn}}, Path=Name}" />
    </i:EventTrigger>
</i:Interaction.Triggers>
</DataGrid>

In ViewModel the code is:

public RelayCommand<string> CalculateAusgabe_Command { get; set; }
...
CalculateAusgabe_Command = new RelayCommand<string>(CalculateAusgabe);

and finally the procedure that does the calculation:

void CalculateAusgabe(string colName)
{
    if (currBetrag.USt == null) return;
    switch (colName)
    {
        case "colBetBtto":
        case "colBetUStS":
            if (currBetrag.BttoBetrag != 0M)
            {
                currBetrag.UStBetrag = Math.Round((currBetrag.BttoBetrag / (100M + currBetrag.USt.UStProz)) * currBetrag.USt.UStProz, 2);
                currBetrag.NttoBetrag = currBetrag.BttoBetrag - currBetrag.UStBetrag;
            }
            break;
        case "colBetUStB":
        ...

The current row of the DataGrid is bound to currBetrag object. Everything works fine, CalculateAusgabe is activated when the event is triggered but no matter what I do the parameter colName always is null. Any help on this problem is appreciated.

Answer 1

You could perform the calculations in the setters of the currBetrag class, eg:

public class currBetrag : INotifyPropertyChanged
{
    private decimal _bttoBetrag;
    public decimal BttoBetrag
    {
        get { return _bttoBetrag; }
        set
        {
            _bttoBetrag = value;
            OnPropertyChanged();

            CalculateAusgabe("BttoBetrag");
        }
    }

    private decimal _uStBetrag;
    public decimal UStBetrag
    {
        get { return _uStBetrag; }
        set
        {
            _uStBetrag = value;
            OnPropertyChanged();

            CalculateAusgabe("UStBetrag");
        }
    }

    private void CalculateAusgabe(string colName)
    {
        if (USt == null) return;
        switch (colName)
        {
            //set the value of all fields...
            _uStBetrag = ?;
        }

        //and raise the PropertyChanged event for all involved properties
        OnPropertyChanged("UStBetrag");
        OnPropertyChanged("NttoBetrag");
        //...
    }

    public event PropertyChangedEventHandler PropertyChanged;
    private void OnPropertyChanged([CallerMemberName] String propertyName = "")
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
     }
}

Note that is considered a bad practice not to use English member names regardless of what your native language is.