绑定到ObservableCollection中items字段的合计，并在值更改时更新

using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Linq;

public class ViewModel : INotifyPropertyChanged
{
    private ObservableCollection<int> items;
    private int total;

    public ObservableCollection<int> Items
    {
        get { return items; }
        set
        {
            items = value;
            UpdateTotal();
            OnPropertyChanged("Items");
        }
    }

    public int Total
    {
        get { return total; }
        set
        {
            total = value;
            OnPropertyChanged("Total");
        }
    }

    public ViewModel()
    {
        Items = new ObservableCollection<int>();
        Items.CollectionChanged += (sender, e) => UpdateTotal();
    }

    private void UpdateTotal()
    {
        Total = Items.Sum();
    }

    public event PropertyChangedEventHandler PropertyChanged;

    protected virtual void OnPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

<Grid>
    <TextBlock Text="{Binding Total}" />
</Grid>

ViewModel viewModel = new ViewModel();
DataContext = viewModel;