Winforms Databinding object containing a List<T>

Question

I'm having trouble with a situation that I know must be pretty common, so I'm hoping the solution is simple. I have an object that contains a List<> of objects. It also has some properties that reflect aggregate data on the objects in the List<> (actually a BindingList<> so I can bind to it). On my form, I have a DataGridView for the List, and some other fields for the aggregate data. I can't figure out how to trigger a refresh of the aggregate data when values in the DataGridView get changed.

I have tried raising a PropertyChanged event when the properties of the objects in the List are changed, but that doesn't seem to refresh the display of the aggregate data. If I access an aggregate property (eg, display it in a messagebox), the textbox on the main form is refreshed.

Here's some simplified code to illustrate what I'm trying to do:

namespace WindowsFormsApplication1 {
public class Person {

    public int Age {
        get;
        set;
    }

    public String Name {
        get;
        set;
    }
}

public class Roster : INotifyPropertyChanged {

    public BindingList<Person> People {
        get;
        set;
    }

    public Roster () {
        People = new BindingList<Person>();
    }

    private int totalage;
    public int TotalAge {
        get {
            calcAges();
            return totalage;
        }
        set {
            totalage = value;
            NotifyPropertyChanged("TotalAge");
        }
    }

    private void calcAges () {
        int total = 0;
        foreach ( Person p in People ) {
            total += p.Age;
        }
        TotalAge = total;
    }

    #region INotifyPropertyChanged Members
    public event PropertyChangedEventHandler PropertyChanged;
    private void NotifyPropertyChanged ( String info ) {
        if ( PropertyChanged != null ) {
            PropertyChanged(this, new PropertyChangedEventArgs(info));
        }
    }
    #endregion
}
}

This looks very similar to stackoverflow.com/questions/601320/… and may be a duplicate of it. — John Saunders

casperOne casperOne · Accepted Answer · 2009-07-13T15:40:25

The calcAges method and the TotalAge property look very suspicious.

First, TotalAge should be read-only. If you allow it to be public and writable, what is the logic for changing the components that make up the age?

Second, every time you get the value, you are firing the PropertyChanged event, which is not good.

Your Roster class should look like this:

public class Roster : INotifyPropertyChanged {

    public Roster ()
    {
        // Set the binding list, this triggers the appropriate
        // event binding which would be gotten if the BindingList
        // was set on assignment.
        People = new BindingList<Person>();
    }

    // The list of people.
    BindingList<Person> people = null;

    public BindingList<Person> People 
    {
        get 
        { 
            return people; 
        }
        set 
        { 
            // If there is a list, then remove the delegate.
            if (people != null)
            {
                // Remove the delegate.
                people.ListChanged -= OnListChanged;
            }

            /* Perform error check here */ 
            people = value;

            // Bind to the ListChangedEvent.
            // Use lambda syntax if LINQ is available.
            people.ListChanged += OnListChanged;

            // Technically, the People property changed, so that
            // property changed event should be fired.
            NotifyPropertyChanged("People");

            // Calculate the total age now, since the 
            // whole list was reassigned.
            CalculateTotalAge();
        }
    }

    private void OnListChanged(object sender, ListChangedEventArgs e)
    {
        // Just calculate the total age.
        CalculateTotalAge();
    }

    private void CalculateTotalAge()
    {
        // Store the old total age.
        int oldTotalAge = totalage;

        // If you can use LINQ, change this to:
        // totalage = people.Sum(p => p.Age);

        // Set the total age to 0.
        totalage = 0;

        // Sum.
        foreach (Person p in People) {
            totalage += p.Age;
        }

        // If the total age has changed, then fire the event.
        if (totalage != oldTotalAge)
        {
            // Fire the property notify changed event.
            NotifyPropertyChanged("TotalAge");
        }
    }

    private int totalage = 0;

    public int TotalAge 
    {
        get 
        {
            return totalage;
        }
    }
    public event PropertyChangedEventHandler PropertyChanged;

    private void NotifyPropertyChanged ( String info ) {
        if ( PropertyChanged != null ) {
            PropertyChanged(this, new PropertyChangedEventArgs(info));
        }
    }
}

Now, when the properties in the list items are changed, the parent object will fire the property changed event, and anything bound to it should change as well.

Winforms Databinding object containing a List<T>

2 Answers