WPF ObservableCollection和PropertyChanged

Question

I have a question regarding updating an observable collection. I am using Visual Studio 2010 c#. I am a relative newbie and may have made a catastrophic error so apologies to start. Basically this code works perfectly when I run this on Windows 7 but on Windows 8 I get a NullReferenceException on PropertyChanged.

I have a class that holds the Tasks observable collection.

Tasks.cs

namespace TimeLocation
{
    public static class TStatic
    {
        static TStatic()
        {
            TaskList = new ObservableCollection<Tasks>(); 
        }
        public static ObservableCollection<Tasks> TaskList { set; get; }
    }


public class Tasks : INotifyPropertyChanged
{
    public int taskID;
    public string taskname;

    public Tasks(string tname)
    {

        this.taskname = tname;
    }



    public string Taskname
    {
        get { return taskname; }
        set
        {
            if (value != "")
            {
                taskname = value; OnPropertyChanged("Taskname");
            }
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    protected void OnPropertyChanged(string info)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(info));

        }
    }
}

I have a grid in MainWindow that is bound to Tasks and when updated updates fine via PropertyChanged above. The user can also edit a Tasks by dragging with a mouse. I want this to update the grid and observable collection as well. When run on Windows 7 this worked but fails on Windows 8.

In Code behind of mainwindow.xaml

public partial class MainWindow : INotifyPropertyChanged
{

    public MainWindow()
    {

        InitializeComponent();

        DataContext = TStatic.TaskList;
    }

    private void dCanvas_PreviewMouseLeftButtonUp(object sender, System.Windows.Input.MouseButtonEventArgs e)
    {
        Tasks t = _selectedtask;
        t.taskname = "Test";
        PropertyChanged(t, new PropertyChangedEventArgs("Taskname"));
    }
}

I am sure that the taskname is updated as the name has changed when I save it to file and add a watch.

_selectedtask has been filled with a task from selecting a grid row.

In Windows 8 the line

PropertyChanged(t, new PropertyChangedEventArgs("Taskname"));

returns the null error. In Windows 7 it is not null and updates the grid properly.

Answer 1

First of all, public fields declaration is a very bad practice. You should make them private and give access to them, if needed, through the public properties.

Secondly, the ui elements like Window shouldn't implement INotifyPropertyChanged interface. You implement it for the view model classes (see the MVVM pattern, the recommended approach to the wpf apps) or at least for the data classes like your Tasks.

In your dCanvas_PreviewMouseLeftButtonUp code the update probably didn't work because you update the field of Tasks object but call the PropertyChanged method of the MainWindow. Setting up TaskName property is enough:

t.TaskName =  "Test"