Question

I have multiple viewModels in my application and am binding/used them in ViewModelLocator mvvm light. I have done button on one of my DailyActivities.xaml page. but when i clicked on it,it gives me error in ViewModelLocator like “Object reference not set to an instance of an object”. and control comes to this line :

public static void ClearActivities()
{
_activities.Cleanup(); //Error here
_activities = null;
}

here is the code for DailyActivitiesViewModel in ViewModelLocator:

private static ActivitiesViewModel _activities;

          public static ActivitiesViewModel ActivitiesStatic
    {
        get
        {
            if (_activities == null)
            {
                CreateActivities();
            }

            return _activities;
        }
    }

    /// <summary>
    /// Gets the ViewModelPropertyName property.
    /// </summary>
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Performance",
        "CA1822:MarkMembersAsStatic",
        Justification = "This non-static member is needed for data binding purposes.")]
    public ActivitiesViewModel Activities
    {
        get
        {
            return ActivitiesStatic;
        }
    }

    /// <summary>
    /// Provides a deterministic way to delete the ViewModelPropertyName property.
    /// </summary>
    public static void ClearActivities()
    {
        _activities.Cleanup(); --Error here
        _activities = null;
    }

    /// <summary>
    /// Provides a deterministic way to create the ViewModelPropertyName property.
    /// </summary>
    public static void CreateActivities()
    {
        if (_activities == null)
        {
            _activities = new ActivitiesViewModel();
        }
    }

Kindly Suggest?

Thanks

Answer 1

You need to determine if the field exists before you can use it: It seems like if the field _activities is already null then no clean-up is required.

why not do something like

public static void ClearActivites()
{
    if (null == _activities) return;
    _activities.Cleanup();
    _activities = null;
}