wpf - 对象引用未设置为 ViewModelLocator Mvvm Light 中的对象实例

Question

我的应用程序中有多个视图模型，并在 ViewModelLocator mvvm light 中绑定/使用它们。我在 DailyActivities.xaml 页面之一上完成了按钮。但是当我单击它时，它在 ViewModelLocator 中出现错误，例如“对象引用未设置为对象的实例”。和控制来到这一行：

公共静态无效ClearActivities（）{_activities.Cleanup（）；//这里出错_activities = null; }

这是 ViewModelLocator 中 DailyActivityViewModel 的代码：

私有静态活动视图模型_活动；

          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();
        }
    }

请建议？

谢谢

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;
}