使用MVVM Light开发日历时ObservableCollection变更后ItemsControl不更新

问题根源 #

ObservableCollection<T> 仅会在集合本身发生添加、删除、替换元素这类整体操作时向UI发送变更通知，集合内部元素的属性变更需要元素类自行实现属性变更通知机制。你当前的Dates类没有实现INotifyPropertyChanged接口，修改IsSelected等属性后，绑定的UI无法收到变更通知，自然不会触发样式更新。

解决方案 #

因为你使用了MVVM Light框架，直接让Dates类继承框架提供的ObservableObject基类即可快速实现属性通知，修改后的Dates类代码如下：

using GalaSoft.MvvmLight;

public class Dates : ObservableObject
{
    public Dates(DateTime day, int currentMonth)
    {
        Date = day;
        Day = day.Day.ToString();
        IsInactive = day.Month != currentMonth;
        IsToday = day.DayOfYear == DateTime.Today.DayOfYear;
    }

    #region Properties
    private DateTime _date;
    public DateTime Date
    {
        get => _date;
        set { _date = value; RaisePropertyChanged(); }
    }

    private string _day;
    public string Day
    {
        get => _day;
        set { _day = value; RaisePropertyChanged(); }
    }

    private bool _isInactive;
    public bool IsInactive
    {
        get => _isInactive;
        set { _isInactive = value; RaisePropertyChanged(); }
    }

    private bool _isToday;
    public bool IsToday
    {
        get => _isToday;
        set { _isToday = value; RaisePropertyChanged(); }
    }

    private bool _isSelected;
    public bool IsSelected
    {
        get => _isSelected;
        set { _isSelected = value; RaisePropertyChanged(); }
    }
    #endregion Properties
}

可选优化 #

你当前的点击命令处理方法遍历整个集合查找匹配日期，可简化为：

private void DayClickedeCommandParameterhandling(DateTime s)
{
    var target = List.FirstOrDefault(d => d.Date == s);
    if (target != null)
    {
        // 如果需要单选逻辑可以先把之前选中的项IsSelected重置为false
        // foreach(var item in List) item.IsSelected = false;
        target.IsSelected = true;
    }
}

修改后重新运行，点击按钮即可触发对应的DataTrigger样式更新。

内容的提问来源于stack exchange，提问作者Andreas Prinz