为什么observablecollection初始化为null

本文关键字：null 初始化 observablecollection 为什么 | 更新日期: 2023-09-27 17:54:28

我使用这个构造函数来初始化这个对象

 ObservableCollection<MyHomeworkModel> allTabs = new ObservableCollection<MyHomeworkModel>();

初始化后allTabs为null。我不能添加MyHomeworkModel对象，因为它。

请帮助我，并提前感谢。

using System.Windows.Input;
using System.Collections.ObjectModel;
using MyHomework;
namespace MyHomework__MVVM_
{
    class MyHomeworkViewModel
    {
        private ObservableCollection<MyHomeworkModel> allTabs;
        private MyHomeworkModel selectedTab;
        public MyHomeworkViewModel()
        {
            ObservableCollection<MyHomeworkModel> allTabs = new ObservableCollection<MyHomeworkModel>();
            selectedTab = new MyHomeworkModel();
            AddCourseCommand = new AddCourseCommand(this);
        }
        public ObservableCollection<MyHomeworkModel> AllTabs
        {
            get
            {
                return allTabs;
            }
            set
            {
                allTabs = value;
            }
        }
        public MyHomeworkModel SelectedTab
        {
            get
            {
                return selectedTab;
            }
            set
            {
                selectedTab = value;
            }
        }
        public ICommand AddCourseCommand
        {
            get;
            private set;
        }
        public void AddNewTab()
        {
            NewCourseName ncn = new NewCourseName();
            ncn.ShowDialog();
            if (ncn.courseName != null)
            {
                MyHomeworkModel newTab = new MyHomeworkModel();
                newTab.Header = ncn.courseName;
                AllTabs.Add(newTab);
            }
        }
    }
}

为什么observablecollection初始化为null

这里(在MyHomeworkViewModel的构造函数中):

ObservableCollection<MyHomeworkModel> allTabs = new ObservableCollection<MyHomeworkModel>();

您创建了一个局部变量来遮蔽类成员，因此在构造函数调用之后它仍然为空。要避免这种情况，只需跳过语句开头的类型:

allTabs = new ObservableCollection<MyHomeworkModel>();

 public MyHomeworkViewModel()
    {
        allTabs = new ObservableCollection<MyHomeworkModel>();
        selectedTab = new MyHomeworkModel();
        AddCourseCommand = new AddCourseCommand(this);
    }

如果创建

ObservableCollection<MyHomeworkModel> allTabs = new ObservableCollection<MyHomeworkModel>();

在构造函数中，它将在用完后被删除。在外部声明它，然后调用

allTabs = new ObservableCollection<MyHomeworkModel>();

ObservableCollection<MyHomeworkModel> allTabs = new ObservableCollection<MyHomeworkModel>();是而不是将allTabs 字段设置为新的集合，它创建了一个新的局部变量并将该局部变量设置为新的集合。要初始化该字段，您应该将该行更改为:

allTabs = new ObservableCollection<MyHomeworkModel>();

这是c#约定在私有类变量前加下划线的原因之一:

private ObservableCollection<MyHomeworkModel> _allTabs;

更多指南在这里:http://msdn.microsoft.com/en-gb/library/vstudio/ms229042%28v=vs.100%29.aspx

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