Question

我使用此构造函数初始化此对象

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

事实证明，初始化后allTabs为null。由于这个原因，我无法将MyHomeworkModel对象添加到它。

请帮助我，并提前致谢。

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);
            }
        }
    }
}

Answer 1

这里（在MyHomeworkViewModel的构造函数中）：

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

创建一个隐藏类成员的局部变量，因此在构造函数调用后它保持为null。为了避免这种情况，只需跳过语句开头的类型：

allTabs = new ObservableCollection<MyHomeworkModel>();

Answer 2

如果你创建

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

在您的构造函数中，它将在用完后删除。在外面声明它并且只调用

allTabs = new ObservableCollection<MyHomeworkModel>();

Answer 3

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

Answer 4

第ObservableCollection<MyHomeworkModel> allTabs = new ObservableCollection<MyHomeworkModel>();行不将allTabs 字段设置为新集合，它创建了一个新的本地变量，将该局部变量设置为新集合。要初始化字段，您应该将该行更改为：

allTabs = new ObservableCollection<MyHomeworkModel>();

Answer 5

这是使用下划线为私有类变量添加前缀的C＃约定的一个原因：

private ObservableCollection<MyHomeworkModel> _allTabs;

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

为什么observablecollection初始化为null？

5 个答案: