Question

I used this constructor to initialize this object

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

It turned out that allTabs was null after initialization. I couldn’t add MyHomeworkModel objects to it because of that.

Please help me and thanks in advance.

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

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