向ObservableCollection中添加元素 - 对象引用错误

Question

我正在使用绑定的ViewModel和Xamarin Forms创建一个秒表类型的应用程序。在代码中，我想要实现的是，在视图中按下“Lap”按钮时触发一个命令，该命令运行一个方法，该方法应该向一个对象添加大量详细信息，然后将其添加到ObservableCollection中。

我的问题是，添加按钮实际上没有添加到集合中。

我在这一行得到错误“System.NullReferenceException: Object reference not set to an instance of an object.”

RaceTimings.Add(lap);

我做错了什么？完整代码如下。

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Diagnostics;
using System.Linq;
using System.Timers;
using System.Windows.Input;
using TechsportiseApp.Models;
using Xamarin.Forms;

namespace TechsportiseApp.ViewModels
{
    public class TimerViewModel : INotifyPropertyChanged
    {

        public Timer myTimer = new Timer();

        public ICommand StartTimerCommand { get; set; }
        public ICommand LapCommand { get; set; }

        public TimerViewModel()
        {
            StartTimerCommand = new Command(StartTimer);
            LapCommand = new Command(LapButton);

            LapPosition = 1;
        }

        //TimeSpan _elapsedTime;
        //public TimeSpan ElapsedTime
        //{
        //    get {              
        //        return _elapsedTime;
        //    }
        //    set
        //    {
        //        if (_elapsedTime == value)
        //            return;

        //        _elapsedTime = value;
        //        OnPropertyChanged("ElapsedTime");
        //    }
        //}

        string _elapsedTime;
        public String ElapsedTime
        {
            get
            {
                //if(ElapsedTime == "")
                //{
                //    _elapsedTime = "00:00:00.00";
                //}
                return _elapsedTime;
            }
            set
            {
                if (_elapsedTime == value)
                    return;

                _elapsedTime = value;
                OnPropertyChanged("ElapsedTime");
            }
        }

        DateTime _currentTime;
        public DateTime CurrentTime
        {
            get { return _currentTime; }
            set
            {
                if (_currentTime == value)
                    return;

                _currentTime = value;
                OnPropertyChanged("CurrentTime");
            }
        }

        DateTime _raceStartTime;
        public DateTime RaceStartTime
        {
            get { return _raceStartTime; }
            set
            {
                if (_raceStartTime == value)
                    return;

                _raceStartTime = value;
                OnPropertyChanged("StartTime");
            }
        }

        DateTime _lapTime;
        public DateTime LapTime
        {
            get { return _lapTime; }
            set
            {
                if (_lapTime == value)
                    return;

                _lapTime = value;
                OnPropertyChanged("LapTime");
            }
        }

        ObservableCollection<Timing> _timings;
        public ObservableCollection<Timing> RaceTimings
        {
            get
            {
                //var sortedtimings = new ObservableCollection<Timing>(_timings.OrderBy(c => c.Position));
                return _timings;
            }
            set
            {
                if (_timings != value)
                {
                    _timings = value;
                    OnPropertyChanged("Timings");
                }
            }
        }

        int _lapPosition;
        public int LapPosition
        {
            get { return _lapPosition; }
            set
            {
                if (_lapPosition == value)
                    return;

                _lapPosition = value;
                OnPropertyChanged("LapPosition");
            }
        }



        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void OnPropertyChanged(string propertyName)
        {
            var changed = PropertyChanged;
            if (changed != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }

        void StartTimer()
        {
            myTimer.Elapsed += new ElapsedEventHandler(DisplayTimeEvent);
            myTimer.Interval = 10; // 1000 ms is one second
            myTimer.Start();
            RaceStartTime = DateTime.Now;
            //ElapsedTime = myTimer.Elapsed.Milliseconds() += new ElapsedEventHandler(DisplayTimeEvent);
        }

        void DisplayTimeEvent(object source, ElapsedEventArgs e)
        {
            TimeSpan _elapsed;
            _elapsed = e.SignalTime - RaceStartTime;

            string hours = _elapsed.Hours.ToString().PadLeft(2, '0');
            string minutes = _elapsed.Minutes.ToString().PadLeft(2, '0');
            string seconds =  _elapsed.Seconds.ToString().PadLeft(2, '0');
            int _hundredths = _elapsed.Milliseconds / 10;

            string hundredths = _hundredths.ToString().PadLeft(2,'0');
            _elapsedTime = hours + ":" + minutes + ":" + seconds + "." + hundredths;
            ElapsedTime = _elapsedTime;
            OnPropertyChanged("ElapsedTime");
            var RaceTimings = new ObservableCollection<Timing>();
        }

        void LapButton()
        {
            var lap = new Timing
            {
                Id = 0,
                RaceId = 0,
                StartTime = RaceStartTime,
                EndTime = DateTime.Now,
                Position = LapPosition,
                Status = 0
            };
            RaceTimings.Add(lap);
            LapPosition = LapPosition + 1;
            var listcheck = RaceTimings.Count;

        }
    }
}

- Matthew Warr

Answer 1

只需在构造函数中初始化_timings 集合，如下所示：

RaceTimings = new ObservableCollection<Timing>();

或者初始化字段：

ObservableCollection<Timing> _timings = new ObservableCollection<Timing>();

- Pawan Nogariya

---

谢谢！就是这个起了作用。另一个选项还是出错了。RaceTimings = new ObservableCollection<Timing>(); - Matthew Warr