c# - 基本功能的奇怪行为

我有ObservableCollection女巫，可以容纳两种不同的类型FirstDerivedType和SecondDerivedType。它们都在Data上具有引用。主要问题是为什么当我在构造函数PlotModel中创建FirstDerivedType()的实例时

Data.PlotModel = new PlotModel();

很好但是当我在PlotModel中创建base.Draw(Data.PlotModel);的实例时，我在null中得到了DataContext。为什么？

namespace WpfApplication3
{

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        DataContext = new CurrentViewModel();
    }
}
public class CurrentViewModel
{
    private ObservableCollection<BaseType> items;

    public ObservableCollection<BaseType> Items
    {
        get
        {
            return this.items;
        }

        set
        {
            this.items = value;
        }
    }

    public CurrentViewModel()
    {
        Items = new ObservableCollection<BaseType>();
        Items.Add(new FirstDerivedType (17));
        Items.Add(new FirstDerivedType (100));
        Items.Add(new FirstDerivedType (50));
        Items.Add(new FirstDerivedType (120));
    }
}

public class Data:INotifyPropertyChanged
{
    public int ID { get; set; }
    public PlotModel PlotModel { get; set; }

    public event PropertyChangedEventHandler PropertyChanged;

    private void RaisePropertyChanged(string info)
    {
        if (PropertyChanged!=null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(info));
        }
    }
}

public abstract class BaseType
{
    public virtual void Draw(PlotModel PlotModel)
    {
        PlotModel = new OxyPlot.PlotModel(); //do not Working
        LineSeries LS= new LineSeries();
        Random rnd = new Random();
        for (int i=0; i<10; i++)
        {

            LS.Points.Add(new DataPoint(i,rnd.Next(1,100)));
        }
        PlotModel.Series.Add(LS);
        PlotModel.InvalidatePlot(false);
    }
}

public class FirstDerivedType : BaseType
{
    public FirstDerivedType(int ID)
    {
        Data = new Data();
        //Data.PlotModel = new PlotModel(); //Working
        Data.ID = ID;
        base.Draw(Data.PlotModel);
    }
    public Data Data { get; set; }
}

public class SecondDerivedType : BaseType
{
    public Data Data { get; set; }
}
}

最佳答案

在不起作用的情况下，您未设置类型PlotModel的实例的属性Data，而只是将方法参数PlotModel的值设置为新的PlotModel实例。请注意，Data.PlotModel仍为空。

这行在这里：

public virtual void Draw(PlotModel PlotModel)
{
    PlotModel = new OxyPlot.PlotModel(); //do not Working

将新实例分配给方法PlotModel的参数Draw，而不是实例PlotModel的属性Data。它不会影响拥有属性Data的实例PlotModel。这行：

base.Draw(Data.PlotModel);

如果在Data.PlotModel方法中重新分配了方法参数，则不会导致属性Draw更改。

尝试将Draw重命名为CreatePlotModel，然后执行以下操作：

public virtual PlotModel CreatePlotModel()
{
    var model = new OxyPlot.PlotModel();
    LineSeries LS= new LineSeries();
    Random rnd = new Random();
    for (int i=0; i<10; i++)
    {

        LS.Points.Add(new DataPoint(i,rnd.Next(1,100)));
    }
    model.Series.Add(LS);
    model.InvalidatePlot(false);
    return model;
}

然后分配数据的PlotModel属性：Data.PlotModel = base.CreatePlotModel()