WPF自定义网格，在ArrangeOverride或MeasureOverride中绘制行和列会导致异常

Question

我正在尝试创建一个自定义网格，根据最大的可用子维度和窗口维度，它可以动态调整大小并重新定位它的子项。这意味着我必须动态地重绘我的行和列定义。下面给出的是我的自定义网格代码

public class GridPanel : Grid
{
    #region Fields

    Size maxItemSize;
    int maxPossibleColumns = 0;
    int maxPossibleRows = 0;

    #endregion

    #region Overrides

    protected override Size MeasureOverride(Size constraint)
    {
        this.ShowGridLines = true;
        GetMaxItemSize(constraint);
        //SetupGridCells(constraint);
        return constraint;
    }

    protected override Size ArrangeOverride(Size arrangeSize)
    {
        //SetupGridCells(arrangeSize); 
        return base.ArrangeOverride(arrangeSize); //Caught a nullReferrence exception
    }



    #endregion

    #region Methods

    private void GetMaxItemSize(Size availableSize)
    {
        foreach (UIElement item in Children)
        {
            item.Measure(availableSize);
            maxItemSize = item.DesiredSize.Width > maxItemSize.Width ? item.DesiredSize : maxItemSize;
        }
    }

    private void SetupGridCells(Size finalSize)
    {
        ColumnDefinition spaceWidth;
        ColumnDefinition cellWidth;

        RowDefinition spaceHeight;
        RowDefinition cellHeight;

        maxPossibleColumns = (int)(finalSize.Width / maxItemSize.Width);
        for (int i = 0; i < maxPossibleColumns; i++)
        {
            spaceWidth = new ColumnDefinition { Width = new GridLength(1, GridUnitType.Star) };
            cellWidth = new ColumnDefinition { Width = new GridLength(maxItemSize.Width) };

            this.ColumnDefinitions.Add(spaceWidth);//***
            this.ColumnDefinitions.Add(cellWidth);//***
        }
        spaceWidth = new ColumnDefinition { Width = new GridLength(1, GridUnitType.Star) };
        this.ColumnDefinitions.Add(spaceWidth);//***

        /////////////////////////////////////////////////////////////////////////////////////////////

        maxPossibleRows = (int)(finalSize.Height / maxItemSize.Height);
        for (int i = 0; i < maxPossibleRows; i++)
        {
            spaceHeight = new RowDefinition { Height = new GridLength(1, GridUnitType.Star) };
            cellHeight = new RowDefinition { Height = new GridLength(1, GridUnitType.Star) };

            this.RowDefinitions.Add(spaceHeight);//***
            this.RowDefinitions.Add(cellHeight);//***
        }
        spaceHeight = new RowDefinition { Height = new GridLength(1, GridUnitType.Star) };
        this.RowDefinitions.Add(spaceHeight);
    }

    #endregion
}

这就是我使用面板的方式

<Panels:GridPanel>
        <Rectangle Height="10" Width="10" Fill="Red" />
        <Rectangle Height="20" Width="20" Fill="Black" />
        <Rectangle Height="30" Width="30" Fill="Green" />
        <Rectangle Height="40" Width="40" Fill="Blue" />
        <Rectangle Height="50" Width="50" Fill="Aqua" />
        <Rectangle Height="60" Width="60" Fill="Wheat" />
        <Rectangle Height="70" Width="70" Fill="Blue" />
        <Rectangle Height="80" Width="80" Fill="DarkBlue" />
        <Rectangle Height="90" Width="90" Fill="DarkGoldenrod" />
        <Rectangle Height="100" Width="100" Fill="Gainsboro" />
        <Rectangle Height="110" Width="110" Fill="Pink" />
        <Rectangle Height="120" Width="120" Fill="Brown" />
    </Panels:GridPanel>

SetupGridCells（）是我正在尝试重绘行和列的方法。问题是即使我在ArrangeOveride（）或MeasureOverride（）中调用此方法，它也会在返回base.ArrangeOverride（arrangeSize）中抛出nullReference异常;在进一步调试时，我注意到this.ColumnDefinitions.Add（spaceWidth）; // *（那些用注释// * 标记的行）导致了这个问题。

有人可以指出问题是什么吗？提前致谢