关于C#“未将对象引用设置到对象的实例”的几个问题。

Question

我是一个超级小白，也是第一次提问，还望各位大佬指教。

首先，我编写了一个WPF窗体和一个TextBox.ID为"aa1"的“TextChanged", 想达到如下目的: 当“aa1”的text值发生改变后，立即实时的将"aa1"的text值赋给TextBox.ID为“aa2”的text。
但是调试时在“aa2.Text = Convert.ToString(x1);”这句话提示“System.NullReferenceException:“未将对象引用设置到对象的实例。”

个人想法是这样的：因为这是“aa1”的TextChanged，所以C#可以知道“aa1”是什么，但是对“aa2”没有说明，导致C#不知道这个“aa2”是什么，问题可能是出在“实例化”与“引用”之间。

想请问各位大佬：
1.对于这种情况我应该怎样去修改？
2.在“未将对象引用设置到对象的实例”这句话中，“对象”指的是什么？

代码片段如下：

         private void Aa1_TextChanged(object sender, TextChangedEventArgs e)
        {
            double x1 = Convert.ToDouble(aa1.Text);
            aa2.Text = Convert.ToString(x1);
        }

谢谢。

Answer 1

自己来结一下帖子，直接上代码。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using Autodesk.Revit.DB;
using Autodesk.Revit.UI;
using Autodesk.Revit.ApplicationServices;
using Autodesk.Revit.UI.Events;
using Autodesk.Revit.DB.Events;
using Autodesk.Revit;
using System.Drawing;

using Rectangle = System.Windows.Shapes.Rectangle;

namespace Bridge_design
{
/// <summary>
/// Cross_section_size.xaml 的交互逻辑
/// </summary>
public partial class Cross_section_size : System.Windows.Window
{
    ExternalCommandData m_revit;
    public Cross_section_size(ExternalCommandData revit)
    {
        InitializeComponent();
        m_revit = revit;
        InitTextbox();
        InitCanvas();

    }

    private void InitCanvas()
    {
        double a1 = Convert.ToDouble(aa1.Text);
        double a2 = Convert.ToDouble(aa2.Text);
        double a3 = Convert.ToDouble(aa3.Text);
        double a4 = Convert.ToDouble(aa4.Text);
        double a5 = Convert.ToDouble(aa5.Text);
        double a6 = Convert.ToDouble(aa6.Text);
        double a7 = Convert.ToDouble(aa7.Text);
        double a8 = Convert.ToDouble(aa8.Text);
        double a9 = Convert.ToDouble(aa9.Text);
        double b1 = Convert.ToDouble(bb1.Text);
        double b2 = Convert.ToDouble(bb2.Text);
        double b3 = Convert.ToDouble(bb3.Text);
        double b4 = Convert.ToDouble(bb4.Text);
        double b5 = Convert.ToDouble(bb5.Text);
        double b6 = Convert.ToDouble(bb6.Text);
        DrawingLine(new System.Windows.Point(0, 0), new System.Windows.Point(2 * a1 + 2 * a2 + 2 * a3 + a5, 0));
        DrawingLine(new System.Windows.Point(0, b1), new System.Windows.Point(a1, b1));
        DrawingLine(new System.Windows.Point(a1, b1), new System.Windows.Point(a1 + a2, b1 + b2));
        DrawingLine(new System.Windows.Point(a1 + a2, b1 + b2), new System.Windows.Point(a6, b1 + b2 + b3 + b4 + b5));
        DrawingLine(new System.Windows.Point(a6, b1 + b2 + b3 + b4 + b5), new System.Windows.Point(a6 + 2 * a7 + a9, b1 + b2 + b3 + b4 + b5));
        DrawingLine(new System.Windows.Point(a6 + 2 * a7 + a9, b1 + b2 + b3 + b4 + b5), new System.Windows.Point(a1 + a2 + 2 * a3 + a5, b1 + b2));
        DrawingLine(new System.Windows.Point(a1 + a2 + 2 * a3 + a5, b1 + b2), new System.Windows.Point(a1 + 2 * a2 + 2 * a3 + a5, b1));
        DrawingLine(new System.Windows.Point(a1 + 2 * a2 + 2 * a3 + a5, b1), new System.Windows.Point(2 * a1 + 2 * a2 + 2 * a3 + a5, b1));
        DrawingLine(new System.Windows.Point(a1 + a2 + a3, b1), new System.Windows.Point(a1 + a2 + a3 + a5, b1));
        DrawingLine(new System.Windows.Point(a1 + a2 + a3 + a5, b1), new System.Windows.Point(a1 + a2 + a3 + a4 + a5, b1 + b2));
        DrawingLine(new System.Windows.Point(a1 + a2 + a3 + a4 + a5, b1 + b2), new System.Windows.Point(a6 + a7 + a8 + a9, b1 + b2 + b3));
        DrawingLine(new System.Windows.Point(a6 + a7 + a9, b1 + b2 + b3 + b4), new System.Windows.Point(a6 + a7, b1 + b2 + b3 + b4));
        DrawingLine(new System.Windows.Point(a6 + a7 - a8, b1 + b2 + b3), new System.Windows.Point(a1 + a2 + a3 - a4, b1 + b6));
        DrawingLine(new System.Windows.Point(a1 + a2 + a3 - a4, b1 + b6), new System.Windows.Point(a1 + a2 + a3, b1));
        if (b4 == 0)
        {
        }
        else
        {
            DrawingLine(new System.Windows.Point(a6 + a7 + a8 + a9, b1 + b2 + b3), new System.Windows.Point(a6 + a7 + a9, b1 + b2 + b3 + b4));
            DrawingLine(new System.Windows.Point(a6 + a7, b1 + b2 + b3 + b4), new System.Windows.Point(a6 + a7 - a8, b1 + b2 + b3));
        }
    }
    private void InitTextbox()
    {
        this.aa1.Text = 17.5.ToString();
        this.aa2.Text = 19.2.ToString();
        this.aa3.Text = 33.ToString();
        this.aa4.Text = 15.ToString();
        this.aa5.Text = 113.ToString();
        this.aa6.Text = 67.ToString();
        this.aa7.Text = 28.ToString();
        this.aa8.Text = 16.ToString();
        this.aa9.Text = 51.ToString();
        this.bb1.Text = 15.ToString();
        this.bb2.Text = 6.5.ToString();
        this.bb3.Text = 110.ToString();
        this.bb4.Text = 9.5.ToString();
        this.bb5.Text = 18.ToString();
        this.bb6.Text = 7.ToString();
        this.aa1.TextChanged += Aa1_TextChanged;

    }

    private void Aa1_TextChanged(object sender, TextChangedEventArgs args)
    {
        mainPane1.Children.Clear();
        System.Windows.Controls.TextBox textBox = sender as System.Windows.Controls.TextBox;
        if (textBox == null)
            return;
        double x1 = Convert.ToDouble(textBox.Text);
        double x2 = Convert.ToDouble(aa2.Text);
        double x3 = Convert.ToDouble(aa3.Text);
        double x4 = Convert.ToDouble(aa4.Text);
        double x5 = Convert.ToDouble(aa5.Text);
        double x6 = Convert.ToDouble(aa6.Text);
        double x7 = Convert.ToDouble(aa7.Text);
        double x8 = Convert.ToDouble(aa8.Text);
        double x9 = Convert.ToDouble(aa9.Text);
        double y1 = Convert.ToDouble(bb1.Text);
        double y2 = Convert.ToDouble(bb2.Text);
        double y3 = Convert.ToDouble(bb3.Text);
        double y4 = Convert.ToDouble(bb4.Text);
        double y5 = Convert.ToDouble(bb5.Text);
        double y6 = Convert.ToDouble(bb6.Text);
        DrawingLine(new System.Windows.Point(0, 0), new System.Windows.Point(2 * x1 + 2 * x2 + 2 * x3 + x5, 0));
        DrawingLine(new System.Windows.Point(0, y1), new System.Windows.Point(x1, y1));
        DrawingLine(new System.Windows.Point(x1, y1), new System.Windows.Point(x1 + x2, y1 + y2));
        DrawingLine(new System.Windows.Point(x1 + x2, y1 + y2), new System.Windows.Point(x6, y1 + y2 + y3 + y4 + y5));
        DrawingLine(new System.Windows.Point(x6, y1 + y2 + y3 + y4 + y5), new System.Windows.Point(x6 + 2 * x7 + x9, y1 + y2 + y3 + y4 + y5));
        DrawingLine(new System.Windows.Point(x6 + 2 * x7 + x9, y1 + y2 + y3 + y4 + y5), new System.Windows.Point(x1 + x2 + 2 * x3 + x5, y1 + y2));
        DrawingLine(new System.Windows.Point(x1 + x2 + 2 * x3 + x5, y1 + y2), new System.Windows.Point(x1 + 2 * x2 + 2 * x3 + x5, y1));
        DrawingLine(new System.Windows.Point(x1 + 2 * x2 + 2 * x3 + x5, y1), new System.Windows.Point(2 * x1 + 2 * x2 + 2 * x3 + x5, y1));
        DrawingLine(new System.Windows.Point(x1 + x2 + x3, y1), new System.Windows.Point(x1 + x2 + x3 + x5, y1));
        DrawingLine(new System.Windows.Point(x1 + x2 + x3 + x5, y1), new System.Windows.Point(x1 + x2 + x3 + x4 + x5, y1 + y2));
        DrawingLine(new System.Windows.Point(x1 + x2 + x3 + x4 + x5, y1 + y2), new System.Windows.Point(x6 + x7 + x8 + x9, y1 + y2 + y3));
        DrawingLine(new System.Windows.Point(x6 + x7 + x9, y1 + y2 + y3 + y4), new System.Windows.Point(x6 + x7, y1 + y2 + y3 + y4));
        DrawingLine(new System.Windows.Point(x6 + x7 - x8, y1 + y2 + y3), new System.Windows.Point(x1 + x2 + x3 - x4, y1 + y6));
        DrawingLine(new System.Windows.Point(x1 + x2 + x3 - x4, y1 + y6), new System.Windows.Point(x1 + x2 + x3, y1));
        if (y4 == 0)
        {
        }
        else
        {
            DrawingLine(new System.Windows.Point(x6 + x7 + x8 + x9, y1 + y2 + y3), new System.Windows.Point(x6 + x7 + x9, y1 + y2 + y3 + y4));
            DrawingLine(new System.Windows.Point(x6 + x7, y1 + y2 + y3 + y4), new System.Windows.Point(x6 + x7 - x8, y1 + y2 + y3));
        }

    }

    protected void DrawingLine(System.Windows.Point startPt, System.Windows.Point endPt)
    {
        LineGeometry myLineGeometry = new LineGeometry();
        myLineGeometry.StartPoint = startPt;
        myLineGeometry.EndPoint = endPt;

        Path myPath = new Path();
        myPath.Stroke = System.Windows.Media.Brushes.Black;
        myPath.StrokeThickness = 1;
        myPath.Data = myLineGeometry;

        this.mainPane1.Children.Add(myPath);
    }
}
}

我的程序是再Revit中运行的，结果就是实时修改画线。