Question

我在WinForm的构造函数中使用ZedGraphControl绘制图表时遇到问题我初始化图形如下：

ZedGraphControl Graph = new ZedGraphControl(); 
Graph.Dock = DockStyle.Fill;
GroupBoxGraph.Controls.Add(Graph);

GraphPane pane = Graph.GraphPane;

/*Initial pane settings*/
pane.XAxis.Type = AxisType.Date;
pane.XAxis.Scale.Format = "HH:mm:ss";
pane.XAxis.Scale.Min = (XDate)(DateTime.Now);
//Shows 30 seconds interval.
pane.XAxis.Scale.Max = (XDate)(DateTime.Now.AddSeconds(30));
pane.XAxis.Scale.MinorUnit = DateUnit.Second;
pane.XAxis.Scale.MajorUnit = DateUnit.Minute;
pane.XAxis.MajorTic.IsBetweenLabels = true;
pane.XAxis.MinorTic.Size = 5;           

RollingPointPairList list = new RollingPointPairList(1200);
LineItem curve = pane.AddCurve("Hmi Mode", list, Color.Blue, SymbolType.None);

Graph.AxisChange();
tickStart = Environment.TickCount;

只是为了测试我想在点击按钮时绘制一个新点。所以在按钮上单击我想执行此代码：

private void button1_Click(object sender, EventArgs e)
    {
        if (Graph != null) {
            // Make sure that the curvelist has at least one curve
            if (Graph.GraphPane.CurveList.Count <= 0)
                return;

            // Get the first CurveItem in the graph
            LineItem curve = Graph.GraphPane.CurveList[0] as LineItem;
            if (curve == null)
                return;

            // Get the PointPairList
            IPointListEdit list = curve.Points as IPointListEdit;
            // If this is null, it means the reference at curve.Points does not
            // support IPointListEdit, so we won't be able to modify it
            if (list == null)
                return;

            // Time is measured in seconds
            double time = (Environment.TickCount - tickStart) / 1000.0;

            // 3 seconds per cycle
            list.Add(time, Math.Sin(2.0 * Math.PI * time / 3.0));

            // Keep the X scale at a rolling 30 second interval, with one
            // major step between the max X value and the end of the axis
            Scale xScale = Graph.GraphPane.XAxis.Scale;
            if (time > xScale.Max - xScale.MajorStep) {
                xScale.Max = time + xScale.MajorStep;
                xScale.Min = xScale.Max - 30.0;
            }

            // Make sure the Y axis is rescaled to accommodate actual data
            Graph.AxisChange();
            // Force a redraw
            Graph.Invalidate();
        }
    }

但是我的Graph Object总是为null！我甚至创建了一个属性并在setter中放置了一个断点。永远不会调用setter，但对象仍为null。（在我发布的第一行代码后，Graph对象不为空。

知道如何发生这种情况吗？谢谢

Answer 1

您的第一个代码块看起来像是在创建一个名为Graph的本地变量，但您的第二个代码块button1_Click看起来像是在尝试使用名为Graph的类字段（或属性）。这是两个不同的实体。因此，初始化代码可能永远不会将ZedGraphControl的实例分配给Graph 字段，因此它总是null。

尝试更改第一个代码以删除局部变量声明并改为引用该字段。也就是说，将第一行更改为：

Graph = new ZedGraphControl();

ZedGraph对象无引用异常

1 个答案: