ZedGraph对象nullreference异常

Question

I have a problem plotting a graph using the ZedGraphControl in the constructor of my WinForm I initialize the graph like this:

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;

And just for testing I want to plot a new point when i click a button. So on button click I want to execute this code:

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();
        }
    }

But my Graph Object is always null! I even created a property and put a breakpoint in the setter. The setter is never called but the object is still null. (After the first line of code i posted the Graph object is not null.

Any idea how this can happen? Thank you

Answer 1

Your first code block looks like it's creating a local variable called Graph , but your second code block button1_Click looks like it's trying to use a class field (or property ) called Graph . These are two different entities. So likely your initializing code is never assigning an instance of ZedGraphControl to the Graph field , thus it's always null .

Try changing your first code to remove the local variable declaration and reference the field instead. That is, change the first line to:

Graph = new ZedGraphControl();