值不能为null。 参数名称：字体

Question

I am developing a an apllication to draw a chart and show legend in the chart.

Here is what I have done so far. Without the legend part graph is drawing correctly. I am trying to draw legend like this MSDN link. At run time I am getting this error.

Tried googling. But I wont get what the exact reason is.

           Chart gammaPlotChart = new Chart();
           ChartArea chart4Area = new ChartArea();
           chart4Area.Name = "Default";
           Series gammaValues = new Series();
           gammaValues.Name = "LogGamma";
           gammaValues.ChartType = SeriesChartType.Line;
           gammaValues.XValueType = ChartValueType.Double;
           gammaValues.YValueType = ChartValueType.Double;
           gammaPlotChart.ChartAreas[0].AxisX.Minimum = 1.4;
           gammaPlotChart.ChartAreas[0].AxisX.Interval = 0.2;
           gammaPlotChart.ChartAreas[0].AxisX.Maximum = 5.0;
           gammaPlotChart.ChartAreas[0].AxisY.Minimum = 0;
           gammaPlotChart.ChartAreas[0].AxisY.Interval = 0.5;
           gammaPlotChart.ChartAreas[0].AxisY.Maximum = 3.0;
           gammaPlotChart.Series.Add(gammaValues);
           gammaPlotChart.Series["LogGamma"].Points.DataBindXY(sRGBValues.greyScaleValues.LogV, sRGBValues.greyScaleValues.LogL);

           gammaPlotChart.Legends.Add(new Legend("Legend1"));
           gammaPlotChart.Legends["Legend1"].DockedToChartArea = "Default";
           gammaPlotChart.Series["LogGamma"].Legend = "Legend1";
           gammaPlotChart.Series["LogGamma"].IsVisibleInLegend = true;


           gammaPlotChart.Legends["Legend1"].CellColumns.Add(new LegendCellColumn("Name", LegendCellColumnType.Text, "LEGENDTEXT"));
           gammaPlotChart.Legends["Legend1"].CellColumns.Add(new LegendCellColumn("Sym", LegendCellColumnType.SeriesSymbol, "Mist"));
           Font font = new Font("Arial", 10);
           gammaPlotChart.Legends["Legend1"].CellColumns.Add(new LegendCellColumn("Avg", LegendCellColumnType.Text, "AVG{N2}"));
           gammaPlotChart.Legends["Legend1"].CellColumns[0].Font = new System.Drawing.Font("Arial", 10);
           gammaPlotChart.Legends["Legend1"].CellColumns[1].Font = font;
           gammaPlotChart.Legends["Legend1"].CellColumns[2].Font = font;

           Rectangle chart4Rect = new System.Drawing.Rectangle(locChart4, chartSize);
           gammaPlotChart.Printing.PrintPaint(e.Graphics, chart4Rect);

The IDE is shown below :

Stak trace :

System.ArgumentNullException was unhandled by user code
  Message=Value cannot be null.
Parameter name: font
  Source=System.Drawing
  ParamName=font
  StackTrace:
       at System.Drawing.Graphics.MeasureString(String text, Font font, SizeF layoutArea, StringFormat stringFormat)
       at System.Drawing.Graphics.MeasureString(String text, Font font)
       at System.Windows.Forms.DataVisualization.Charting.GdiGraphics.MeasureString(String text, Font font)
       at System.Windows.Forms.DataVisualization.Charting.ChartGraphics.MeasureStringAbs(String text, Font font)
       at System.Windows.Forms.DataVisualization.Charting.Legend.GetHeaderSize(ChartGraphics chartGraph, LegendCellColumn legendColumn)
       at System.Windows.Forms.DataVisualization.Charting.Legend.GetOptimalSize(ChartGraphics chartGraph, SizeF maxSizeRel)
       at System.Windows.Forms.DataVisualization.Charting.Legend.CalcLegendPosition(ChartGraphics chartGraph, RectangleF& chartAreasRectangle, Single elementSpacing)
       at System.Windows.Forms.DataVisualization.Charting.LegendCollection.CalcInsideLegendPosition(ChartGraphics chartGraph, Single elementSpacing)
       at System.Windows.Forms.DataVisualization.Charting.ChartPicture.Resize(ChartGraphics chartGraph, Boolean calcAreaPositionOnly)
       at System.Windows.Forms.DataVisualization.Charting.ChartPicture.Paint(Graphics graph, Boolean paintTopLevelElementOnly)
       at System.Windows.Forms.DataVisualization.Charting.PrintingManager.PrintPaint(Graphics graphics, Rectangle position)
       at ReportPrinter.PrintReport.DrawChart(PrintPageEventArgs e, InputData sRGBValues, InputData adobeRGBValues)
       at ReportPrinter.PrintReport.printDoc_PrintPage(Object sender, PrintPageEventArgs e)
       at System.Drawing.Printing.PrintDocument.OnPrintPage(PrintPageEventArgs e)
       at System.Drawing.Printing.PrintDocument._OnPrintPage(PrintPageEventArgs e)
       at System.Drawing.Printing.PrintController.PrintLoop(PrintDocument document)
       at System.Drawing.Printing.PrintController.Print(PrintDocument document)
       at System.Drawing.Printing.PrintDocument.Print()
       at System.Windows.Forms.PrintPreviewControl.ComputePreview()
       at System.Windows.Forms.PrintPreviewControl.CalculatePageInfo()
  InnerException: 

Answer 1

I had the same problem: I was using the MSDN page you linked to to make a table legend and i experienced the same error as you received.

The solution for me was setting the LegendCellColumn.HeaderFont property. For example in Zigma's case:

LegendCellColumn lcc = new LegendCellColumn("Name", LegendCellColumnType.Text, "LEGENDTEXT");
lcc.HeaderFont = new System.Drawing.Font("Trebuchet MS", 12F, System.Drawing.FontStyle.Bold);
gammaPlotChart.Legends["Legend1"].CellColumns.Add(lcc);

Explanation:

The problem is using the constructor with parameters:

LegendCellColumn("Name", LegendCellColumnType.Text, "LEGENDTEXT")

Because in the first parameter you set the HeaderText property of the LegendCellColumn but the HeaderFont property is "null" for default. When the chart tries to draw the legend it discovers it has no font for the header and throws an error.