where did i mistake?

Question

I m using telerik radchart control in my window programm. below the code for populate chart. when i m using arrary then it give error , but if i simply declare chartseries then it working.

if i m using chartseries array which is show in rad color , then it gives error, and if i using green code then it working properly both code are shame only array difference.
error occuring on a[0].AddItem(Convert.ToInt32(ds.Tables["Marks"].Rows[0][0]));
error is: - nullreferenceexception was unhandled ,Object reference not set to an instance of an object.

SqlCommand cmd1 = new SqlCommand("select Paper1,Paper2,Paper3,Paper4,Paper5 from Student_Exams_Marks_Detail where S_ID='" + comboBox3.Text + "'and Section='" + comboBox2.Text + "' and Class='" + comboBox1.Text + "' ", con);
                SqlDataAdapter da = new SqlDataAdapter(cmd1);
                SqlDataAdapter da1 = new SqlDataAdapter(cmd3);
                da.Fill(ds, "Marks");
                da1.Fill(ds, "SubjectNames");
                radChart1.PlotArea.XAxis.AxisLabel.TextBlock.Text = "Years";
                //radChart1.DataSource = ds.Tables["v"];

                radChart3.DataSource = ds.Tables["Marks"];
                radChart4.DataSource = ds.Tables["Marks"];
                //radChart1.Skin = "LightBlue";


                radChart3.DataBind();
                radChart4.DataBind();


                //ChartSeries[] a;
                //a = new ChartSeries[5];
                //a[0].AddItem(Convert.ToInt32(ds.Tables["Marks"].Rows[0][0]));
                //a[1].AddItem(Convert.ToInt32(ds.Tables["Marks"].Rows[0][1]));
                //a[2].AddItem(Convert.ToInt32(ds.Tables["Marks"].Rows[0][2]));
                //a[3].AddItem(Convert.ToInt32(ds.Tables["Marks"].Rows[0][3]));
                //a[4].AddItem(Convert.ToInt32(ds.Tables["Marks"].Rows[0][4]));
                //a[0].Name = Convert.ToString(ds.Tables["SubjectNames"].Rows[0][3]);
                //a[1].Name = Convert.ToString(ds.Tables["SubjectNames"].Rows[0][4]);
                //a[2].Name = Convert.ToString(ds.Tables["SubjectNames"].Rows[0][5]);
                //a[3].Name = Convert.ToString(ds.Tables["SubjectNames"].Rows[0][6]);
                //a[4].Name = Convert.ToString(ds.Tables["SubjectNames"].Rows[0][7]);
                //radChart1.Clear();
                //radChart1.Series.Add(a[0]);
                //radChart1.Series.Add(a[1]);
                //radChart1.Series.Add(a[2]);
                //radChart1.Series.Add(a[3]);
                //radChart1.Series.Add(a[4]);

                ChartSeries s1 = new ChartSeries();
                ChartSeries s2 = new ChartSeries();
                ChartSeries s3 = new ChartSeries();
                ChartSeries s4 = new ChartSeries();
                ChartSeries s5 = new ChartSeries();
                s1.AddItem(Convert.ToInt32(ds.Tables["Marks"].Rows[0][0]));
                s2.AddItem(Convert.ToInt32(ds.Tables["Marks"].Rows[0][1]));
                s3.AddItem(Convert.ToInt32(ds.Tables["Marks"].Rows[0][2]));
                s4.AddItem(Convert.ToInt32(ds.Tables["Marks"].Rows[0][3]));
                s5.AddItem(Convert.ToInt32(ds.Tables["Marks"].Rows[0][4]));
                s1.Name = Convert.ToString(ds.Tables["SubjectNames"].Rows[0][3]);
                s2.Name = Convert.ToString(ds.Tables["SubjectNames"].Rows[0][4]);
                s3.Name = Convert.ToString(ds.Tables["SubjectNames"].Rows[0][5]);
                s4.Name = Convert.ToString(ds.Tables["SubjectNames"].Rows[0][6]);
                s5.Name = Convert.ToString(ds.Tables["SubjectNames"].Rows[0][7]);
                radChart1.Clear();
                radChart1.Series.Add(s1);
                radChart1.Series.Add(s2);
                radChart1.Series.Add(s3);
                radChart1.Series.Add(s4);
                radChart1.Series.Add(s5);

                radChart1.PlotArea.XAxis.AxisLabel.TextBlock.Text = "vk";

                radChart1.DataBind();

please help me what is wrong in it.

Sam Hobbs · Answer

To debug the problem when you get the error use the debugger to see what is null. You can put the mouse over something and then click and you will see the values. Find the item that is null. Then determine why it is null. It will make diagnosis much easier when you know what is null. Something else I do is to make separate lines of code. You have one line that is doing many things. If you do the same things except using separate lines then it will be easier to find the part that is not working. In the line: a = new ChartSeries[5]; You are allocating an array but you have not allocated any ChartSeries instances. You must do something such as the following for each item in a: a[i] = new ChartSeries();

Sam Hobbs · Answer

Good. And thank you for marking my assistance as the answer. When I said that I make lines separate I mean I do that for diagnostic purposes. I often combine multiple functions into one line as you do but for diagnostic purposes it helps to make functions separate. Then after I have figured it out I might make the code the same as I had it with everything on one line.

Vikas Ahlawat · Answer

thanks for your reply. My problem is solved . by a[i] = new ChartSeries();