private double sum  = 0;
    private double sum1 = 0;
    private double sum2 = 0;
    private double sum3 = 0;
    private double sum4 = 0;
    private double sum5 = 0;
    private double sum6 = 0;
    private double sum7 = 0;
    private double sum8 = 0;
    private double sum9 = 0;
----------------------------------------
sum += Convert.ToDouble(e.Row.Cells[2].Text);
            sum1 += Convert.ToDouble(e.Row.Cells[3].Text);
            sum2 += Convert.ToDouble(e.Row.Cells[4].Text);
            sum3 += Convert.ToDouble(e.Row.Cells[5].Text);
            sum4 += Convert.ToDouble(e.Row.Cells[6].Text);
            sum5 += Convert.ToDouble(e.Row.Cells[7].Text);
            sum6 += Convert.ToDouble(e.Row.Cells[8].Text);
            sum7 += Convert.ToDouble(e.Row.Cells[9].Text);
            sum8 += Convert.ToDouble(e.Row.Cells[10].Text);
            sum9 += Convert.ToDouble(e.Row.Cells[11].Text);
---------------------------------------------------------------
e.Row.Cells[0].Text = "合计：";
            e.Row.Cells[2].Text = sum.ToString();
            e.Row.Cells[3].Text = sum1.ToString();
            e.Row.Cells[4].Text = sum2.ToString("f");
            e.Row.Cells[5].Text = sum3.ToString("f");
            e.Row.Cells[6].Text = sum4.ToString("f");
            e.Row.Cells[7].Text = sum5.ToString("f");
            e.Row.Cells[8].Text = sum6.ToString("f");
            e.Row.Cells[9].Text = sum7.ToString("f");
            e.Row.Cells[10].Text = sum8.ToString("f");
            e.Row.Cells[11].Text = sum9.ToString("f");

变量用数组，这样就能循环了
private double []sum  = new double[]{0,0,0,0,0,0,0,0,0,0};

for(int i=0;i<10;i++)
{
sum[i] += Convert.ToDouble(e.Row.Cells[i+2].Text);
}

e.Row.Cells[0].Text = "合计：";
for(int i=0;i<10;i++)
{
   e.Row.Cells[i+2].Text = sum[i].ToString();
} 

这是在CSDN C#论坛的回复，有学习的朋友可以参考。