for (int i = 0; i <= dataGridView1.RowCount ; i++)
         {
               mark = Convert.ToInt32(dataGridView1.CurrentRow.Cells[0].Value);
              unit  = Convert.ToInt32(dataGridView1.CurrentRow.Cells[2].Value);
              result += ( s1 * s);

         } 

但这段代码只计算第一行。

我如何更改行索引？

but this code calculate just first row.
how i change row index?

我解决了！

i solve it!

for (int i = 0; i < dataGridView1.RowCount ; i++)
   {
         mark = Convert.ToInt32(dataGridView1.Rows[i].Cells[3].Value);
         unit = Convert.ToInt32(dataGridView1.Rows[i].Cells[2].Value);
         sumunit += unit;
        result += ( mark * unit);

   }
   result /= sumunit;
   label2.Text = result.ToString();

}