C#中怎么让dataGridView里一列的值相加

写了个例子，求Northwind数据库中“Orders”表中“Freight”列（列索引为7）的值相加：

private void Form1_Load(object sender, EventArgs e)
{
sqlDataAdapter1.Fill(dataSet11);
}

private void button1_Click(object sender, EventArgs e)
{
float s=0;

for (int i = 0; i < dataGridView1.RowCount; i++)
{
if (!dataGridView1.Rows[i].IsNewRow)
s=s+float.Parse(dataGridView1[7, i].Value.ToString());
}
MessageBox.Show(s.ToString()); //显示结果
}