在下面尝试，转换为int（我假设你有整数值）并取总和

try below, convert to int ( I assume you have integer values) and take the sum

dataGridView5.Rows[2].Cells[i].Value=Convert.ToInt32(dataGridView5.Rows[2].Cells[i].Value.ToString())+Convert.ToInt32(dataGridView5.Rows[2].Cells[i-1].Value.ToString())

在对这些值求和之前，将这些值转换为数值类型（整数或十进制）。

Convert these values into numeric types (integer or decimal) before doing a sum on these values.

使用单元格的Te xt属性并将其转换为整数。

Use the cell's Text property and convert it to Integer.

dataGridView5.Rows[2].Cells[i].Value=Convert.ToInt32(dataGridView5.Rows[2].Cells[i].Text)+Convert.ToInt32(dataGridView5.Rows[2].Cells[i-1].Text)