c＃在将excel单元格值复制到数据行时获取无效的强制转换异常

我正在尝试使用以下代码

int noofrow = 1;

for (int r = noofrow; r <= rows; r++)
{
    DataRow dr = dt.NewRow();
    for (int c = 1; c <= cols; c++)
    {
        if (((microsoft.Office.Interop.Excel.Range)objSHT.Cells[r,c]).Value2 != null)
        {
            //string s11 = ((microsoft.Office.Interop.Excel.Range)objSHT.Cells[r,c]).Value2;
            dr[c - 1] = ((microsoft.Office.Interop.Excel.Range)objSHT.Cells[r,c]).Value2;
        }
        else
        {
            dr[c - 1] = "";
        }
    }
    dt.Rows.Add(dr);
    dt.acceptChanges();
}