读取DataTable值时，输入字符串不是正确的格式

Question

我正在使用DataTable执行两列的总和，第一次转换并执行添加但第二次它给我输入字符串格式不正确 我的代码在这里：

Dim dt As New DataTable()
dt.Columns.Add("BillingAmount")
dt.Columns.Add("RecievedAmount")
dt.Columns.Add("Balance")

For i As Integer = 0 To mstr_DataTable.Rows.Count - 1
   Dim BillingAmount As Decimal = Decimal.Parse(drw("BillingAmount").ToString()) + Decimal.Parse(mstr_DataTable.Rows(0)("BillAmount").ToString()) 
   '^ I am getting exception here .
   drw("BillingAmount") = BillingAmount.ToString()
   drw("RecievedAmount") = mstr_DataTable.Rows(0)("PaymentAmount").ToString()
   Dim Balance As Decimal = Decimal.Parse(drw("BillingAmount").ToString()) - Decimal.Parse(mstr_DataTable.Rows(0)("PaymentAmount").ToString())
   '^ I am getting exception here .
   drw("Balance") = Balance.ToString()
Next