have you tried to cast your fields?

drSales["invAmt"] = (decimal)drSales["invAmt"] * (decimal)dr["rate"];

>How can I perform +-*/ operation with datarow, eg.
>
>
>//drSales is datarow
>//dr is datarow
>
>drSales["invAmt"] = drSales["invAmt"] * dr["rate"];
>
>
>give me Error 2 Operator '*' cannot be applied to operands of type 'object' and 'object'
>
>
>Thanks