A few ways to achieve this:
1. Perform calculations in SQL and summarize this field like this:
SELECT Quantity, Amount, Quantity * Amount As TotalAmount FROM MyTable
Then just use the TotalAmount field in the Detail line and sum it in the footer.
2. Create a second dataset that calculates the total for you and uses it in the footer instead of the sum:
=Sum(Fields!TotalAmount.Value, "MyTotalingDataset")
3. Do this using custom code. Right-click on the layout area, select Properties and go to the Code tab. Enter the following code:
Public Dim TotalAmount As Double = 0
Public Function CalculateRowTotal(ThisValue As Double, ThatValue As Double) As Double
TotalAmount = TotalAmount + (ThisValue * ThatValue)
Return ThisValue * ThatValue
End Function
In the Details dialog box, make the column in which you sum the field this expression:
=Code.CalculateRowTotal(Fields!Quantity.Value, Fields!Amount.Value)
This will execute the code above and complete your calculation plus calculate the total amount in this process.
, :
=Code.TotalAmount
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