datacolumn expression

Can you not use the Expression property with a column mapped to the
datatable from the database. I set the expression property of a
column ( datacolumn("TOTALCOST).expression = "col6.columnname +
col5.columnname" ) after filling the dataadapter and try to do an
update .
But I am getting an error

The column mapping from SourceColumn 'TOTALCOST' failed because the
DataColumn 'TOTALCOST' is a computed column.
Any idea what is wrong?

How can you set the value of a column as the sum of values from two
other columns the user enters.

I tried to do this in columnchanged and rowchanged events but it is
getting into a recursion cos I am changing the column again inside
the event.
I know there should be an easy solution to this.
Any help apreciated