Formula Field

I'm using the crystal report of VB.Net 2005 I've tried to write
ReportInstance.DataDefinition.FormulaFields(index).Text = "{DataTable.ColumnName}"
but it didn't work ,I've found that FormulaFields is read only,is there a way to do my point,I mean I need to assign column names of a datatble to the FormulaField(index).Try this, the following code has always worked for me:

For Each crxFormulaField In crxReport.FormulaFields
Select Case crxFormulaField.Name
Case "{@.YourFormulaName}"
crxFormulaField.Text = "{DataTable.ColumnName}"
End Select
Next crxFormulaField

Good luck!
-- Heather