'Declaration Public ReadOnly Property Expression As Expression
public Expression Expression {get;}
If this expression is not empty, the field is a calculated field (as opposed to a data field).
'Declaration Public ReadOnly Property Expression As Expression
public Expression Expression {get;}
This property is never null (Nothing in VB), it always returns an Expression object, which has empty Expression.Text by default. (A non-empty Text string indicates a calculated field.)
To specify an expression, set its Expression.Text property, or the DataSetField.ExpressionText property on the current field.
Either this property or DataSetField.DataField, but not both, should be defined on a DataSetField. In case both properties are defined, DataSetField.DataField is used, and Expression is ignored.