Смотрите книгу "Developing Custom Delphi Components" от Рэя Конопки.
Здесь немного исправленный пример из этой книги -
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function
TMyClass.CreateCalcField( const AFieldName: string; AFieldClass:
TFieldClass; ASize: Word ): TField;
begin
Result :=
FDataSet.FindField( AFieldName ); // Field may already
exists!
if Result<>nil then Exit;
if
AFieldClass = nil then
begin
DBErrorFmt( SUnknownFieldType,
[AFieldName] );
end;
Result := FieldClass.Create( Owner
);
with Result do
try
FieldName :=
AFieldName;
if ( Result is TStringField ) or ( Result
is TBCDField ) or ( Result is TBlobField ) or (
Result is TBytesField ) or ( Result is TVarBytesField )
then
begin
Size := ASize;
end;
Calculated :=
True;
DataSet := FDataset;
Name := FDataSet.Name +
AFieldName;
except
Free; // We must release
allocated memory on error!
raise;
end;
end; | | 
