using (var tr = db.TransactionManager.StartOpenCloseTransaction())
{
var dim = (Dimension)tr.GetObject(dimId, OpenMode.ForWrite);
if(dim.HasFields)
{
var fld = (Field)tr.GetObject(obj.GetField(), OpenMode.ForWrite);
var code = fld.GetFieldCode(FieldCodeFlags.AddMarkers | FieldCodeFlags.FieldCode);
code = "TESTING" + code;
fld.SetFieldCode(code);
fld.Evaluate();
}
tr.Commit();
}