I need to get all the dataobjects of a certain kind and perform actions on each of them, not inside the page controller but in the normal page class.
So I have:

$myobjectset = DataObject::get("MyObject");
foreach ($myobjectset as $singleobject) {

...

}

Silvertripe says that I give the wrong argument, and i may be understaing why, DataObject set it's not an array. What's the correct method of looping through a dataobject set?

A foreach is the correct way. It could be throwing an error if $myobjectset is an empty set so wrap the foreach in if($myobjectset) { .. }.

Also make sure its not something inside the loop throwing the error.

the error disappeared when I tried to use DataObject::getIterator("MyObject")
anyway i'm trying to use this structure to remove the tabs and fields in a certain page from an administration page . I created a dataobject with a field for the tab/field name, the tab path, and a checkbox

So, in the getCMSFields function i do:

$customizeInterface = DataObject::getIterator("FieldsTabs"); //in this object are stored the names of the fields to be removed

if ($customizeInterface){
foreach ($customizeInterface as $myobj) {

if ($myobj->Enabled == '0'){

$fields->removeFieldFromTab($myobj->TabPath, $myobj->NameOfField);
}

}
}

No errors, but it's not working.

You shouldn't have to use getIterator(), a simple DataObject::get() should work fine as long as FieldsTabs is a subclass of DataObject. Can you post the full file and the actual error message you got originally.

You're right WillR. Removing getIterator make the whole function work neatly and perfectly! Just a DataObject::Get and a control over the fact that the set may be empty.
Thank you so much!