How update metadata of a Class

Hello,

A question please....

If we want to update a property of a document like this.. it's ok... the update text of the property is done..

var objID = new MFilesAPI.ObjID();
objID.SetIDs(
    ObjType: (int)MFBuiltInObjectType.MFBuiltInObjectTypeDocument,
    ID: 5);

// Check out the object.
var checkedOutObjectVersion = vault.ObjectOperations.CheckOut(objID);

BUT... which value have we to put for a class ??...

i have try to put the ID of the class... tu put 1.. but seems to have an issue during the checkout...( the code exit the void without exception)

any help is possible by Mfiles team??