Hi, Oleg!

Attributes are "constants" computed at compile time. They are
even "more static" than the static members. It is too late to
change them when the program (your control with some attributes
assigned to some properties) is used in another application
(and is already compiled, i suppose).

You have only one chance -- implementing the needed logic *in*
the converter (dynamically, at the time, that is "runtime" for
the converter and "design time" for the designed component
instance).

Look at the GetStandardValuesExclusive(). It must not deliver
the same value each time it is called. You can compute
the return value considering what ever you want (the context
argument gives access at least to the designed control instance,
designer host and many other things that help).

Im not 120% sure, but i hope, GetStandardValuesExclusive()
is called each time the user starts editing the property
and you can decide: to allow or not to allow.

Another question. You say you must allow entering new names
when the list is empty. And what is when there is only one name
in the list? Are you sure, that the user does not want to append
one more? I mean: (i) maybe it must be always allowed to enter
new names or (ii) you need different evidences (not simply
NameList.Count == 0) to decide whether the list is complete
and may not be extended.

HTH,
Dima.