"like a centipede with a wooden leg: 99 times tic and 1 toc
the stream is then truncated and still requires extra coding"

That's the whole point!
If we turncate the stream to $$$MaxStringLength, no matter where (in a calculated property or in a GETter method), just to present this (string)value to a variable, a function or whatever, then we can instantly stick to a string.

By the way, it's possible to store $$$MaxStringLength bytes in a local or global variable.
Properties of classes are stored in globals as $listelements, so the maxlength for a single string property depends on things like count and types of other properties in the class and not least, does this class extends %Persistent or extends another class(es) which extends %Persistent - in this case we need some extra bytes for those (extended) class names too.

Supposed, you have a magical datatype of %StringOrMaybeStream and your property would be something like this:

Property TheText As %StringOrMaybeStream

So the next question is, how would your application use this magical property?
The very first question is, how would you put your data into this magical property?

Give me some (hypothetical) examples of use and I will try to give you a solution, if there is any.

Hello Kevin,

if I anderstand you correctly, several your Cache systems (development, life, ...) will store their files on a third server (possibly on a file server) and you want to put the files from each (source) Cache (instance) into a different (target) folder, where the folder name being the "name" of the source Cache system - am I correct?

If yes, I think, your best choice ist:

...\hostNameOfCache\instanceNameofCache...

or, as John Murray suggested

...\GUIDofCacheInstance...

or just a fixed string like

...\kevinNotebook(development)...

You can put (John's) GUID or my "fixed string"
in a (possibly system) Global like ^%Zconfig("path")=...

Why?
Things like MAC- or IP-Addresses can change more often then you think. Especially IP-Addresses will change, hence are there DNS servers.

On the other hand, it's uncommon to change host- or instance names (of Cache or of whatever installation).

Your IP-Address preference has an other downside too. Many servers have more then one IP-Address (and if they do not have one today, maybe tomorrow!).

If you persist to take the systems IP-Address, here is, what you wanted:

set iplist = $system.INetInfo.GetListOfConfiguredInterfaces(0)
for i = 1:1:$listlength(iplist) zw $list(iplist,i)

Regards and
have a nice day
Julius

You are comparing apples wih oranges!

The line  (your case 1):

set dat=$lg(^TestD(id))    //dat=$lb("a","b","c","d","e")

sets dat to the FIRST list item (if present, or to "", if the list item is NOT present) of ^TestD(id)  but ONLY if ^TestD(id) is defined AND it is a Cache list.

In elsecase, you will get an <UNDEF> if ^TestD(id) does not exists or an <LIST> error, if ^TestD(id) exists but the content is not a list!

The line (your case 2):

 set dat=$g(^TEST2(id))   //dat = "a#b#c#d#e"

sets dat to the content of ^TEST2(id) , if  it exists or to "", if there is no ^TEST2(id)

If you want to compare collections, then you have to know what do you compare.
Let look on this examples:

1) Hobbies
   old: music, sport
   new: sport, music
   
   One could say, there is no difference (if all hobbies are equally preferred) 
   
2) Work instructions
   old: format_disk, create_backup
   new: create_backup, format_disk
   
   In this example, if the work is done in a wrong sequence, the data are lost.

In other words, if you compare collections, you have to take in account the
importance or unimportance of sequencies, which means, the compare function
needs a third input argument

compare(old, new, relyOnSequence, ...)

By the way, your test() method has his very special "point of view" of lists:

set old1=$lb("blue","red","green"),    new1=$lb("green","red","blue")
set null=$lb("", "", "")

do ##class(...).test(old, new, .add, .del) write add=nul, del=nul --> 11
do ##class(...).test(old, old, .add, .del) write add=nul, del=nul --> 11
do ##class(...).test(nul, nul, .add, .del) write add=nul, del=nul --> 11

Is this an expected behavior?
 

Class your.class
{
Property Colors As list Of %String;


ClassMethod toJson(list) As %String [ SqlProc ]
{
    s json=""
    f i=1:1:$ll(list) {
        s:i>1 json=json_"," s item=$lg(list,i)
        if '$ld(list,i) { s json=json_"null" }
        elseif item="" { s json=json_"""""" }
        elseif $lv(item) { s json=json_..toJson(item) }
        elseif $num(item,".")=item { s json=json_$fn(item,"N") }
        else {
            f c="\","/","""",$c(8),$c(9),$c(10),$c(12),$c(13) s item=$replace(item,c,"\"_$tr(c,$c(8,9,10,12,13),"btnfr"))
            f  q:'$locate(item,"[:cntrl:]",0,j,v)  s $e(item,j-1)="\u"_$e($zh($a(v)+65536),2,5)
            s json=json_""""_item_""""
        }
    }
    
    q "["_json_"]"
}
}

select your.class_toJson(Colors) from your.class

gives you the expected result . If you prefer  a direct  use of globals, then use

write ##class(your.class).toJson($lg(^your.global(theOID),theSlotNumber))

my quick test says there are two chars, CR and LF, on an AIX and on Ubuntu

USER>s str=##class(%GlobalCharacterStream).%New()
USER>d str.WriteLine("Test")
USER>d str.Rewind()
USER>zzdump str.Read(100)
0000: 54 65 73 74 0D 0A                                       Test..
USER>
​USER>w $zv
Cache for UNIX (IBM AIX for System Power System-64) 2017.1 (Build 792U) Mon Mar 20 2017 19:19:37 EDT
USER>

-----------------

USER>s str=##class(%GlobalCharacterStream).%New()
USER>d str.WriteLine("Test")
USER>d str.Rewind()
USER>zzdump str.Read(100)
0000: 54 65 73 74 0D 0A                                       Test..
USER>
​USER>w $zv
Cache for UNIX (Ubuntu Server LTS for x86-64) 2017.1 (Build 792U) Mon Mar 20 2017 19:22:31 EDT
USER>

Maybe, your problem has nothing to do with Cache?

Regards,

Julius