powershell - Unexpected ConvertTo-Json results? Answer: it has a default -Depth of 2

Question

Why do I get unexpected ConvertTo-Json results, why do I get values like System.Collections.Hashtable and/or why does a round-trip ($Json | ConvertFrom-Json | ConvertTo-Json) fail?

Meta issue

Stackoverflow has a good mechanism to prevent duplicate questions but as far as I can see there is no mechanism to prevent questions that have a duplicate cause. Take this question as a an example: almost every week a new question comes in with the same cause, yet it is often difficult to define it as a duplicate because the question itself is just a slightly different. Nevertheless, I wouldn't be surprised if this question/answer itself ends up as a duplicate (or off-topic) but unfortunately stackoverflow has no possibility to write an article to prevent other programmers from continuing writing questions caused by this “known” pitfall.

Duplicates

A few examples of similar questions with the same common cause:

• PowerShell ConvertTo-Json does not convert Array as expected (yesterday)
• Powershell ConvertTo-json with embedded hashtable
• powershell “ConvertTo-Json” has messed json format output
• Nested arrays and ConvertTo-Json
• Powershell ConvertTo-JSON missing nested level
• How to save a JSON object to a file using Powershell?
• Cannot convert PSCustomObjects within array back to JSON correctly
• ConvertTo-Json flattens arrays over 3 levels deep
• Add an array of objects to a PSObject at once
• Why does ConvertTo-Json drop values
• How to round-trip this JSON to PSObject and back in Powershell
• …

Different

So, were does this “self-answered” question differ from the above duplicates?
It has the common cause in the title and with that it might better prevent repeating questions due to the same cause.

Answer 1

ConvertTo-Json has a -Depth parameter:

Specifies how many levels of contained objects are included in the JSON representation.
The default value is 2.

Example

To do a full round-trip with a JSON file you need to increase the -Depth for the ConvertTo-Json cmdlet:

$Json | ConvertFrom-Json | ConvertTo-Json -Depth 9

TL;DR

Probably because ConvertTo-Json terminates branches that are deeper than the default -Depth (2) with a (.Net) full type name, programmers assume a bug or a cmdlet limitation and do not read the help or about.
Personally, I think a string with a simple ellipsis (three dots: …) at the end of the cut off branch, would have a clearer meaning (see also: Github issue: 8381)

Why?

This issue often ends up in another discussion as well: Why is the depth limited at all?

Some objects have circular references, meaning that a child object could refer to a parent (or one of its grandparents) causing a infinitive loop if it would be serialized to JSON.

Take for example the following hash table with a parent property that refers to the object itself:

$Test = @{Guid = New-Guid}
$Test.Parent = $Test

If you execute: $Test | ConvertTo-Json it will conveniently stop at a depth level of 2 by default:

{
    "Guid":  "a274d017-5188-4d91-b960-023c06159dcc",
    "Parent":  {
                   "Guid":  "a274d017-5188-4d91-b960-023c06159dcc",
                   "Parent":  {
                                  "Guid":  "a274d017-5188-4d91-b960-023c06159dcc",
                                  "Parent":  "System.Collections.Hashtable"
                              }
               }
}

This is why it is not a good idea to automatically set the -Depth to a large amount.