Problem using Powershell to extract a value from a Json structure - json

Question

Using Powershell, I get a json snippet returned into my First variable (this always works fine);
# Initialise variables...
$nMessage_id = "";
$whatStatusJsonContent = "";
# Function https://abc.googleapis.com/abc/load call here and returns...
$whatStatusJsonContent = '{"message_id":9093813071099257562}'
Then I call the convert function into a temp variable like this;
$ResponseBody = ConvertFrom-Json $whatStatusJsonContent;
which puts the Json into a nice little data structure like this;
message_id
----------
9093813071099257562
From which I can select the value I want by calling this;
$nMessage_id = $ResponseBody.message_id;
Usually, this works fine and I get the value into my second variable;
$nMessage_id = 9093813071099257562
The problem is: Sometimes I get nothing in $nMessage_id, even though $whatStatusJsonContent is definitely logged as having the Json returned correctly from the function.
My question is: Do I have to ConvertFrom-Json, or can I read it raw from the First variable..?
COMBINED SOLUTION: Thanks to #mklement() and #Bernard-Moeskops
# Initialise variables...
$nMessage_id = "";
$whatStatusJsonContent = "";
# Function https://abc.googleapis.com/abc/load call here and returns...
$whatStatusJsonContent = '{"message_id":9093813071099257562}'
$ResponseBody = ConvertFrom-Json $whatStatusJsonContent;
if($ResponseBody.message_id){
# ConvertFrom-Json got the value!
$nMessage_id = $ResponseBody.message_id
}else{
# ConvertFrom-Json didn't work!
$nMessage_id = = ($whatStatusJsonContent -split '[:}]')[1]
}

Answer 1

There's nothing overtly wrong with your code.
ConvertFrom-Json should work as expected and return a [pscustomobject] instance with a .message_id property.
In your example, the message_id JSON property value is a number that is an integer, for which ConvertTo-Json automatically chooses a suitable integer data type as follows: the smallest signed type >= [int] (System.Int32)[1] that can accommodate the value ([int] -> [long] (System.Int64) -> [decimal] (System.Decimal)); the caveat is that if the value can't even fit into a [decimal], an - inexact - [double] is used.[2]
With the sample JSON in your question, [long] is chosen.
In a follow-up comment you state:
The routine makes over 1000 calls/hour and for most of them the Json comes back and the $nMessage_id is yielded perfectly. Then, suddenly, the $nMessage_id is empty, even though the Json is logged as coming back fine. So, somewhere in the ConvertFrom-Json or $ResponseBody.message_id the value is going missing...
I have no explanation, but if - for whatever reason - ConvertFrom-Json is the culprit, you can try string manipulation as a workaround to extract the message ID and see if that helps:
$whatStatusJsonContent = '{"message_id":9093813071099257562}'
# Extract the message_id property value as a *string*
# (which you can cast to a numeric type if/as needed).
$message_id = ($whatStatusJsonContent -split '[:}]')[1]
The above stores a string with content 9093813071099257562 in $message_id; note that, as written, the input string must have the exact format as above with respect to whitespace; while it is possible to make the text parsing more robust, not having to worry about format variations is one good reason to use a dedicated parser such as ConvertFrom-Json.
Another option is to try a different JSON parser to see if that helps.
Json.NET is the preeminent JSON parser in the .NET world (which now underlies the JSON cmdlets in PowerShell Core):
$whatStatusJsonContent = '{"message_id":9093813071099257562}'
$message_id = [NewtonSoft.Json.Linq.JObject]::Parse($whatStatusJsonContent).message_id.Value
Note: Json.NET - like ConvetFrom-Json in PowerShell _Core - commendably uses the arbitrary large [bigint] type as well once a number is too large to fit into a [long].
Use of the Json.NET assembly has the added advantage of better performance than the ConvertFrom-Json cmdlet.
In PowerShell Core, you can run the above code as-is (the assembly is preloaded); in Windows PowerShell you'll have to download the package via the link above and add the assembly (NewtonSoft.Json.dll) to your session with Add-Type -LiteralPath.
[1] Curiously, in PowerShell Core, as of (at least) v6.2.0, the smallest type chosen is [long] (System.Int64).
[2] More helpfully, PowerShell Core, as of (at least) v6.2.0, creates an arbitrarily large [bigint] (System.Numerics.BigInteger) instance once a value doesn't fit into a [long] anymore; that is, the [decimal] type is skipped altogether.

Answer 2

You are going to have to convert it, so that PowerShell can understand it. It will convert from a string to a PSCustomObject. Just check by asking the type of the variable before and after.
$ResponseBody.message_id.GetType()
If sometimes the output is nothing, you could do something like:
if($ResponseBody.message_id){
$nMessage_id = $ResponseBody.message_id
}else{
throw "No message id found"
}
Hope this helps.