% : You cannot call a method on a null-valued expression - powershell

Question

I think I've found unexplainable in PowerShell scripting. Let me describe the problem below:
I created a function that will query VSTS and will output back the amount of bugs that contain specific tags.
function GetBugsFromVSTS($applicationName, $first_url, $second_url, $query) {
#Building the body of the HTTP request to VSTS endpoint
$bodyOfRequest = $query | ConvertTo-Json
#Building headers of the HTTP request to VSTS endpoint
$headers = #{
'Content-Type'='application/json'
'Accept'='application/json'
'Authorization' = '' + $vsts_access_token + ''
}
$getBugs = Invoke-RestMethod -Uri $first_url `
-Method Post `
-Body $bodyOfRequest `
-Headers $headers | ConvertTo-Json -Depth 100
$json_decoded = $getBugs | ConvertFrom-Json
if ($json_decoded.workItems -eq $null) {
Write-Host "$applicationName Json decoded workitems are equal null" -ForegroundColor Red
} else {
Write-Host "$applicationName Json decoded workitems are not null" -ForegroundColor Green
$json_decoded.workItems | % {
$bug_id = $_.id
Write-Host $bug_id
}
}
$json_decoded.workItems | % {
$bug_id = $_.id
$url = ($second_url +$bug_id)
$getVstsBugState = Invoke-RestMethod -Uri $url -Method Get -Headers $headers
[PSCustomObject]#{
"Application Name" = $applicationName
"Id" = $bug_id;
"State" = $getVstsBugState.fields.'System.State';
"Reason" = $getVstsBugState.fields.'System.Reason';
"Severity" = $getVstsBugState.fields.'Microsoft.VSTS.Common.Severity'.Substring(4);
"AssignedTo" = $getVstsBugState.fields.'System.AssignedTo'.displayName
}
}
}
When I try to run it
$application = "MyBeautifulApp"
$application_first_url = "https://test.visualstudio.com/72L80E4b-1583-45c1-b669-d8de6133V895/_apis/wit/wiql?api-version=1.0"
$application_second_url = "https://test.visualstudio.com/72L80E4b-1583-45c1-b669-d8de6133V895/_apis/wit/workItems/"
$application_query = #{"query" = "SELECT [System.Id],[System.WorkItemType],[System.Title],[System.AssignedTo],[System.State] FROM WorkItems WHERE [System.TeamProject] = #project AND [System.WorkItemType] = 'Bug' AND [System.Title] CONTAINS 'Test' AND [System.TAGS] CONTAINS 'Test'"}
GetBugsFromVSTS $application $application_first_url $application_second_url $application_query
} catch {
Write-Warning "##vso[task.logissue type=warning;]Some problem occured querying MyBeautifulApp application. Please see below for error details"
$_
}
The most interesting part here is that I'm getting this really weird error message
WARNING: ##vso[task.logissue type=warning;]Some problem occured
querying MyBeautifulApp application. Please see below for error
details
% : You cannot call a method on a null-valued expression.
For the sake of test I added if..else clause to test that $json_decoded.workItems are not equal null, and that verification passes. If I run the code, full output will be
MyBeautifulApp Json decoded workitems are not null
9805
10330
10331
10371
10372
10373
10374
WARNING: ##vso[task.logissue type=warning;]Some problem occured querying MyBeautifulApp application. Please see below for error details
% : You cannot call a method on a null-valued expression. line 29
+ $json_decoded.workItems | % {
+ ~~~
+ CategoryInfo : InvalidOperation: (:) [ForEach-Object], RuntimeException
+ FullyQualifiedErrorId : InvokeMethodOnNull,Microsoft.PowerShell.Commands.ForEachObjectCommand
How is it possible that the first verification for $json_decoded.workItems (line 23) is not indicating that variable equals null, whereas the second statement (line 29) shows that it's null?

Answer 1

The problem isn't $json_decoded.workItems, the problem happens inside the script block passed to % / ForEach-Object:
Since you're using an enclosing try / catch handler, the offending individual statement inside the block is, unfortunately, not pinpointed in the error message.
However, given that there's only one method call in your block, the offending line can be inferred:
"Severity" = $getVstsBugState.fields.'Microsoft.VSTS.Common.Severity'.Substring(4);
In other words: $getVstsBugState.fields.'Microsoft.VSTS.Common.Severity' evaluates to $null, which is why the .Substring() method call fails.