Intermittent PowerShell "You cannot call a method on a null-valued expression" exception

user_15323023 0
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Very infrequently (less than 0.1% of function calls) I see an exception from inside a function. The function writes a log entry to a common log file and the intent is that it will wait until it can obtain the mutex that controls writing to the common file.

Relevant code snippet:

function WriteSummary ([string]$fnContent)  
{
    $fnTimeStamp = Get-Date -Format "HH:mm:ss.ff"
    Try {
        [void]$SummaryLogMutex.WaitOne()
        [System.IO.File]::AppendAllText($script:SummaryLog,("$fnTimeStamp $fnContent") + ([Environment]::NewLine))
    } Catch {
        $FullException = ($_ | Format-List -Force | Out-String)
        WriteActivity -fnContent ("$FullException")
    } Finally {
        [void]$SummaryLogMutex.ReleaseMutex()
    }
}

$SummaryLogMutex = New-Object System.Threading.Mutex($false, "Global\SummaryLogMutex")

WriteSummary -fnContent ("Some text")

The exception is as follows:

Exception             : System.Management.Automation.RuntimeException: You 
                        cannot call a method on a null-valued expression.
                           at System.Management.Automation.ExceptionHandlingOps
                        .CheckActionPreference(FunctionContext funcContext, 
                        Exception exception)
                           at System.Management.Automation.Interpreter.ActionCa
                        llInstruction`2.Run(InterpretedFrame frame)
                           at System.Management.Automation.Interpreter.EnterTry
                        CatchFinallyInstruction.Run(InterpretedFrame frame)
                           at System.Management.Automation.Interpreter.EnterTry
                        CatchFinallyInstruction.Run(InterpretedFrame frame)

The trace indicates it is "[void]$SummaryLogMutex.WaitOne()" causing the exception.

I would be grateful for help with understanding and resolving what is going on.

Modified code to be tested to create object inside try block and avoid invoking method on variable set to null:

function WriteSummary ([string]$fnContent)  
{
    $fnTimeStamp = Get-Date -Format "HH:mm:ss.ff"
    Try {
        If (!$script:SummaryLogMutex) {
            $script:SummaryLogMutex = New-Object System.Threading.Mutex($false, "Global\SummaryLogMutex")
            WriteActivity -fnContent ("Created Summary Log Mutex")
        }
        [void]$script:SummaryLogMutex.WaitOne()
        [System.IO.File]::AppendAllText($script:SummaryLog,("$fnTimeStamp $fnContent") + ([Environment]::NewLine))
    } Catch {
        $FullException = ($_ | Format-List -Force | Out-String)
        WriteActivity -fnContent ("$FullException")
    } Finally {
        [void]$script:SummaryLogMutex.ReleaseMutex()
    }
}

WriteSummary -fnContent ("Some text")
mutex powershell exception
1 Answers

I'm not able to reproduce this behavior ($SummaryLogMutex intermittently resolves to $null inside WriteSummary even though it exists in the callers scope), but I suspect you can avoid it entirely by using your named mutex differently.

Since it has a name, we can easily obtain a handle to it without relying on $SummaryLogMutex:

function WriteSummary ([string]$fnContent)  
{
    $fnTimeStamp = Get-Date -Format "HH:mm:ss.ff"
    Try {
        [void][System.Threading.Mutex]::OpenExisting("Global\SummaryLogMutex").WaitOne()
        [System.IO.File]::AppendAllText($script:SummaryLog,("$fnTimeStamp $fnContent") + ([Environment]::NewLine))
    }

Now we can reliably catch failures:

catch [System.Threading.WaitHandleCannotBeOpenedException] {
   # handle non-existing mutex (eg. create it)
}
user_7126490
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