Different result when running PowerShell script with C#

Question

I am using C# to execute a PowerShell script:

using (PowerShell powerShellInstance = PowerShell.Create())
{
    string query = string.Format(@"$server = ""{0}""
        $db = ""master""
        $sql = ""SELECT DISTINCT hostprocess FROM master.dbo.sysprocesses""
        $user = ""{1}""
        $pass = ""{2}""
        $processId = Invoke-sqlcmd -Server $server -Database $db -Username $user -Password $pass -Query $sql | select -ExpandProperty hostprocess
        Write-Output $processId", Server, DbUserName, DbPass);
    powerShellInstance.AddScript(query);
    var results = powerShellInstance.Invoke();
}

When I run the script in the PowerShell console, it returns 2 string values. But in C#, results has 2 elements but they are null. What is the problem here and why is the behavior different when executing it via C#?

I've tried to fix the formatting - you might want to check that I haven't changed the meaning; if you want to tweak it further, click "edit" — Marc Gravell
– Marc Gravell, Commented Apr 18, 2018 at 8:37
Please don't delete a question once you have received the answer you want. That deprives future readers of potentially good material to learn from, and prevents existing answerers from getting credit for the help they have rendered. — halfer
– halfer, Commented Apr 18, 2018 at 17:42

poke · Accepted Answer · 2018-04-18 09:00:06Z

PowerShell.Invoke only returns PSObject objects for the return value of the executed PowerShell commands. When you write to the host using Write-Output, then there will be no return value, so you cannot see the result within the results collection. Instead, the written output will be written to the Information stream of the PowerShell object:

using (var ps = PowerShell.Create())
{
    // commands that writes to the output
    ps.AddScript("Write-Host 'output value'");
    ps.Invoke();

    var output = (ps.Streams.Information.First().MessageData as HostInformationMessage).Message;
    Console.WriteLine(output); // output value
}

using (var ps = PowerShell.Create())
{
    // command that returns a value, e.g. a string literal
    ps.AddScript("'return value'");
    var results = ps.Invoke();

    string stringResult = results.First().BaseObject as string;
    Console.WriteLine(stringResult); // return value
}

Within the PowerShell console itself, of course you will see written output directly and return values that are not collected will also often be displayed directly.

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