Weird behavior of powercfg.exe when I execute it programatically - vb.net

Question

To programtically set display brightness I had two options :
Use PowerWriteACValueIndex and PowerWriteDCValueIndex API
Use PowerCfg.exe
Now, I tried both, and both gave me the same (weird) result. Here's the code I'm using :
With psiInfo
.FileName = "powercfg"
.UseShellExecute = True
.WindowStyle = ProcessWindowStyle.Hidden
.Arguments = "-setacvalueindex " & guidCurScheme.ToString() & " " & SUBGROUP_GUID.GUID_VIDEO_SUBGROUP & " " & SETTING_GUID.GUID_DISPBRIGHTNESS & " " & psValue.ValueAC
End With
pProc = Process.Start(psiInfo)
Where :
Public Const GUID_VIDEO_SUBGROUP As String = "7516b95f-f776-4464-8c53-06167f40cc99"
Public Const GUID_DISPBRIGHTNESS As String = "aded5e82-b909-4619-9949-f5d71dac0bcb"
Public Structure POWERSETTING
Public ValueAC As String
Public ValueDC As String
End Structure
Dim psValue As POWERSETTING
And guidCurScheme is the GUID of the active power scheme, as returned by PowerGetActiveScheme API function. I do the samething for DC part, with "-setdcvalueindex".
Now the weird behavior is as follow :
If I call this code in a button click handler event, it will set these values, but not the "general" screen brightness slider (and for that matter, the display brightness). (somebody edits the pictures links inside the post please)
If I the user clicks the button a second time, it will only then apply the setting.
If i put the code in a loop for it to run twice when the user click the button, it will still behave as in (1)
If I try to run the same command but manually from command-line, it will run work just fine.
This is totally unexpected, I don't know what I'm doing wrong. It gives the exact same result when I use the PowerWriteACValueIndex and PowerWriteDCValueIndex functions from Windows API.

Answer 1

maybe double-check the syntax of your argument by letting the app copy the string to clipboard. Sometimes just a tiny typo ruins it for me.
Also i'd check if the application that gets called is the one from syswow or from system32 (that depends on how you compile your app)
You can also check the exitcode or look at the error-output you get.
The error-output can be captured like this:
Dim so As System.IO.StreamReader
Dim se As System.IO.StreamReader
se = myproc.StandardError
so = myproc.StandardOutput
myproc.WaitForExit()
MsgBox(so.ReadToEnd & vbcrlf & se.ReadToEnd)