Powershell to run PostMessage Windows API function failing - powershell

Question

I need to write a script to install Java runtime of a specific version, add to the system path variable then go on to run the Java program. I want to do this all in one go without having to restart the target computer.
I found this so question and thought I could try to change to my needs.
SendMessage is causing script to hang
So I tried with PostMessage as per the answer. Obviously I needed to change the message.
So I tried code like this:
echo Start of experiment
$NewPath = (Get-ItemProperty -Path "HKLM:\System\CurrentControlSet\Control\Session Manager\Environment" -Name Path).Path
$NewPath = "%JAVA_BIN%;" + $NewPath
$RegKey ="HKLM:\SYSTEM\CurrentControlSet\Control\Session Manager\Environment"
Set-ItemProperty -Path $RegKey -Name Path -Value $NewPath
echo Updated Path environment variable
# Next bit which isn't working
$HWND_BROADCAST = 0xffff
$WM_SETTINGCHANGE = 0x001A
$ENVIRON = "Environment"
#Store the C# signature of the PostMessage function.
$signature = #"
[DllImport("user32.dll")]
public static extern int PostMessage(int hWnd, int hMsg, int wParam, int lParam);
"#
#Add the SendMessage function as a static method of a class
$PostMessage = Add-Type -MemberDefinition $signature -Name "Win32PostMessage" -Namespace Win32Functions -PassThru
#Invoke the PostMessage Function
$PostMessage::PostMessage($HWND_BROADCAST, $WM_SETTINGCHANGE, 0, $ENVIRON)
echo end of experiment
This is my output:
Start
of
experiment
Cannot convert argument "3", with value: "Environment", for "PostMessage" to type "System.Int32": "Cannot convert value "Environment" to type "System.Int32".
Error: "Input string was not in a correct format.""
At C:\share\tvm_drivers\PED\setenv.ps1:43 char:26
+ $PostMessage::PostMessage <<<< ($HWND_BROADCAST, $WM_SETTINGCHANGE, 0, $ENVIRON)
+ CategoryInfo : NotSpecified: (:) [], MethodException
+ FullyQualifiedErrorId : MethodArgumentConversionInvalidCastArgument
end
of
experiment
I am running this on Windows 7.
Can anyone help?
For a bit of background, if you reboot the target PC after setting the path env variable, then the setting works. But if you don't reboot, it is necessary to send a broadcast message as above to tell any other Window, including the command prompt, about the change. That is why I need the message.

Answer 1

This is the powershell that successfully updates all the open windows about the env change.
# Notifies other processes that the global environment block has changed.
# This lets other processes see changes to ENV: without having to reboot
# or logoff/logon. A non-zero result from SendMessageTimeout indicates success.
if (-not ("win32.nativemethods" -as [type])) {
# import sendmessagetimeout from win32
add-type -Namespace Win32 -Name NativeMethods -MemberDefinition #"
[DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Auto)]
public static extern IntPtr SendMessageTimeout(
IntPtr hWnd, uint Msg, UIntPtr wParam, string lParam,
uint fuFlags, uint uTimeout, out UIntPtr lpdwResult);
"#
}
$HWND_BROADCAST = [intptr]0xffff;
$WM_SETTINGCHANGE = 0x1a;
$result = [uintptr]::zero
# notify all windows of environment block change
[win32.nativemethods]::SendMessageTimeout($HWND_BROADCAST, $WM_SETTINGCHANGE,
[uintptr]::Zero, "Environment", 2, 5000, [ref]$result);