param(
  [switch]$Enable,
  [switch]$Disable,
  [string]$ProxyServer = '127.0.0.1:8888',
  [string]$Override = '<local>'
)

$ErrorActionPreference = 'Stop'
$key = 'HKCU:\Software\Microsoft\Windows\CurrentVersion\Internet Settings'

if ($Enable -eq $Disable) {
  throw 'Pass exactly one of -Enable or -Disable.'
}

if ($Enable) {
  Set-ItemProperty -Path $key -Name ProxyEnable -Type DWord -Value 1
  Set-ItemProperty -Path $key -Name ProxyServer -Type String -Value $ProxyServer
  Set-ItemProperty -Path $key -Name ProxyOverride -Type String -Value $Override
  Write-Host "Enabled Windows proxy: $ProxyServer"
} else {
  Set-ItemProperty -Path $key -Name ProxyEnable -Type DWord -Value 0
  Write-Host 'Disabled Windows proxy.'
}

$source = @"
using System;
using System.Runtime.InteropServices;
public static class WinInetNative {
  [DllImport("wininet.dll", SetLastError = true)]
  public static extern bool InternetSetOption(IntPtr hInternet, int dwOption, IntPtr lpBuffer, int dwBufferLength);
}
"@

Add-Type -TypeDefinition $source -ErrorAction SilentlyContinue | Out-Null
[WinInetNative]::InternetSetOption([IntPtr]::Zero, 39, [IntPtr]::Zero, 0) | Out-Null
[WinInetNative]::InternetSetOption([IntPtr]::Zero, 37, [IntPtr]::Zero, 0) | Out-Null