gistfile1.cs

public static class ProcessExtensions
{
    public static Process GetParent(this Process child)
    {
        int parentPid = 0;

        IntPtr hnd = Kernel32.CreateToolhelp32Snapshot(Kernel32.TH32CS_SNAPPROCESS, 0);

        if (hnd == IntPtr.Zero)
            return null;

        Kernel32.PROCESSENTRY32 processInfo = new Kernel32.PROCESSENTRY32 { dwSize = (uint)Marshal.SizeOf(typeof(Kernel32.PROCESSENTRY32)) };

        if (Kernel32.Process32First(hnd, ref processInfo) == false) return null;

        do
        {
            if (child.Id == processInfo.th32ProcessID)
                parentPid = (int)processInfo.th32ParentProcessID;
        }
        while (parentPid == 0 && Kernel32.Process32Next(hnd, ref processInfo));

        if (parentPid > 0)
            return Process.GetProcessById(parentPid);
        else
            return null;            
    }
}

internal static class Kernel32
{
    public static uint TH32CS_SNAPPROCESS = 2;

    [StructLayout(LayoutKind.Sequential)]
    public struct PROCESSENTRY32
    {
        public uint dwSize;
        public uint cntUsage;
        public uint th32ProcessID;
        public IntPtr th32DefaultHeapID;
        public uint th32ModuleID;
        public uint cntThreads;
        public uint th32ParentProcessID;
        public int pcPriClassBase;
        public uint dwFlags;
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 260)]
        public string szExeFile;
    };

    [DllImport("kernel32.dll", SetLastError = true)]
    public static extern IntPtr CreateToolhelp32Snapshot(uint dwFlags, uint th32ProcessID);

    [DllImport("kernel32.dll")]
    public static extern bool Process32First(IntPtr hSnapshot, ref PROCESSENTRY32 lppe);

    [DllImport("kernel32.dll")]
    public static extern bool Process32Next(IntPtr hSnapshot, ref PROCESSENTRY32 lppe);
}