[DllImport("kernel32.dll")]
static extern bool QueryFullProcessImageName(IntPtr hProcess, uint dwFlags, StringBuilder lpExeName, ref int lpdwSize);
[DllImport("kernel32.dll")]
static extern IntPtr OpenProcess(ProcessAccessRights dwDesiredAccess, bool bInheritHandle, int dwProcessId);
[Flags]
enum ProcessAccessRights
{
    PROCESS_QUERY_INFORMATION = 0x0400,
    PROCESS_QUERY_LIMITED_INFORMATION = 0x1000,
    PROCESS_CREATE_THREAD = 0x0002,
    PROCESS_VM_OPERATION = 0x0008,
    PROCESS_VM_WRITE = 0x0020,
    PROCESS_VM_READ = 0x0010
}
[DllImport("kernel32.dll")]
static extern bool CloseHandle(IntPtr hObject);

private string[] GetProcessesPaths(string processName)
{
    //Ottiene i processi in base al nome
    Process[] processes = Process.GetProcessesByName(processName);
    //Crea l'array del risultato
    string[] result = new string[processes.Length];
    //Itera fra i processi trovati
    for (int i = 0; i < processes.Length; i++)
    {
        //Ottiene l'handle di sistema per il processo
        IntPtr handle = OpenProcess(ProcessAccessRights.PROCESS_QUERY_INFORMATION, false, processes[i].Id);
        //la dimensione massima del percorso
        int sz = 512;
        //Crea uno stringBuilder che riceverà il nome del processo
        StringBuilder sb = new StringBuilder(sz);
        //Ottiene il percorso completo processo
        QueryFullProcessImageName(handle, 0, sb, ref sz);
        //imposta il percorso
        result[i] = sb.ToString();
        //Rilascia l'handle del processo
        CloseHandle(handle);
    }
    //Ritorna il risultato
    return result;
}