Cannot resolve UnauthorizedAccessException

public void WriteToErrorLog(Exception ex, string src)
{
    string name = Environment.UserName;
    string domain = System.Security.Principal.WindowsIdentity.GetCurrent().Name;
    NTAccount act = new NTAccount(domain);
    DirectoryInfo dirinfo = new DirectoryInfo(logFile);
    DirectorySecurity sec = dirinfo.GetAccessControl();

    // FileSecurity sec = File.GetAccessControl(logFile);
    sec.AddAccessRule(
        new FileSystemAccessRule(
            act,
            FileSystemRights.Write | FileSystemRights.ListDirectory,
            InheritanceFlags.ContainerInherit,
            PropagationFlags.InheritOnly,
            AccessControlType.Allow
        )
    );
    //File.SetAccessControl(logFile, sec); 
    //if (permissionset.IsSubsetOf(AppDomain.CurrentDomain.PermissionSet))
    //{
    StreamWriter streamwr = new StreamWriter(logFile, true); //Error: access denied

    //new FileStream(logFile, FileMode.Append, FileAccess.Write)
    streamwr.WriteLine("------{0}-------", DateTime.Now);
    if (ex.InnerException != null)
    {
        streamwr.Write("Inner exeption Type: ");
        streamwr.WriteLine(ex.InnerException.GetType().ToString());
        streamwr.Write("Inner exeption: ");
        streamwr.WriteLine(ex.InnerException.Message);
        streamwr.Write("Inner Source: ");
        streamwr.WriteLine(ex.InnerException.Source);
        streamwr.WriteLine(ex.InnerException.StackTrace);
    }

    streamwr.Write("Exception Type: ");
    streamwr.WriteLine(ex.GetType().ToString());
    streamwr.WriteLine("Exception: " + ex.Message);
    streamwr.WriteLine("Source: " + src);
    streamwr.WriteLine("Stack Trace: ");
    if (ex.StackTrace != null)
    {
        streamwr.WriteLine(ex.StackTrace);
        streamwr.WriteLine();
    }
    streamwr.Close();
}