递归方法解决的问题:

[HttpPost]
public async Task<JsonResult> DeleteComment([FromBody] DeleteCommentViewModel obj)
{
    if (ModelState.IsValid)
    {
       var comment = await 
      _commentRepository.GetAll().SingleOrDefaultAsync(m => m.Id == obj.CommentId);
      if (comment != null)
      {
          await RemoveChildren(comment.Id);
         _commentRepository.Delete(comment);
      }
      if (Request.IsAjaxRequest())
      {
          return Json(1);
      }
   }
   return Json(new { code = 0 });
}


async Task RemoveChildren(int i)
{
    var children = await _commentRepository.GetAll().Where(c => c.ParentId = i).ToListAsync();
        foreach (var child in children)
    {
       await RemoveChildren(child.Id);
       _commentRepository.Delete(child);
    }
}