Model state not valid

So I have a view called index that lays out all of the threads in my database. Then inside that view I'm loading all the comments on the threads. When I call on my form that is supposed to create a new comment it keeps telling me that my model state is invalid. It tells me that it cannot convert from type string to type profile or comment or tag. Originally I had this as my code:

 public ActionResult AddComment(Thread thread, string commentBody)
    {
        if (ModelState.IsValid)
        {
            _repository.AddComment(thread, comment);
            TempData["Message"] = "Your comment was added.";
            return RedirectToAction("Index");
        }

Then I changed it to this:

 public ActionResult AddComment(Thread thread, string commentBody)
    {
        Profile profile = _profileRepository.Profiles.FirstOrDefault(x => x.Id ==       thread.ProfileId);
        Tag tag = _tagRepository.Tags.FirstOrDefault(t => t.Id == thread.TagId);
        thread.ThreadTag = tag;
        thread.Profile = profile;
        Comment comment = new Comment()
                              {
                                  CommentBody = commentBody,
                                  ParentThread = thread
                              };
        if (ModelState.IsValid)
        {
            _repository.AddComment(thread, comment);
            TempData["Message"] = "Your comment was added.";
            return RedirectToAction("Index");
        }

This still tells me that my model state is invalid. How do I get it so that it will not try to change the state?

Also here is the form that is being used to call this action:

@using(Html.BeginForm("AddComment", "Thread", mod))
            {
                <input type="text" name="AddComment" id="text" />
                <input type="submit" value="Save"/>
            }

In the instance of code above mod is the model which is a thread. And as requested here is everything inside of thread:

 public Thread()
    {
        this.ChildComments = new HashSet<Comment>();
    }

    public int Id { get; set; }
    public string TopicHeader { get; set; }
    public string TopicBody { get; set; }
    public Nullable<int> UpVotes { get; set; }
    public Nullable<int> DownVotes { get; set; }
    public int ProfileId { get; set; }
    public int TagId { get; set; }

    public virtual Profile Profile { get; set; }
    public virtual ICollection<Comment> ChildComments { get; set; }
    public virtual Tag ThreadTag { get; set; }

And finally the comment class:

 public partial class Comment
{
    public int Id { get; set; }
    public string CommentBody { get; set; }
    public int UpVotes { get; set; }
    public int DownVotes { get; set; }

    public virtual Thread ParentThread { get; set; }
}

回答1:

Use the code below to iterate through the errors. Then you can see what field and what object is failing on validation. And then you can go from there. Just looking at the IsValid property is not going to give enough information.

var errors = ModelState.Values.SelectMany(v => v.Errors);

And then loop through the errors.

回答2:

Before checking your error, you need to know why is the model state invalid. You can easily do that by debugging and seeing the list of errors.

The second error should be a separate questions as I believe to be written in stackoverflow guidelines.