MVC Core. Templates can be used only with field access, property access, single-dimension array index, or single-parameter custom indexer expressions - asp.net-mvc

Question

I am passing in my view this model
public class BlogViewModel
{
public List<CommentModel> commentModels { get; set; }
public BlogPostLayoutModel blogPostLayoutModel { get; set; }
public CommentModel commentModel { get; set; }
public int pageNumber { get; set; }
}
CommentModel class
public class CommentModel
{
public int IDComment { get; set; }
public int IDPost { get; set; }
[Required()]
public string Username { get; set; }
[Required()]
public string Content { get; set; }
public static CommentModel CommentToCommentModel(Comment comment)
{
CommentModel commentModel = new CommentModel();
commentModel.IDComment = comment.IDComment;
commentModel.IDPost = comment.IDPost;
commentModel.Username = comment.Username;
commentModel.Content = comment.Content;
return commentModel;
}
public static List<CommentModel> CommentsToCommentModels(List<Comment> comments)
{
List<CommentModel> commentModels = new List<CommentModel>();
foreach (var comment in comments)
{
CommentModel commentModel = new CommentModel();
commentModel.IDComment = comment.IDComment;
commentModel.IDPost = comment.IDPost;
commentModel.Username = comment.Username;
commentModel.Content = comment.Content;
commentModels.Add(commentModel);
}
return commentModels;
}
}
And my view, where the error is thrown (fifth line)
<form asp-action="AddComment">
<div asp-validation-summary="ModelOnly" class="text-danger"></div>
#{ var pageNumber = Model.pageNumber;
#Html.HiddenFor(x => pageNumber)
int IDPost = Model.blogPostLayoutModel.IDPost;
#Html.HiddenFor(x => x.commentModel.IDPost == IDPost)
}
<div class="row">
<div class="form-group col-md-4">
<label asp-for="commentModel.Username" class="control-label"></label>
<input asp-for="commentModel.Username" class="form-control" />
<span asp-validation-for="commentModel.Username" class="text-danger"></span>
</div>
</div>
<div class="row">
<div class="form-group col-md-12">
<label class="control-label">Comment</label>
<textarea asp-for="commentModel.Content" class="form-control"></textarea>
<span asp-validation-for="commentModel.Content" class="text-danger"></span>
</div>
<div class="form-group col-md-4">
<input type="submit" value="Create" class="btn btn-default" />
</div>
</div>
</form>
I have searched for the error, but seems like it's mostly people trying to return a value from a method and getting told to pass the value to a local variable and then put that local variable inside the tag helper, but that doesn't seem to be the case here.
And also blogPostLayoutModel.IDPost is defined as
public int IDPost { get; set; }
EDIT: My commentModel was null, so that was the problem.
Working form
<form asp-action="AddComment">
<div asp-validation-summary="ModelOnly" class="text-danger"></div>
#{
#Html.HiddenFor(x => Model.pageNumber)
Model.commentModel.IDPost = Model.blogPostLayoutModel.IDPost;
#Html.HiddenFor(x => x.commentModel.IDPost)
}
<div class="row">
<div class="form-group col-md-4">
<label asp-for="commentModel.Username" class="control-label"></label>
<input asp-for="commentModel.Username" class="form-control" />
<span asp-validation-for="commentModel.Username" class="text-danger"></span>
</div>
</div>
<div class="row">
<div class="form-group col-md-12">
<label class="control-label">Comment</label>
<textarea asp-for="commentModel.Content" class="form-control"></textarea>
<span asp-validation-for="commentModel.Content" class="text-danger"></span>
</div>
<div class="form-group col-md-4">
<input type="submit" value="Create" class="btn btn-default" />
</div>
</div>
</form>