UserOwnsPostAsync.cs

public async Task<bool> UserOwnsPostAsync(Guid postId, string userId)
{
    var post = await _dataContext.Posts.AsNoTracking().SingleOrDefaultAsync(x => x.Id == postId);

    if (post == null)
    {
        return false;
    }

    if (post.UserId != userId)
    {
        return false;
    }

    return true;
}