Why doesn't asp.net core model binding work?

Question

This is my view

@model MyProject.ViewModel.CourseViewModel

@{
    ViewData["Title"] = "AddCourseView";
    Layout = "~/Views/Shared/_Layout.cshtml";
}

<h1>Add Course</h1>

<div class="row">
    <div class="col-md-12">
        <form method="post">
            <div asp-validation-summary="All" class="text-danger"></div>
            <div class="form-group">
                <label asp-for="Course.CourseName"></label>
                <input asp-for="Course.CourseName" class="form-control" />
                <span asp-validation-for="Course.CourseName" class="text-danger"></span>
            </div>
            <label>Select lecturers to the course</label>
            @{
                var lst = Model.List;
                @for (int i = 0; i<lst.Count(); i++)
                {
                    var lecturer = lst.ElementAt(i);
                    <div class="checkbox">
                        <label><input type="checkbox" value="" id="checkbox @i" [email protected]>@lecturer.LecturerId</label>
                    </div>


                }

                @if (lst.Count() == 0)
                {
                    <div class="checkbox">
                        <span class="badge badge-danger" style="font-size:medium">Lecturers list is empty</span>
                    </div>
                }
            }
            <p></p>
            <button type="submit" class="btn btn-primary">Create</button>
        </form>
        </div>
    </div>

The view call AddCourse

This is my controller:

  [HttpGet]
        public IActionResult AddCourse()
        {
            var lst = userService.GetAllLecturers();
            CourseViewModel courseViewModel = new CourseViewModel();
            courseViewModel.List = new List<IsCheckedLecturer>();

            foreach(var lecture in lst)
            {
                courseViewModel.List.Add(new IsCheckedLecturer
                {
                    IsChecked = false,
                    LecturerId = lecture.UserId
                });
            }


            return View(courseViewModel);
        }

        [HttpPost]
        public IActionResult AddCourse(CourseViewModel viewModel)
        {
            return View(viewModel);
        }

HttpGet work fine, but the HttpPost method(the second method) get a null list instead of get the model from the view(by model binding)

This is the courseViewModel

    public class CourseViewModel
    {
        public Course Course { get; set; }
        public IList<IsCheckedLecturer> List { get; set; }

    }

    public class IsCheckedLecturer
    {
        public bool IsChecked { get; set; }
        public string LecturerId { get; set; }
    }

why doesn't model binding work? and how i fix it? Maybe because its complex view model?

Chris Pratt · Accepted Answer · 2020-04-16 15:25:20Z

Your question isn't very clear, but I'm assuming you just mean that the List property of viewModel is null, not viewModel itself. The reason for that is that you're not binding the input(s) correctly.

var lst = Model.List;
@for (int i = 0; i<lst.Count(); i++)
{
    var lecturer = lst.ElementAt(i);
    <div class="checkbox">
        <label><input type="checkbox" value="" id="checkbox @i" [email protected]>@lecturer.LecturerId</label>
    </div>
}

The code above will result in the input having a name of lecturer.IsChecked. There's nothing to bind this to when it gets back server-side (there's no lecturer param), so the model binder just drops it, leaving your List property null.

Instead, you need to change your code to something like:

@for (int i = 0; i < Model.List.Count(); i++)
{
    <div class="checkbox">
        <label><input type="checkbox" id="checkbox@i" asp-for="List[i].IsChecked">@Model.List[i].LecturerId</label>
    </div>
}

With that, the name of the input will end up as something like List[0].IsChecked, which will then bind to the List property on your model.

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