在 ASP.NET 核心 Razor 页面中填充下拉列表时获取 null 引用异常

Question

I am new to ASP.Net core. I am trying to populate dropdown list in class Createmodel from the Instructor model and getting a "Null reference" exception.

This is the code in cshtml.cs file:

public SelectList Instructors { get; set; }

public async Task OnGetAsync()
{
    var students = from s in _context.User_Profile
                   select s;

    if (!string.IsNullOrEmpty(SearchString))
    {
        students = students.Where(s => s.FirstName.Contains(SearchString));
    }

    UserProfile = await students.ToListAsync();

    var instructors = from i in _context.Instructor
                      orderby i.FirstName
                      select i;
       
    Instructors = new SelectList(instructors, "ID", "FirstName");
}

I have the following markup in my cshtml view:

<div class="form-group">
    <label asp-for="Class_Info.Instructor" class="control-label"></label>
    <select asp-for="Class_Info.Instructor" class="form-control" asp-items="@Model.Instructors">
         <option value="">-- Select --</option>
    </select>
</div>

I am getting the error at:

@Model.Instructors

I am getting this exception

Please advise...

Answer 1

I had misspelled the field name here:

Instructors = new SelectList(instructors, "ID", "FirstName");

Changed it to:

Instructors = new SelectList(instructors, "InstructorID", "FirstName");

And it started working. Thank you everyone for your help!!