What is a Nullreferenceexception and how do I fix it?

Question

System.Nullreferenceexception Hresult=0x80004003 Message=The object reference was not defined as an instance of an object. Source=App_web_cyyu2ydb Stacktrace: in ASP. _Page_views_vacinacao_index_cshtml.Execute() in D: In Development Projects AR Gestcentros Gestcentros Views Vacinacao Index.cshtml:line 35 in System.Web.Webpages.WebPageBase.Executepagehierarchy() in System.Web.Mvc.WebViewPage.Executepagehierarchy() in System.Web.Webpages.StartPage.Runpage() in System.Web.Webpages.StartPage.Executepagehierarchy() in System.Web.Webpages.WebPageBase.Executepagehierarchy(Webpagecontext pageContext, Textwriter Writer, Webpagerenderingbase startPage) in System.Web.Mvc.RazorView.Renderview(Viewcontext viewContext, Textwriter Writer, Object instance) in System.Web.Mvc.BuildManagerCompiledView.Render(Viewcontext viewContext, Textwriter Writer) in System.Web.Mvc.ViewResultBase.Executeresult(Controllercontext context) in System.Web.Mvc.ControllerActionInvoker.Invokeactionresult(Controllercontext controllerContext, Actionresult actionResult) in System.Web.Mvc.ControllerActionInvoker.Invokeactionresultfilterrecursive(Ilist1 filters, Int32 filterIndex, ResultExecutingContext preContext, ControllerContext controllerContext, ActionResult actionResult) em System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilterRecursive(IList1 Filters, Int32 filterIndex, Resultexecutingcontext preContext, Controllercontext controllerContext, Actionresult actionResult)

I intend to return the child’s name with the vaccines already administered. what happens is that it does not return anything to me, because if I call the Id it returns the Id of the Child

 using GestCentros.Models;
using System.Collections.Generic;

namespace GestCentros.ViewModels
{
    public class VacinacaoIndexData
    {
       public IEnumerable<Vacinacao> Vacinacoes { get; set; }
        public IEnumerable<Vacina> Vacinas { get; set; }
        public IEnumerable<Enrollment> Enrollments { get; set; }

    }
}

public ActionResult Index(int? id, int? Idvacina)
{
    var user = db.Users.Where(u => u.UserName == User.Identity.Name).FirstOrDefault();

    if (user == null)
    {
        return RedirectToAction("Index", "Home");
    }    
    var viewModel = new VacinacaoIndexData();    
    viewModel.Vacinacoes = db.Vacinacao
        .Include(v => v.Vacinas)
        .Include(v => v.Crianca)
        .Where(c => c.IdCentro == user.IdCentro);  

    if (id != null)
    {
        ViewBag.IdVacinacao = id.Value;
        viewModel.Vacinas = viewModel.Vacinacoes
            .Single(i => i.IdVacinacao == id.Value).Vacinas;
    }    
    if (Idvacina == null) return View(viewModel);           
        ViewBag.IdVacina = Idvacina.Value;
        // Lazy loading
        viewModel.Enrollments = viewModel.Vacinas.Single(x => x.IdVacina == Idvacina).Enrollments;
    // Explicit loading
    Vacina selectedVacina = viewModel.Vacinas.Single(x => x.IdVacina == Idvacina);    
    db.Entry(selectedVacina).Collection(x => x.Enrollments).Load();

        foreach (var enrollment in selectedVacina.Enrollments)
        {
            db.Entry(enrollment).Reference(x => x.Centro).Load();
        }    
        viewModel.Enrollments = selectedVacina.Enrollments;            

    return View(viewModel);
}



    @model GestCentros.ViewModels.VacinacaoIndexData

@{
    ViewBag.Title = "Index";
}

<h2 class="text-center">Lista de Crianças Vacinadas</h2>

<table class="table">
    <tr>
        <th>Nome</th>
        <th>Data Vacinação</th>
        <th>Vacinas</th>
        <th>Operações</th>
    </tr>

    @foreach (var item in Model.Vacinacoes)
     {
        string selectedRow = "";
        if (item.IdVacinacao == ViewBag.IdVacinacao)
        {
            selectedRow = "success";
        }
        <tr class="@selectedRow">
            <td>
                @Html.DisplayFor(model => item.Crianca.Nome)

            </td>
            <td>
                @item.DataVacina.ToString("dd/MM/yyyy")
            </td>
            <td>
                @{
                    foreach (var vacina in item.Vacinas)
                    {
                        @:  @vacina.Nome <br />
                    }
                }
            </td>
        </tr>
     }
</table>

@if (Model.Vacinas != null)
{
    <h3>Vacinas Ministradas na Campanha</h3>
    <table class="table">
        <tr>
            <th>Operações</th>
            <th>Vacina</th>
        </tr>

        @foreach (var item in Model.Vacinas)
        {
            var selectedRow = "";
            if (item.IdVacina == ViewBag.IdVacina)
            {
                selectedRow = "success";
            }
            <tr class="@selectedRow">
                <td>
                    @Html.ActionLink("Selecionar", "Index", new { IdVacina = item.IdVacina }, new { @class = "btn btn-sm btn-secundary" })
                    | @Html.ActionLink("Voltar a Lista", "Index", new { }, new { @class = "btn btn-sm btn-default" })
                </td>                
                <td>
                    @item.Nome
                </td>
            </tr>
        }

    </table>
}

@if (Model.Enrollments != null)
{
    <h3>
        Students Enrolled in Selected Course
    </h3>
    <table class="table">
        <tr>
            <th>Name</th>
            <th>Grade</th>
        </tr>
        @foreach (var item in Model.Enrollments)
        {
            <tr>
                <td>
                    @item.Centro.Name
                </td>
                <td>
                    @Html.DisplayFor(modelItem => item.Grade)
                </td>
            </tr>
        }
    </table>
}

Answer 1

TL;DR:

You are trying to access a variable from an instantiated object. On the line item.Crianca.Nome or the item is null, or the prop Crianca of item is null. Instantiating what is not instantiated solves the problem.

Explanation

There are two types of value for variables: Tipos de valor and Tipos de referência.

A Value Type stores the variable in memory Stack. Examples of Value Type: Int32, bool, double. When vc creates a value type, a memory space is allocated to store the value and the variable stores the value directly.

The Reference Type stores the object in memory Heap and creates in the Stack only a reference to the address of the object in Heap, but not the object directly. Basically a pointer. Example: An object generated from a class

When we create an object (Crianca filho;), is generated in memory Stack a space to store the instance address (in this case, the variable is filled with the address, but the location the address points to is null, as the instance has not yet been created).

When instantiating the object(filho = new Crianca();) a space in memory Heap is allocated to store variable data.

If we attempt to access a property of an object not yet instantiated, the Stack will point to a value that is null, so will have a NullReferenceException

Useful links:

Value type vs Reference type

Difference Heap vs Stack

Answer 2

As stated in the comments, Exception NullReferenceException occurs when an attempt to access an attribute or method of a non-einstantiated object occurs.

It would occur, for example, when trying to access an attribute after instantiating an object in the form of Objeto instanciaObjeto; instead of Objeto instanciaObjeto = new Objeto();.

From the image that was sent, it seems that the error occurs when accessing an attribute of the Child object. I suggest you check, with a debug, if the object is being loaded normally with the Vaccination.