Hide Show Divs on Index causing errors

Question

I am new to asp.net mvc and am having some issues. What I am trying to do is display a div on the Index that allows a user to enter name and choose values from drop down lists. Then on "Generate Password" Hide the initial div and display a div "Hi 'UserName' you entered 'value1', 'value2'. Eventually what I want is to capture the UserName and use the value from the drop downs to create a random password (haven't started that module yet "ugh". What it will do is say Hello 'UserName' your new password is 'password'. and the password will contain the length that they have picked and the max num of alpha characters that they want. Every time I try I get the following error.

System.NullReferenceException was unhandled by user code

on this

 @Html.DropDownListFor(m => m.LenOfPass, Model.Lengths, "Please select...", new { @class = "form-control" })

[UPDATED CODE BASED ON @Haney's Response]

New code same error;

Controller;

using System;
using PassGen.Models;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using System.ComponentModel.DataAnnotations;


namespace PassGen.Controllers
{
public class HomeController : Controller
{

    private IEnumerable<int> GetLengths()
    {
        return new List<int>
        {
            8,10,15,20,25
        };
    }

    private IEnumerable<int> GetAlpha()
    {
        return new List<int>
        {
            1,2,3,4,5
        };
    }
    private IEnumerable<SelectListItem> GetLengthList(IEnumerable<int> elements)
    {
        var passLens = new List<SelectListItem>();
        foreach (var len in elements)
        {
            string lengths = Convert.ToString(len);
            passLens.Add(new SelectListItem { Value = lengths, Text = lengths });
        }
        return passLens;
    }
    private IEnumerable<SelectListItem> GetAlphaList(IEnumerable<int> element)
    {
        var alphaMax = new List<SelectListItem>();
        foreach (var alpha in element)
        {
            string alphas = Convert.ToString(alpha);
            alphaMax.Add(new SelectListItem { Value = alphas, Text = alphas });
        }
        return alphaMax;
    }

    [HttpGet]
    public ViewResult Index()
    {
        ViewBag.ShowDetails = false;
        var LenOfPass = GetLengths();
        var MaxOfAlpha = GetAlpha();
        var model = new PassGenerator();

        model.LenOfPass = new List<SelectListItem>(); // Note initialization of the property
        model.Lengths = GetLengthList(LenOfPass);
        model.Alphas = GetAlphaList(MaxOfAlpha);

        return View(model);
    }

    [HttpPost]
    public ActionResult Index(PassGenerator response)
    {
        ViewBag.ShowDetails = false; 
        if (ModelState.IsValid)
        {
            ViewBag.ShowDetails = true;

            ViewBag.UserName = response.UserName;
            ViewBag.PassLength = response.LenOfPass;
            ViewBag.AlphaMax = response.MaxOfAlpha;

            return PartialView ("Index",response);

        }
        else
        {
            //There is a validation error
            return View();
        }

    }
  }
}

Model;

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using System.Security;
using System.Security.Cryptography;
using System.ComponentModel.DataAnnotations;
using System.Text;

namespace PassGen.Models
{
public class PassGenerator
{
    [Required(ErrorMessage = "Please enter your name!")]
    public string UserName { get; set; }

    //This property will hold Length of password chosen by User.
    [Required(ErrorMessage = "Please choose a length for your password!")]
    public int LenOfPass { get; set; }

    //This will hold Lengths for selection
    public IEnumerable<SelectListItem> Lengths { get; set;}

    //This property will hold Maximum number of Alphanumeric characters chosen by User.
    [Required(ErrorMessage = "Please choose the maximum number of alphanumeric characters that you want included!")]
    public int MaxOfAlpha { get; set; }

    //This will hold values for Max Alphanumeric.
    public IEnumerable<SelectListItem> Alphas { get; set;}

    //This property will hold the Random Password that is generated
    public string RandPass { get; set;}

  }
}

Index not changed, please excuse me for being a newbie on this. Index:

@model PassGen.Models.PassGenerator

@{
Layout = null;
}

<!DOCTYPE html>

<html>
<head>
<meta name="viewport" content="width=device-width" />
<title>Index</title>
<script src="~/Scripts/jquery-2.1.1.min.js" type="text/javascript"></script>
</head>
<body>
<div class="inital-form">
    @using (Html.BeginForm()) { 
        @Html.ValidationSummary()
        <p>Your Name: @Html.TextBoxFor(x => x.UserName)</p>
        <p>Please choose the length of your password:
            @Html.DropDownListFor(m => m.LenOfPass, Model.Lengths, "Please select...", new { @class = "form-control" })
        </p>
        <p>Please choose the Maximum number of Alphanumeric characters you would like included:
            @Html.DropDownListFor(m => m.MaxOfAlpha, Model.Alphas, "Please select...", new { @class = "form-control" }) 
        </p>
        <input type="submit" value="Generate Your Password" />
    }
</div>
@if (ViewBag.ShowDetails) { 
<div class="response-section">
    <h1>Hello @Html.DisplayFor(m => m.UserName)</h1>
    <p>You have entered the following information;</p>
    <ul>
        <li>Your requested length for your password: @Html.DisplayFor(m => m.LenOfPass)</li>
        <li>Maximum number of Alphanumeric Characters required: @Html.DisplayFor(m => m.MaxOfAlpha)</li>
    </ul>
</div>
}
</body>
</html>

Answer 1

You never initialize the property LenOfPass on your PassGenerator instance, so it remains null. Then, when referencing it in the View, a NullReferenceException is thrown. Alternative for your controller:

public ViewResult Index()
{
    ViewBag.ShowDetails = false;
    var LenOfPass = GetLengths();
    var MaxOfAlpha = GetAlpha();
    var model = new PassGenerator();
    model.LenOfPass = new List<SelectListItem>(); // Note initialization of the property

    model.Lengths = GetLengthList(LenOfPass);
    model.Alphas = GetAlphaList(MaxOfAlpha);


    return View(model);
}

In most programming languages and C# for certain, trying to reference a null variable, field, or property will cause a NullReferenceException.

Also of note: in general a select list will result in a single value. LenOfPass thus should not be IEnumerable<SelectListItem> and instead be string, int, or whatever type you expect the value to be set to.

public int LenOfPass { get; set; }

This will not cause an exception because int is a struct AKA value type and cannot be null.