Variable used in LINQ method is throwing NullReferenceException on initialization?

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  I am writing a program (for personal use) that stores card names into a database. I want to implement a way to see if a name already exists and if it does it will go in a "Do_have_in_db_listbox" else it will go in a "do_not_have_list_box". This is my original method for doing so:

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  I've tried this without the While loop, I've tried variations of if, else, else if and conditions. But for whatever reason the else statement NEVER executes no mater what the first condition is. I've been looking and trying to trouble shoot but the only thing that worked for me was an exception handler:

  retrieve.Read();
  try
            {
                if(retrieve["Card"] != DBNull.Value)
                {
                    listBox_haveInDB.Items.Add(word_for_query);
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            }
            catch
            {
                listBox_notINDb.Items.Add(word_for_query);
            }
  

  This is my way of getting around it. What am I doing wrong?
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• How to filter DataGrid with SelectedItem from ItemsView using WPF?

  Quick Summary

  I have managed to code up this output so far

  

  XAML

  <Grid>
      <Grid.ColumnDefinitions>
          <ColumnDefinition />
          <ColumnDefinition />
      </Grid.ColumnDefinitions>
  
      <StackPanel Grid.Column="0">
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                      <GridViewColumn Header="Id" DisplayMemberBinding="{Binding Name}" />
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  </Grid>
  

  PersonModel

  public class PersonModel
  {
      public int Id { get; set; }
      public string Name { get; set; }
  
      public ObservableCollection<PersonModel> GetPersonModels()
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          ObservableCollection<PersonModel> people = new ObservableCollection<PersonModel>();
          people.Add(new PersonModel { Id = 1, Name = "John" });
          people.Add(new PersonModel { Id = 2, Name = "Max" });
          people.Add(new PersonModel { Id = 3, Name = "Ed" });
  
          return people;
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  AddressModel

  public class AddressModel
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      public int Id { get; set; }
      public string Address { get; set; }
      public int PersonId { get; set; }
  
      public ObservableCollection<AddressModel> GetAddressModels()
      {
          ObservableCollection<AddressModel> addresses = new ObservableCollection<AddressModel>();
          addresses.Add(new AddressModel { Id = 1, Address = "Address 1", PersonId = 1 });
          addresses.Add(new AddressModel { Id = 2, Address = "Address 1", PersonId = 2 });
          addresses.Add(new AddressModel { Id = 3, Address = "Address 2", PersonId = 3 });
         
          return addresses;
      }
  }
  

  ViewModel

  public class ViewModel : BaseViewModel
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          CopyOfAddressModel = new AddressModel();
  
          RefreshData();
      }
  
      // Person Models properties
      private ObservableCollection<PersonModel> _personModels;
      public ObservableCollection<PersonModel> PersonModels
      {
          get { return _personModels; }
          set
          {
              _personModels = value;
              OnPropertyChanged();
          }
      }
  
      public PersonModel CopyOfPersonModel { get; set; }
  
      // Person View - Used by the View
      private ICollectionView _personView;
      public ICollectionView PersonView
      {
          get { return _personView; }
          set
          {
              _personView = value;
              OnPropertyChanged();
          }
      }
  
  
  
      // Address Models properties
      private ObservableCollection<AddressModel> _addressModels;
      public ObservableCollection<AddressModel> AddressModels
      {
          get { return _addressModels; }
          set
          {
              _addressModels = value;
              OnPropertyChanged();
          }
      }
      public AddressModel CopyOfAddressModel { get; set; }
  
      // Address View - Used by the View
      private ICollectionView _addressView;
      public ICollectionView AddressView
      {
          get { return _addressView; }
          set
          {
              _addressView = value;
              OnPropertyChanged();
          }
      }
  
      // Method to refresh the data
      public void RefreshData()
      {
          // Refresh the collections from the Model method
          PersonModels = CopyOfPersonModel.GetPersonModels();
          AddressModels = CopyOfAddressModel.GetAddressModels();
  
          // Refresh the views
          PersonView = CollectionViewSource.GetDefaultView(PersonModels);
          AddressView = CollectionViewSource.GetDefaultView(AddressModels);
      }
  
  }
  

  Goal

  My current goal is to filter the Address View on the left hand side of the MainWindow via the the ListView on the right hand side.

  For example: Selected Id from the ListView is 1. Then filter the Address View in the DataGrid where PersonId = 1.

  What I have tried

  I added a Filter method and a Search property:

  // Filtering
  private bool Filter(AddressModel address)
  {
      return Search == address.PersonId;
  }
  
  private int search;
  public int Search
  {
      get { return search; }
      set
      {
          search = value;
          OnPropertyChanged();
          AddressView.Refresh();  
      }
  }
  

  and within my RefreshData() at the bottom I have added this line of code to enable filtering.

  // Filter
  AddressView.Filter = new Predicate<object>(o => Filter(o as AddressModel));
  

  Problem

  This fully works if I bind the Search property to a TextBox

  <TextBox Text="{Binding Search, UpdateSourceTrigger=PropertyChanged}" />
  

  And the DataGrid filters as intended.

  But if I try to filter via selected item from ListView like so.. it does not work.

  <ListView ItemsSource="{Binding PersonView}" SelectedItem="{Binding Search}" SelectedValue="{Binding Id}">
      <ListView.View>
          <GridView>
              <GridViewColumn Header="Id" DisplayMemberBinding="{Binding Id}" />
              <GridViewColumn Header="Id" DisplayMemberBinding="{Binding Name}" />
          </GridView>
      </ListView.View>
  </ListView>
  
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              break;
          case 3:
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              break;
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              System.out.println("Good Bye");
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      while (choice != 4);    
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  }
  

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  Thank you (any help is appreciated)
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  Firstly i tried to research on my problem and read these solutions but none of them made me understand the problem in this code. I am getting this error when i try to compile my program but only the 39th line of my code is initializing "null" variable. I can't change that because i want to use it that way and i don't know if it is letting me trouble.

  Customer customer = null;
  

  read articles:

  Request.UrlReferrer.ToString() ->System.NullReferenceException: Object reference not set to an instance of an object

  System.NullReferenceException: Object reference not set to an instance of an object

  What is a NullReferenceException, and how do I fix it?

  my code:

  using System;
  using System.Collections.Generic;
  using System.IO;
  using System.Linq;
  using System.Text;
  using System.Threading.Tasks;
  using Newtonsoft.Json;
  
  namespace ConsoleApplication123
  {
      class Program
      {
          static void Main(string[] args)
          {
              List<Customer> cList = new List<Customer>();
              using (StreamReader sr = new StreamReader("C:/Mehmet/ConsoleApp42/firstDisk.json"))
              {
                  string cjson = sr.ReadToEnd();
                  cList = JsonConvert.DeserializeObject<List<Customer>>(cjson);
              }
              Console.WriteLine("There are " + cList.Count + " Customers Name in Input json file.");
              Console.WriteLine("Details are as follows: ");
              foreach (Customer c in cList)
              {
                  Console.WriteLine("Id: " + c.Id);
                  Console.WriteLine("Name: " + c.Name);
                  Console.WriteLine("Contact Info: ");
                  string[] contacts = c.Contact.Split(new string[] { " " }, StringSplitOptions.RemoveEmptyEntries);
                  foreach (string s in contacts)
                  {
                      Console.WriteLine(s);
                  }
                  Console.WriteLine("Bill: " + c.Bill);
                  Console.WriteLine("Email: " + c.Email);
              }
              string option = "";
              int Id = 0;
              string searchText = string.Empty;
              Customer customer = null;
              Console.WriteLine("Select Option Display, Search, Create Email, Exit");
              while (!(option = Console.ReadLine()).Equals("Exit"))
              {
                  switch (option)
                  {
                      case "Display":
                          Console.Write("Enter Id of the Customer: ");
                          int.TryParse(Console.ReadLine(), out Id);
                          customer = (from c in cList where c.Id == Id select c).FirstOrDefault();
                          if (customer != null)
                          {
                              Console.WriteLine("Contact Info for Id: " + Id);
                              Console.WriteLine("Id: " + customer.Id);
                              Console.WriteLine("Name: " + customer.Name);
                              string[] contacts = customer.Contact.Split(new string[] { " " }, StringSplitOptions.RemoveEmptyEntries);
                              foreach (string s in contacts)
                              {
                                  Console.WriteLine(s);
                              }
                              Console.WriteLine("Bill: " + customer.Bill);
                              Console.WriteLine("Email: " + customer.Email);
                          }
                          else
                          {
                              Console.WriteLine("No Contact Info for Id: " + Id);
                          }
                          Console.WriteLine("Select Option Display, Search, Create Email, Exit");
                          break;
                      case "Search":
                          Console.Write("Enter Name of the Customer: ");
                          searchText = Console.ReadLine();
                          customer = (from c in cList where c.Name.Equals(searchText) select c).FirstOrDefault();
                          if (customer != null)
                          {
                              Console.WriteLine("Contact Info Name: " + searchText);
                              Console.WriteLine("Id: " + customer.Id);
                              Console.WriteLine("Name: " + customer.Name);
                              string[] contacts = customer.Contact.Split(new string[] { " " }, StringSplitOptions.RemoveEmptyEntries);
                              foreach (string s in contacts)
                              {
                                  Console.WriteLine(s);
                              }
                              Console.WriteLine("Bill: " + customer.Bill);
                              Console.WriteLine("Email: " + customer.Email);
                          }
                          else
                          {
                              Console.WriteLine("No Contact Info for Id: " + Id);
                          }
                          Console.WriteLine("Select Option Display, Search, Create Email, Exit");
                          break;
                      case "Create Email":
                          Console.Write("Enter Id of the Customer: ");
                          int.TryParse(Console.ReadLine(), out Id);
                          customer = (from c in cList where c.Id == Id select c).FirstOrDefault();
                          if (customer != null)
                          {
                              string[] cparts = customer.Name.Split(new string[] { " " }, StringSplitOptions.RemoveEmptyEntries);
                              string emailId = cparts[0][0] + cparts[1] + customer.Id + "@grandcankaya.com";
                              Console.WriteLine("Email Id Just Created Is: " + emailId);
                              int index = cList.IndexOf(customer);
                              if (index >= 0)
                              {
                                  cList[index].Email = emailId;
                              }
  
                              using (StreamWriter sw = new StreamWriter("C:/Mehmet/ConsoleApp42/finalDisk.json"))
                              {
                                  sw.WriteLine(JsonConvert.SerializeObject(cList));
                              }
                              Console.WriteLine("Contact Info for Id: " + Id);
                              Console.WriteLine("Id: " + customer.Id);
                              Console.WriteLine("Name: " + customer.Name);
                              string[] contacts = customer.Contact.Split(new string[] { " " }, StringSplitOptions.RemoveEmptyEntries);
                              foreach (string s in contacts)
                              {
                                  Console.WriteLine(s);
                              }
                              Console.WriteLine("Bill: " + customer.Bill);
                              Console.WriteLine("Email: " + customer.Email);
                          }
                          else
                          {
                              Console.WriteLine("No Contact Info for Id: " + Id);
                          }
                          Console.WriteLine("Select Option Display, Search, Create Email, Exit");
                          break;
                      case "Exit":
                          Console.WriteLine("Select Option Display, Search, Create Email, Exit");
                          break;
                      default:
                          Console.WriteLine("Select Option Display, Search, Create Email, Exit");
                          break;
                  }
              }
              Console.Write("Press Any Key To Exit.");
              Console.ReadKey();
          }
      }
  
      public class Customer
      {
          public int Id { get; set; }
          public string Name { get; set; }
          public decimal Bill { get; set; }
          public string Contact { get; set; }
          public string Email { get; set; }
      }
  }
  

  the issue i am having:

  

  this is my json file at first:

      [
    {
      "FirstName": "Kaan",
      "LastName": "OZAYDIN",
      "EmployeeId": 101,
      "Email": "",
      "Password": "",
      "Address": {
        "ZipCode": "06460",
        "State": "Ankara",
        "Country": "Turkey"
      }
    },
    {
      "FirstName": "Erkan",
      "LastName": "YILDIZ",
      "EmployeeId": 102,
      "Email": "",
      "Password": "",
      "Address": {
        "ZipCode": "06460",
        "State": "Ankara",
        "Country": "Turkey"
      }
    }
  ]
  

  what is wrong with the code? please let me know, if you can. Also that was a homework but since i don't get the coding, i won't upload it. Time doesn't matter, just wanted to learn.
• Attempt to invoke virtual method 'void android.widget.ProgressBar.setVisibility(int)' on a null object reference

  I am trying to make a progress load bar before data loading and making it invisible after load. below is my code for this. I have tried all possible but still exception doesn't seems to getting resolve.

          progressBar.setVisibility(INVISIBLE)
          viewModel.init(args.referenceType, args.referenceValue)
          progressBar.setVisibility(VISIBLE)
  

  init function inside viewmodel call api and fetch data.

  .xml is:

       android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:layout_centerInParent="true"
          android:id="@+id/progressBar"
          android:progress="0"
          android:visibility="invisible"
          />
  

  error is:

         Attempt to invoke virtual method 'void android.widget.ProgressBar.setVisibility(int)' on a null object reference
  
• I have been getting a NullReferenceException error and it just doesn't go! Can someone help me out with this situation?

  Hey guys I am getting this error in my unity console I want to make a 2d sword combat game. But I am not able to damage the enemy. Pls Help!

  NullReferenceException: Object reference not set to an instance of an object
  DoDamage.doDamage () (at Assets/Scripts/DoDamage.cs:16)
  PlayerAttack.SwordCombat () (at Assets/Scripts/PlayerAttack.cs:54)
  PlayerAttack.Update () (at Assets/Scripts/PlayerAttack.cs:25)
  

  My PlayerAttack script This script is attached to player All the public GameObjects in this script are filled

  public class PlayerAttack : MonoBehaviour {
  
      public Animator anim;
      private Vector2 movement;
      private Vector2 sprint;
      private float doubleClickTime = 0.2f;
      private float lastClickTime;
      [SerializeField]
      public bool punched;
      public bool isArmed = false;
      private GameObject sword;
  
      void Start(){
          movement = gameObject.GetComponent<PlayerMove>().movement;
          sprint = gameObject.GetComponent<PlayerMove>().sprinting;
      }
  
      void Update () {
          if(isArmed){
              sword = gameObject.GetComponent<PickupWeapon>().sword;
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          }else{
              HandCombat();
          }
      }
  
      public void HandCombat(){
          if(Input.GetKeyDown(KeyCode.Mouse0)){
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                  anim.SetTrigger("DoublePunch");
                  punched = false;
              }else{
                  anim.SetTrigger("Punch 1");
                  punched = true;
              }
              lastClickTime = Time.time;
          }
      }
  
      public void SwordCombat(){
          if(Input.GetKeyDown(KeyCode.Mouse0)){
              float timeSinceLastClick = Time.time - lastClickTime;
              if(timeSinceLastClick < doubleClickTime && punched){
                  anim.SetTrigger("DoubleSwordAttack");
                  sword.GetComponent<DoDamage>().doDamage();
                  punched = false;
              }else{
                  anim.SetTrigger("SwordAttack");
                  sword.GetComponent<DoDamage>().doDamage();
                  punched = true;
              }
              lastClickTime = Time.time;
          }
      }
  }
  

  My DoDamage script

  This script is responsible to damage the enemy

  public class DoDamage : MonoBehaviour {
  
      public Transform attackPoint;
      public float attackRange = 0.5f;
      public LayerMask enemyLayers;
      private GameObject damageTaker;
  
      public void doDamage(){
          Collider2D[] hitEnemies = Physics2D.OverlapCircleAll(attackPoint.position, attackRange, enemyLayers);
  
          foreach(Collider2D enemy in hitEnemies){
              enemy.gameObject.GetComponent<Control>().TakeDamage(50f);
          }
      }
  
      void OnDrawGizmosSelected(){
          if(attackPoint == null){
              return;
          }
          Gizmos.DrawWireSphere(attackPoint.position, attackRange);
      }
  
  }
  

  My Control script

  This script is attached to enemy and contain its key features

  using System.Collections;
  using System.Collections.Generic;
  using UnityEngine;
  
  public class Control : MonoBehaviour {
  
      public float speed = 8f;
      public float maxHealth = 100;
      float currentHealth;
  
      void Start(){
          currentHealth = maxHealth;
      }
  
      public void TakeDamage(float damage){
          currentHealth -= damage;
          if(currentHealth <= 0){
              Die();
          }
      }
  
      public void Die(){
          Debug.Log("Enemy Died!");
      }
  
  }
  

  I have tried number of solutions. I have wrote this code using one of the brackeys tutorial on 2D combat system. It has the same code as mine but I am getting errors.

  I dont know what does the error mean and not getting a perfect solution. Pls Help!