Set value to a sub class from main class [duplicate]

This question already has an answer here:

I am trying to set value to StorageFile who lives inside MainFolderDisplay as a list. But the problem is i am getting exception like bellow. Whats wrong i am doing here?

StorageFile

MainFolderDisplay

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

Controller:

var mainFolder = new MainFolderDisplay();

mainFolder.StorageFile.Add(new StorageFile

AwsUniqueFileName = "asasas",
FileId = 0,
FolderType = 0,
RelatedFolderId = 0,
UserDisplayFileName = "",
UserId = 0,
);


Model:

using Test.Models;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace Test.ViewModels

public class MainFolderDisplay

public int MainFolderId get; set;
public string MainFolderName get; set;
public int UserId get; set;

public List<SubFolder> SubFolders get; set;
public List<StorageFile> StorageFile get; set;


public class SubFolderDisplay

public int SubFolderId get; set;
public string SubFolderName get; set;
public int MainFolderId get; set;
public List<StorageFile> StorageFile get; set;


public class StorageFileDisplay


public int FileId get; set;
public string AwsUniqueFileName get; set;
public string UserDisplayFileName get; set;
public int UserId get; set;
public int FolderType get; set;
public int RelatedFolderId get; set;




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2 Answers
2

Your "StorgaeFile" property is not initialized before using :

var mainFolder = new MainFolderDisplay();

mainFolder.StorageFile = new List<StorageFile>

new StorageFile

AwsUniqueFileName = "asasas",
FileId = 0,
FolderType = 0,
RelatedFolderId = 0,
UserDisplayFileName = "",
UserId = 0,

;


The better way is to change the "MainFolderDisplay" class implementation to handle initialization :

public class MainFolderDisplay

private List<StorageFile> _storageFile;

public int MainFolderId get; set;
public string MainFolderName get; set;
public int UserId get; set;

public List<StorageFile> StorageFile

get => _storageFile ?? (_storageFile = new List<StorageFile>());
set => _storageFile = value;




and now use it without worry :

mainFolder.StorageFile.Add(
new StorageFile

AwsUniqueFileName = "asasas",
FileId = 0,
FolderType = 0,
RelatedFolderId = 0,
UserDisplayFileName = "",
UserId = 0,
);


The StorageFile list isn't initialised.

mainFolder.StorageFile = new List<StorageFile>();


I would suggest renaming this property to StorageFiles to imply it's a collection.

mainFolder.StorageFiles = new List<StorageFile>();