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FurnitureAssembly/FurnitureAssemFileImplement/DataFileSingleton.cs

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+using FurnitureAssemFileImplement.Models;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using System.Xml.Linq;
+
+namespace FurnitureAssemFileImplement
+{
+    public class DataFileSingleton
+    {
+        private static DataFileSingleton? instance;
+        private readonly string ComponentFileName = "Component.xml";
+        private readonly string OrderFileName = "Order.xml";
+        private readonly string FurnitureFileName = "Furniture.xml";
+        public List<Component> Components { get; private set; }
+        public List<Order> Orders { get; private set; }
+        public List<Furniture> Furnitures { get; private set; }
+        public static DataFileSingleton GetInstance()
+        {
+            if (instance == null)
+            {
+                instance = new DataFileSingleton();
+            }
+            return instance;
+        }
+        public void SaveComponents() => SaveData(Components, ComponentFileName, "Components", x => x.GetXElement);
+        public void SaveFurnitures() => SaveData(Furnitures, FurnitureFileName, "Furnitures", x => x.GetXElement);
+        public void SaveOrders() => SaveData(Orders, OrderFileName, "Orders", x => x.GetXElement);
+        private DataFileSingleton()
+        {
+            Components = LoadData(ComponentFileName, "Component", x => Component.Create(x)!)!;
+            Furnitures = LoadData(FurnitureFileName, "Furniture", x => Furniture.Create(x)!)!;
+            Orders = LoadData(OrderFileName, "Order", x => Order.Create(x)!)!;
+        }
+        private static List<T>? LoadData<T>(string filename, string xmlNodeName,
+       Func<XElement, T> selectFunction)
+        {
+            if (File.Exists(filename))
+            {
+                return XDocument.Load(filename)?.Root?.Elements(xmlNodeName)?.Select(selectFunction)?.ToList();
+            }
+            return new List<T>();
+        }
+        private static void SaveData<T>(List<T> data, string filename, 
+            string xmlNodeName, Func<T, XElement> selectFunction)
+        {
+            if (data != null)
+            {
+                new XDocument(new XElement(xmlNodeName,
+               data.Select(selectFunction).ToArray())).Save(filename);
+            }
+        }
+    }
+}

FurnitureAssembly/FurnitureAssemFileImplement/FurnitureAssemFileImplement.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net6.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\FurnitureAssemblyContracts\FurnitureAssemblyContracts.csproj" />
+    <ProjectReference Include="..\FurnitureAssemblyDataModels\FurnitureAssemblyDataModels.csproj" />
+  </ItemGroup>
+
+</Project>