Создан синглтон класс

LawFirm/LawFirmFileImplement/DataFileSingleton.cs

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+using LawFirmFileImplement.Models;
+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using System.Xml.Linq;
+
+namespace LawFirmFileImplement
+{
+    public class DataFileSingleton
+    {
+        private static DataFileSingleton? instance;
+        private readonly string BlankFileName = "Blank.xml";
+        private readonly string OrderFileName = "Order.xml";
+        private readonly string DocumentFileName = "Document.xml";
+        public List<Blank> Blanks { get; private set; }
+        public List<Order> Orders { get; private set; }
+        public List<Document> Documents { get; private set; }
+        public static DataFileSingleton GetInstance()
+        {
+            if (instance == null)
+            {
+                instance = new DataFileSingleton();
+            }
+            return instance;
+        }
+        public void SaveBlanks() => SaveData(Blanks, BlankFileName, "Blanks", x => x.GetXElement);
+        public void SaveDocuments() => SaveData(Documents, DocumentFileName, "Documents", x => x.GetXElement);
+        public void SaveOrders() => SaveData(Orders, OrderFileName, "Orders", x => x.GetXElement);
+        private DataFileSingleton()
+        {
+            Blanks = LoadData(BlankFileName, "Blank", x => Blank.Create(x)!)!;
+            Documents = LoadData(DocumentFileName, "Document", x => Document.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);
+            }
+        }
+    }
+}