добавила реализацию для воторого хранилища...

SushiBarFileImplement/DataFileSingleton.cs

@@ -17,11 +17,14 @@ namespace SushiBarFileImplement
 
         private readonly string ImplementerFileName = "Implementer.xml";
 
+        private readonly string MessageInfoFileName = "MessageInfo.xml";
+
         public List<Component> Components { get; private set; }
         public List<Order> Orders { get; private set; }
         public List<Sushi> Sushis { get; private set; }
         public List<Client> Clients { get; private set; }
         public List<Implementer> Implementers { get; private set; }
+        public List<MessageInfo> Messages { get; private set; }
 
         public static DataFileSingleton GetInstance()
         {
@@ -37,6 +40,7 @@ namespace SushiBarFileImplement
         public void SaveOrders() => SaveData(Orders, OrderFileName, "Orders", x => x.GetXElement);
         public void SaveClients() => SaveData(Clients, ClientFileName, "Clients", x => x.GetXElement);
         public void SaveImplementers() => SaveData(Implementers, ImplementerFileName, "Implementers", x => x.GetXElement);
+        public void SaveMessageInfo() => SaveData(Messages, MessageInfoFileName, "Messages", x => x.GetXElement);
 
         private DataFileSingleton()
         {
@@ -45,6 +49,7 @@ namespace SushiBarFileImplement
             Orders = LoadData(OrderFileName, "Order", x => Order.Create(x)!)!;
             Clients = LoadData(ClientFileName, "Client", x => Client.Create(x)!)!;
             Implementers = LoadData(ImplementerFileName, "Implementer", x => Implementer.Create(x)!)!;
+            Messages = LoadData(MessageInfoFileName, "MessageInfo", x => MessageInfo.Create(x)!)!;
         }
 
         private static List<T>? LoadData<T>(string filename, string xmlNodeName, Func<XElement, T> selectFunction)

SushiBarFileImplement/Implements/MessageInfoStorage.cs

@@ -0,0 +1,49 @@
+using SushiBarContracts.BindingModel;
+using SushiBarContracts.SearchModel;
+using SushiBarContracts.StoragesContracts;
+using SushiBarContracts.ViewModels;
+using SushiBarFileImplement.Models;
+
+namespace SushiBarFileImplement.Implements
+{
+    public class MessageInfoStorage : IMessageInfoStorage
+    {
+        private readonly DataFileSingleton source;
+        public MessageInfoStorage()
+        {
+            source = DataFileSingleton.GetInstance();
+        }
+
+        public MessageInfoViewModel? GetElement(MessageInfoSearchModel model)
+        {
+            if (string.IsNullOrEmpty(model.MessageId)) return null;
+            return source.Messages.FirstOrDefault(x => x.MessageId.Equals(model.MessageId))?.GetViewModel;
+        }
+
+        public List<MessageInfoViewModel> GetFilteredList(MessageInfoSearchModel model)
+        {
+            if (!model.ClientId.HasValue)
+            {
+                return new();
+            }
+            return source.Messages.Where(x => x.ClientId == model.ClientId).Select(x => x.GetViewModel).ToList();
+        }
+
+        public List<MessageInfoViewModel> GetFullList()
+        {
+            return source.Messages.Select(x => x.GetViewModel).ToList();
+        }
+
+        public MessageInfoViewModel? Insert(MessageInfoBindingModel model)
+        {
+            var newMessage = MessageInfo.Create(model);
+            if(newMessage == null)
+            {
+                return null;
+            }
+            source.Messages.Add(newMessage);
+            source.SaveMessageInfo();
+            return newMessage.GetViewModel;
+        }
+    }
+}

SushiBarFileImplement/Models/MessageInfo.cs

@@ -0,0 +1,73 @@
+using SushiBarContracts.BindingModel;
+using SushiBarContracts.ViewModels;
+using SushiBarDataModels;
+using System.Xml.Linq;
+
+namespace SushiBarFileImplement.Models
+{
+    public class MessageInfo : IMessageInfoModel
+    {
+        public string MessageId { get; set; } = string.Empty;
+
+        public int? ClientId { get; set; }
+
+        public string SenderName { get; set; } = string.Empty;
+
+        public DateTime DateDelivery { get; set; } = DateTime.Now;
+
+        public string Subject { get; set; } = string.Empty;
+
+        public string Body { get; set; } = string.Empty;
+
+        public static MessageInfo? Create(MessageInfoBindingModel model)
+        {
+            if (model == null) return null;
+            return new MessageInfo
+            {
+                MessageId = model.MessageId,
+                ClientId = model.ClientId,
+                SenderName = model.SenderName,
+                DateDelivery = model.DateDelivery,
+                Subject = model.Subject,
+                Body = model.Body,
+            };
+        }
+
+        public static MessageInfo? Create(XElement element)
+        {
+            if (element == null)
+            {
+                return null;
+            }
+            return new MessageInfo()
+            {
+                MessageId = element.Attribute("MessageId")!.Value,
+                ClientId = Convert.ToInt32(element.Attribute("ClientId")!.Value),
+                SenderName = element.Element("SenderName")!.Value,
+                DateDelivery = Convert.ToDateTime(element.Element("DateDelivery")!.Value),
+                Subject = element.Attribute("Subject")!.Value,
+                Body = element.Attribute("Body")!.Value,
+            };
+        }
+
+        public MessageInfoViewModel GetViewModel => new()
+        {
+            MessageId = MessageId,
+            ClientId = ClientId,
+            SenderName = SenderName,
+            DateDelivery = DateDelivery,
+            Subject = Subject,
+            Body = Body,
+        };
+
+        public XElement GetXElement =>
+            new("MessageInfo",
+            new XAttribute("MessageId", MessageId),
+            new XElement("ClientId", ClientId),
+            new XElement("SenderName", SenderName),
+            new XElement("DateDelivery", DateDelivery),
+            new XElement("Subject", Subject),
+            new XElement("Body", Body)
+            );
+    }
+}