Добавьте файлы проекта.

PetProjectDog.sln

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+
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio Version 17
+VisualStudioVersion = 17.14.36202.13 d17.14
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "PetProjectDog", "PetProjectDog\PetProjectDog.csproj", "{5528D01A-B792-4076-B8D0-D8F8D812FCAC}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{5528D01A-B792-4076-B8D0-D8F8D812FCAC}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{5528D01A-B792-4076-B8D0-D8F8D812FCAC}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{5528D01A-B792-4076-B8D0-D8F8D812FCAC}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{5528D01A-B792-4076-B8D0-D8F8D812FCAC}.Release|Any CPU.Build.0 = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+	GlobalSection(ExtensibilityGlobals) = postSolution
+		SolutionGuid = {B520792F-6A33-4531-BC98-82FCA29ED32E}
+	EndGlobalSection
+EndGlobal

PetProjectDog/AddNoteForm.cs

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+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using System.Data;
+using System.Drawing;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using System.Windows.Forms;
+
+namespace PetProjectDog
+{
+    public partial class AddNoteForm : Form
+    {
+
+        public string NoteText { get; private set; }
+
+        public AddNoteForm()
+        {
+            InitializeComponent();
+        }
+
+        private void AddNoteForm_Load(object sender, EventArgs e)
+        {
+
+        }
+
+        private void textBox1_TextChanged(object sender, EventArgs e)
+        {
+
+        }
+
+        private void btnSave_Click(object sender, EventArgs e)
+        {
+            if(!string.IsNullOrWhiteSpace(txtNote.Text))
+            {
+                NoteText = txtNote.Text.Trim();
+                DialogResult = DialogResult.OK;
+                Close();
+            }
+            else
+            {
+                MessageBox.Show("Введите текст примечания.", "Внимание", MessageBoxButtons.OK, MessageBoxIcon.Warning);
+            }
+        }
+    }
+}

PetProjectDog/AddNoteForm.resx

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+<?xml version="1.0" encoding="utf-8"?>
+<root>
+  <!--
+    Microsoft ResX Schema
+
+    Version 2.0
+
+    The primary goals of this format is to allow a simple XML format
+    that is mostly human readable. The generation and parsing of the
+    various data types are done through the TypeConverter classes
+    associated with the data types.
+
+    Example:
+
+    ... ado.net/XML headers & schema ...
+    <resheader name="resmimetype">text/microsoft-resx</resheader>
+    <resheader name="version">2.0</resheader>
+    <resheader name="reader">System.Resources.ResXResourceReader, System.Windows.Forms, ...</resheader>
+    <resheader name="writer">System.Resources.ResXResourceWriter, System.Windows.Forms, ...</resheader>
+    <data name="Name1"><value>this is my long string</value><comment>this is a comment</comment></data>
+    <data name="Color1" type="System.Drawing.Color, System.Drawing">Blue</data>
+    <data name="Bitmap1" mimetype="application/x-microsoft.net.object.binary.base64">
+        <value>[base64 mime encoded serialized .NET Framework object]</value>
+    </data>
+    <data name="Icon1" type="System.Drawing.Icon, System.Drawing" mimetype="application/x-microsoft.net.object.bytearray.base64">
+        <value>[base64 mime encoded string representing a byte array form of the .NET Framework object]</value>
+        <comment>This is a comment</comment>
+    </data>
+
+    There are any number of "resheader" rows that contain simple
+    name/value pairs.
+
+    Each data row contains a name, and value. The row also contains a
+    type or mimetype. Type corresponds to a .NET class that support
+    text/value conversion through the TypeConverter architecture.
+    Classes that don't support this are serialized and stored with the
+    mimetype set.
+
+    The mimetype is used for serialized objects, and tells the
+    ResXResourceReader how to depersist the object. This is currently not
+    extensible. For a given mimetype the value must be set accordingly:
+
+    Note - application/x-microsoft.net.object.binary.base64 is the format
+    that the ResXResourceWriter will generate, however the reader can
+    read any of the formats listed below.
+
+    mimetype: application/x-microsoft.net.object.binary.base64
+    value   : The object must be serialized with
+            : System.Runtime.Serialization.Formatters.Binary.BinaryFormatter
+            : and then encoded with base64 encoding.
+
+    mimetype: application/x-microsoft.net.object.soap.base64
+    value   : The object must be serialized with
+            : System.Runtime.Serialization.Formatters.Soap.SoapFormatter
+            : and then encoded with base64 encoding.
+
+    mimetype: application/x-microsoft.net.object.bytearray.base64
+    value   : The object must be serialized into a byte array
+            : using a System.ComponentModel.TypeConverter
+            : and then encoded with base64 encoding.
+    -->
+  <xsd:schema id="root" xmlns="" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:msdata="urn:schemas-microsoft-com:xml-msdata">
+    <xsd:import namespace="http://www.w3.org/XML/1998/namespace" />
+    <xsd:element name="root" msdata:IsDataSet="true">
+      <xsd:complexType>
+        <xsd:choice maxOccurs="unbounded">
+          <xsd:element name="metadata">
+            <xsd:complexType>
+              <xsd:sequence>
+                <xsd:element name="value" type="xsd:string" minOccurs="0" />
+              </xsd:sequence>
+              <xsd:attribute name="name" use="required" type="xsd:string" />
+              <xsd:attribute name="type" type="xsd:string" />
+              <xsd:attribute name="mimetype" type="xsd:string" />
+              <xsd:attribute ref="xml:space" />
+            </xsd:complexType>
+          </xsd:element>
+          <xsd:element name="assembly">
+            <xsd:complexType>
+              <xsd:attribute name="alias" type="xsd:string" />
+              <xsd:attribute name="name" type="xsd:string" />
+            </xsd:complexType>
+          </xsd:element>
+          <xsd:element name="data">
+            <xsd:complexType>
+              <xsd:sequence>
+                <xsd:element name="value" type="xsd:string" minOccurs="0" msdata:Ordinal="1" />
+                <xsd:element name="comment" type="xsd:string" minOccurs="0" msdata:Ordinal="2" />
+              </xsd:sequence>
+              <xsd:attribute name="name" type="xsd:string" use="required" msdata:Ordinal="1" />
+              <xsd:attribute name="type" type="xsd:string" msdata:Ordinal="3" />
+              <xsd:attribute name="mimetype" type="xsd:string" msdata:Ordinal="4" />
+              <xsd:attribute ref="xml:space" />
+            </xsd:complexType>
+          </xsd:element>
+          <xsd:element name="resheader">
+            <xsd:complexType>
+              <xsd:sequence>
+                <xsd:element name="value" type="xsd:string" minOccurs="0" msdata:Ordinal="1" />
+              </xsd:sequence>
+              <xsd:attribute name="name" type="xsd:string" use="required" />
+            </xsd:complexType>
+          </xsd:element>
+        </xsd:choice>
+      </xsd:complexType>
+    </xsd:element>
+  </xsd:schema>
+  <resheader name="resmimetype">
+    <value>text/microsoft-resx</value>
+  </resheader>
+  <resheader name="version">
+    <value>2.0</value>
+  </resheader>
+  <resheader name="reader">
+    <value>System.Resources.ResXResourceReader, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
+  </resheader>
+  <resheader name="writer">
+    <value>System.Resources.ResXResourceWriter, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
+  </resheader>
+</root>

PetProjectDog/DataStorage.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Text.Json;
+using System.Threading.Tasks;
+
+namespace PetProjectDog
+{
+    public static class DataStorage
+    {
+        public static List<Dog> Dogs { get; private set; } = new List<Dog>();
+        public static List<Owner> Owners { get; private set; } = new List<Owner>();
+        private static string FilePath => "dogs.json";
+        public static void SaveDogsToFile()
+        {
+                string path = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "dogs.json");
+
+                var options = new JsonSerializerOptions { WriteIndented = true };
+                string json = JsonSerializer.Serialize(Dogs, options);
+
+                File.WriteAllText(path, json);
+
+        }
+
+        public static void LoadDogsFromFile()
+        {
+            if (!File.Exists(FilePath)) return;
+
+            string json = File.ReadAllText(FilePath);
+            try
+            {
+                Dogs = JsonSerializer.Deserialize<List<Dog>>(json) ?? new List<Dog>();
+            }
+            catch
+            {
+                Dogs = new List<Dog>();
+            }
+        }
+
+
+
+
+
+
+    }
+}

PetProjectDog/Dog.cs

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+using System;
+using System.Collections.Generic;
+using System.Diagnostics.Eventing.Reader;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace PetProjectDog
+{
+    public class Dog
+    {
+        public string Name { get; set; } // имя пса
+        public string Age { get; set; } // возраст
+        public double Weight { get; set; } // вес
+
+        public string Colour { get; set; } // Цвет
+
+        public bool IsFriendly { get; set; } // дружелюбна?
+
+        public string Breed { get; set; } // порода
+
+        public string SpecialFeatures { get; set; } // особые приметы
+
+        public List<string> Notes { get; set; } = new List<string>(); // список примечаний
+
+        public bool IsVaccinated { get; set; }
+        public DateTime LastVaccination { get; set; } // дата последней вакцины
+
+        public DateTime NextVaccination { get; set; } // дата следуюущей вакцины
+
+        public DateTime LastWalk { get; set; } // 
+        public DateTime NextWalk { get; set; } // 
+        public DateTime DateLastUpdate { get; set; } // Дата последнего обновления карточки
+
+        public string PhotoPath { get; set; }  // Путь к фотографии
+
+        public Owner Owner { get; set; } = new Owner(); // поле для владельца
+
+        public Dog(string name, string age, double weight, string color, string breed, bool isFriendly, bool isVaccinated)
+        {
+            Name = name;
+            Age = age;
+            Weight = weight;
+            Colour = color;
+            Breed = breed;
+            IsFriendly = isFriendly;
+            IsVaccinated = isVaccinated;
+        }
+        public Dog()
+        {
+        }
+
+
+        public void ShowMainInfo()
+        {
+            Console.WriteLine($"Имя: {Name}\nВозраст: {Age}\nПорода: {Breed}\n" +
+                $"Дружелюбна: {IsFriendly}\nВакцинирована: {IsVaccinated}");
+        }
+        public override string ToString()
+        {
+            return $"Имя питомца: {Name}, Порода: {Breed},Возраст: {Age} лет, Дата регистрации: {DateTime.Now.ToString("yyyy-MM-dd")}";
+        }
+
+        public void AddNote(string note)
+        {
+            Notes.Add(note);
+        }
+        public void MarkAsWalked()
+        {
+            LastWalk = DateTime.Now;
+        }
+
+
+
+
+
+    }
+}