Сделал все до DataModels

PapaCarloProject/PapaCarloContracts/DataModels/BuyerDataModel.cs

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+using PapaCarloContracts.Extensions;
+using PapaCarloContracts.infrastructure;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Numerics;
+using System.Text;
+using System.Text.RegularExpressions;
+using System.Threading.Tasks;
+using static System.Runtime.InteropServices.JavaScript.JSType;
+using System.Xml;
+using PapaCarloContracts.Exceptions;
+
+namespace PapaCarloContracts.DataModels;
+
+public class BuyerDataModel(string id, string fio, string phoneNumber, double
+discountSize) : IValidation
+{
+    public string Id { get; private set; } = id;
+    public string FIO { get; private set; } = fio;
+    public string PhoneNumber { get; private set; } = phoneNumber;
+    public double DiscountSize { get; private set; } = discountSize;
+    public void Validate()
+    {
+        if (Id.IsEmpty())
+            throw new ValidationException("Field Id is empty");
+        if (!Id.IsGuid())
+            throw new ValidationException("The value in the field Id is not a unique identifier");
+        if (FIO.IsEmpty())
+            throw new ValidationException("Field FIO is empty");
+        if (PhoneNumber.IsEmpty())
+            throw new ValidationException("Field PhoneNumber is empty");
+        if (!Regex.IsMatch(PhoneNumber, @"^((8|\+7)[\- ]?)?(\(?\d{3}\)?[\-]?)?[\d\- ]{7,10}$"))
+            throw new ValidationException("Field PhoneNumber is not a phone number");
+    }
+}

PapaCarloProject/PapaCarloContracts/Exceptions/IValidationException.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace PapaCarloContracts.Exceptions;
+
+public class ValidationException(string message) : Exception(message)
+{
+}

PapaCarloProject/PapaCarloContracts/Extensions/StringExtensions.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace PapaCarloContracts.Extensions;
+
+public static class StringExtensions
+{
+    public static bool IsEmpty(this string str)
+    {
+        return string.IsNullOrWhiteSpace(str);
+    }
+    public static bool IsGuid(this string str)
+    {
+        return Guid.TryParse(str, out _);
+    }
+}

PapaCarloProject/PapaCarloContracts/infrastructure/IValidation.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Net.Http.Headers;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace PapaCarloContracts.infrastructure;
+
+public interface IValidation
+{
+    void Validate();
+}