Initial commit.

- Created and set up main projects
- Created classes for Domain

PhonebookService.Domain/Models/PhonebookRecord.cs

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+namespace PhonebookService.Domain.Models;
+
+#pragma warning disable CS8618 // Non-nullable property must contain a non-null value when exiting constructor. Consider declaring the property as nullable.
+
+/// <summary>
+/// Entity model that represents a record in a phonebook
+/// </summary>
+public class PhonebookRecord
+{
+	public int Id { get; set; }
+
+	public string Email { get; set; }
+
+	public string PhoneNumber { get; set; }
+
+	public string FirstName { get; set; }
+
+	public string LastName { get; set; }
+
+	public string StreetAddress { get; set; }
+
+	public string City { get; set; }
+
+	public string ZipCode { get; set; }
+}

PhonebookService.Domain/PhonebookService.Domain.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+	<PropertyGroup>
+		<TargetFramework>net6.0</TargetFramework>
+		<ImplicitUsings>enable</ImplicitUsings>
+		<Nullable>enable</Nullable>
+	</PropertyGroup>
+
+	<ItemGroup>
+	  <PackageReference Include="FluentValidation" Version="11.5.1" />
+	</ItemGroup>
+
+</Project>

PhonebookService.Domain/Queries/PhonebookFilterQuery.cs

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+namespace PhonebookService.Domain.Queries;
+
+/// <summary>
+/// A model that represents a search/filter query for phonebook
+/// </summary>
+public class PhonebookFilterQuery
+{
+	/// <summary>
+	/// Number of a page to show. Starts from one.
+	/// </summary>
+	/// <value>Default is 1</value>
+	public int Page { get; set; } = 1;
+
+	/// <summary>
+	/// Optional property to filter records that contain the value in their <see cref="Models.PhonebookRecord.FirstName"/>
+	/// </summary>
+	public string? FirstName { get; set; }
+
+	/// <summary>
+	/// Optional property to filter records that match the city name
+	/// </summary>
+	public string? City { get; set; }
+
+	/// <summary>
+	/// Optional property to filter records that match the phone number
+	/// </summary>
+	public string? Phone { get; set; }
+
+	/// <summary>
+	/// Optional property to filter records that match the zip code
+	/// </summary>
+	public string? ZipCode { get; set; }
+
+	/// <summary>
+	/// Optional property to sort the result collection by <see cref="FirstName"/>
+	/// </summary>
+	/// <value>Default is 0 (None)</value>
+	public SortMode Sort { get; set; } = SortMode.None;
+}

PhonebookService.Domain/Queries/SortMode.cs

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+namespace PhonebookService.Domain.Queries;
+
+/// <summary>
+/// Sorting options for <see cref="PhonebookFilterQuery"/>
+/// </summary>
+public enum SortMode
+{
+	Ascending = 1,
+	None = 0,
+	Descending = -1
+}

PhonebookService.Domain/Repositories/IPhonebookRepository.cs

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+using PhonebookService.Domain.Models;
+using PhonebookService.Domain.Queries;
+
+namespace PhonebookService.Domain.Repositories;
+
+/// <summary>
+/// Interface for the phonebook repository
+/// </summary>
+public interface IPhonebookRepository
+{
+	/// <summary>
+	/// Get items list based on the provided query
+	/// </summary>
+	/// <param name="query">Parameters for filtering and pagination</param>
+	/// <returns>Collection of <see cref="Models.PhonebookRecord"/></returns>
+	Task<ICollection<PhonebookRecord>> GetItemsAsync(PhonebookFilterQuery query);
+
+	/// <summary>
+	/// Get one item by its ID
+	/// </summary>
+	/// <param name="id">Id of the item</param>
+	/// <returns>Matched <see cref="Models.PhonebookRecord"/> or null if none found</returns>
+	Task<PhonebookRecord?> GetItemByIdAsync(int id);
+
+	/// <summary>
+	/// Add new item to a database
+	/// </summary>
+	/// <param name="item"><see cref="Models.PhonebookRecord"/> entity</param>
+	/// <returns>The result <see cref="Models.PhonebookRecord"/> with assigned Id</returns>
+	Task<PhonebookRecord> CreateItemAsync(PhonebookRecord item);
+
+	/// <summary>
+	/// Updates provided item in a database
+	/// </summary>
+	/// <param name="item">item to update. Id is required</param>
+	/// <returns>The result <see cref="Models.PhonebookRecord"/> with assigned Id</returns>
+	Task<PhonebookRecord> UpdateItemAsync(PhonebookRecord item);
+
+	/// <summary>
+	/// Deletes provided item from a database
+	/// </summary>
+	/// <param name="item"><see cref="Models.PhonebookRecord"/> to delete</param>
+	Task DeleteItemAsync(PhonebookRecord item);
+}