Add project files.

MyOffice.Services/MyOffice.Services.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net6.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\MyOffice.Core\MyOffice.Core.csproj" />
+    <ProjectReference Include="..\MyOffice.Data.Repositories\MyOffice.Data.Repositories.csproj" />
+  </ItemGroup>
+
+</Project>

MyOffice.Services/Users/Domain/AddUserExternalStatusEnum.cs

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+namespace MyOffice.Services.Users.Domain;
+
+public enum AddUserExternalStatusEnum
+{
+	success,
+	externalid_used,
+	user_not_valid
+}

MyOffice.Services/Users/UserService.cs

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+namespace MyOffice.Services.Users;
+
+using Domain;
+using Core;
+using Data.Models.Users;
+using Data.Repositories.Users;
+
+public class UserService
+{
+	private readonly IUserRepository _userRepository;
+	private readonly IUserExternalRepository _userExternalRepository;
+
+	public UserService(
+		IUserRepository userRepository,
+		IUserExternalRepository userExternalRepository
+	)
+	{
+		_userRepository = userRepository;
+		_userExternalRepository = userExternalRepository;
+	}
+
+	public async Task<User?> Get(Guid id)
+	{
+		return await _userRepository.GetUserAsync(id);
+	}
+
+	public async Task<List<UserExternal>> GetUserExternals(Guid userId)
+	{
+		return await _userExternalRepository.GetUserExternalsByUserIdAsync(userId);
+	}
+
+	public async Task<UserExternal?> GetUserExternal(Guid userId, string provider)
+	{
+		return await _userExternalRepository.GetByUserIdAsync(userId, provider);
+	}
+
+	public async Task<Exec<UserExternal>> RemoveUserExternal(Guid userId, string provider)
+	{
+		var result = new Exec<UserExternal>();
+
+		var userExternal = await _userExternalRepository.GetByUserIdAsync(userId, provider);
+		if (userExternal == null) return result.Set(GeneralExecStatus.not_found);
+
+		_userExternalRepository.RemoveUserExternal(userExternal);
+
+		return result.Set(userExternal);
+	}
+
+	private readonly object _lock = new();
+
+	public Exec<UserExternal, AddUserExternalStatusEnum> AddUserExternal(
+		Guid userId,
+		string provider,
+		string externalId,
+		string email
+	)
+	{
+		var result = new Exec<UserExternal, AddUserExternalStatusEnum>(AddUserExternalStatusEnum.success);
+
+		var externalLogin = _userExternalRepository.GetByUserId(userId, provider);
+
+		if (externalLogin != null) return result.Set(externalLogin);
+
+		lock (_lock)
+		{
+			externalLogin = _userExternalRepository.GetByUserId(userId, provider);
+			// externalLogin exists
+			if (externalLogin != null) return result.Set(externalLogin);
+
+			externalLogin = _userExternalRepository.GetByExternalId(externalId, provider);
+			// externalId connected to another user
+			if (externalLogin != null) return result.Set(AddUserExternalStatusEnum.externalid_used);
+
+			var user = _userRepository.GetByUserUserName(email);
+			// there are another user with that email
+			if (user?.Id != userId) return result.Set(AddUserExternalStatusEnum.user_not_valid);
+
+			externalLogin = new UserExternal
+			{
+				UserId = userId,
+				CreatedOn = DateTime.UtcNow,
+				Provider = provider.ToLower(),
+				ExternalId = externalId,
+				Email = email
+			};
+			_userExternalRepository.AddUserExternal(externalLogin);
+		}
+
+		return result;
+	}
+}