Implementing a system that restricts users from logging into multiple devices simultaneously and logs them out from the last logged-in machine upon a new login attempt requires a combination of backend logic, session management, and user interface updates. Here's a high-level overview of how you could achieve this using C# and .NET Core:

1. Database Setup: Maintain a database table to store active user sessions. Each session should be associated with a unique token or session ID.

2. User Authentication: Implement user authentication using ASP.NET Core Identity or a custom authentication system. Upon successful login, generate a unique session token for the user.

3. Session Management:

   • When a user logs in, store the session token in the database along with the user ID and device information (e.g., IP address, user agent).
   • For each request, check if the session token is valid and associated with the user's account. This can be done using middleware or filters.

4. Multiple Device Handling:

   • When a user logs in from a new device, before creating a new session, check if there are any existing active sessions for the user.
   • If there are active sessions, send a notification (e.g., email or in-app notification) to the user informing them of the new login attempt.
   • Prompt the user to confirm the new login or deny it. If confirmed, proceed with the login and update the existing sessions.

5. Logging Out Previous Session:

   • If the user confirms the new login, invalidate the session tokens associated with the previous sessions.
   • This can be achieved by updating the session records in the database to mark them as inactive or by simply deleting the previous sessions.

6. Implementing the UI:

   • On the frontend, when the user logs in from a new device, display a notification indicating the new login attempt.
   • Provide options to confirm or deny the new login.
   • If the user confirms, send a request to the server to proceed with the login, and the server will handle session management as described above.

7. Session Expiry and Cleanup:

   • Implement session expiry logic to automatically log out users after a certain period of inactivity.
   • Periodically clean up inactive sessions from the database to prevent it from growing too large.

8. Security Considerations:

   • Implement secure token generation and storage to prevent session hijacking.
   • Use HTTPS to secure data transmission.
   • Employ security best practices to protect user data and prevent unauthorized access.

Remember that this is a high-level overview, and the implementation details will depend on your specific application structure and requirements. You might also need to handle scenarios like user session timeouts, password changes, and more. Always follow security best practices and consider using established libraries for tasks like user authentication and session management.

Simplified example of how you could implement the logic described using C# and ASP.NET Core. Keep in mind that this example is for illustrative purposes and may need to be adapted to your specific application's architecture and requirements.

Let's start with the database table structure. You would typically have a table to store user sessions:

Session Table:

Here's a simplified implementation of the logic in C# ASP.NET Core:

1. Models:

csharpCopy code
// Session model
public class Session
{
    public int SessionId { get; set; }
    public int UserId { get; set; }
    public string Token { get; set; }
    public string DeviceInfo { get; set; }
    public bool IsActive { get; set; }
}

// User model (if not using ASP.NET Identity)
public class User
{
    public int UserId { get; set; }
    public string Username { get; set; }
    // Other user properties
}

2. Database Context:

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using Microsoft.EntityFrameworkCore;

public class AppDbContext : DbContext
{
    public DbSet Sessions { get; set; }
    // Other DbSets

    public AppDbContext(DbContextOptions options)
        : base(options)
    {
    }
}

3. Session Management Service:

csharpCopy code
public class SessionService
{
    private readonly AppDbContext _dbContext;

    public SessionService(AppDbContext dbContext)
    {
        _dbContext = dbContext;
    }

    public void CreateSession(int userId, string token, string deviceInfo)
    {
        var newSession = new Session
        {
            UserId = userId,
            Token = token,
            DeviceInfo = deviceInfo,
            IsActive = true
        };

        _dbContext.Sessions.Add(newSession);
        _dbContext.SaveChanges();
    }

    public void InvalidateSessions(int userId)
    {
        var sessionsToInvalidate = _dbContext.Sessions
            .Where(s => s.UserId == userId && s.IsActive)
            .ToList();

        foreach (var session in sessionsToInvalidate)
        {
            session.IsActive = false;
        }

        _dbContext.SaveChanges();
    }
}

4. Controller:

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using Microsoft.AspNetCore.Mvc;

[ApiController]
[Route("api/[controller]")]
public class AuthController : ControllerBase
{
    private readonly SessionService _sessionService;

    public AuthController(SessionService sessionService)
    {
        _sessionService = sessionService;
    }

    [HttpPost("login")]
    public IActionResult Login(LoginModel model)
    {
        // Perform authentication
        // ...

        // Assume authentication was successful
        var userId = authenticatedUser.UserId;
        var token = GenerateUniqueToken();
        var deviceInfo = GetDeviceInfoFromRequest();

        // Invalidate previous sessions and create a new session
        _sessionService.InvalidateSessions(userId);
        _sessionService.CreateSession(userId, token, deviceInfo);

        return Ok(new { Token = token });
    }

    // Other actions
}

Please note that this example simplifies many aspects for clarity. In a real-world application, you would need to handle token generation, device information retrieval, authentication, user confirmation, and many other details. Additionally, consider using libraries like ASP.NET Identity for user management and authentication.

Lastly, security should be a top priority when implementing such features. Always follow best practices for authentication, session management, and data protection.