以下是一个示例，展示如何将用户身份链接到联系人的ASP.Net Core API代码：

首先，在Startup.cs文件中，配置身份验证和授权服务：

public void ConfigureServices(IServiceCollection services)
{
    // 添加身份验证服务
    services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
        .AddJwtBearer(options =>
        {
            options.TokenValidationParameters = new TokenValidationParameters
            {
                ValidateIssuer = true,
                ValidateAudience = true,
                ValidateLifetime = true,
                ValidateIssuerSigningKey = true,
                ValidIssuer = Configuration["Jwt:Issuer"],
                ValidAudience = Configuration["Jwt:Audience"],
                IssuerSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(Configuration["Jwt:Key"]))
            };
        });

    // 添加授权服务
    services.AddAuthorization();

    // 添加数据库上下文
    services.AddDbContext(options =>
        options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));

    // 添加身份相关服务
    services.AddIdentity()
        .AddEntityFrameworkStores()
        .AddDefaultTokenProviders();

    // 添加MVC服务
    services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_2);
}

接下来，创建一个Contact模型类，并在数据库上下文类中添加一个联系人的DbSet：

public class Contact
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Email { get; set; }
    public string UserId { get; set; }
    public ApplicationUser User { get; set; }
}

public class ContactContext : DbContext
{
    public ContactContext(DbContextOptions options) : base(options)
    {
    }

    public DbSet Contacts { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);

        modelBuilder.Entity()
            .HasOne(c => c.User)
            .WithMany(u => u.Contacts)
            .HasForeignKey(c => c.UserId)
            .IsRequired();
    }
}

然后，创建一个ContactsController，其中包含用于创建联系人、获取联系人列表和获取单个联系人的API：

[Route("api/[controller]")]
[ApiController]
[Authorize]
public class ContactsController : ControllerBase
{
    private readonly ContactContext _context;
    private readonly UserManager _userManager;

    public ContactsController(ContactContext context, UserManager userManager)
    {
        _context = context;
        _userManager = userManager;
    }

    // POST: api/Contacts
    [HttpPost]
    public async Task CreateContact([FromBody] Contact contact)
    {
        if (!ModelState.IsValid)
        {
            return BadRequest(ModelState);
        }

        // 获取当前用户的ID
        var userId = _userManager.GetUserId(User);

        // 关联联系人和用户
        contact.UserId = userId;

        _context.Contacts.Add(contact);
        await _context.SaveChangesAsync();

        return CreatedAtAction("GetContact", new { id = contact.Id }, contact);
    }

    // GET: api/Contacts
    [HttpGet]
    public async Task>> GetContacts()
    {
        // 获取当前用户的ID
        var userId = _userManager.GetUserId(User);

        // 获取当前用户的所有联系人
        var contacts = await _context.Contacts.Where(c => c.UserId == userId).ToListAsync();

        return contacts;
    }

    // GET: api/Contacts/5
    [HttpGet("{id}")]
    public async Task> GetContact(int id)
    {
        // 获取当前用户的ID
        var userId = _userManager.GetUserId(User);

        // 获取指定ID的联系人，并确保它属于当前用户
        var contact = await _context.Contacts.FirstOrDefaultAsync(c => c.Id == id && c.UserId == userId);

        if (contact == null)
        {
            return NotFound();
        }

        return contact;
    }
}

以上就是将用户身份链接到联系人的ASP.Net Core API的示例代码。