如何在ASP.NET核心标识中获取/加载现有用户

services.AddIdentity<IdentityUser, IdentityRole>()
    .AddEntityFrameworkStores<ApplicationDbContext>()
    .AddDefaultTokenProviders();

app.UseAuthentication();

private readonly UserManager<IdentityUser> _userManager;
private readonly SignInManager<IdentityUser> _signInManager;

public MyController(UserManager<IdentityUser> userManager, SignInManager<IdentityUser> signInManager)
{
    _userManager = userManager;
    _signInManager = signInManager;
}

var user = await _userManager.GetUserAsync(User);

var user = await _userManager.FindByNameAsync(username);

var user = await _userManager.FindByIdAsync(userId);

var result = await _signInManager.PasswordSignInAsync(username, password, rememberMe, lockoutOnFailure);
if (result.Succeeded)
{
    // 登录成功
}

var result = await _signInManager.ExternalLoginSignInAsync(loginProvider, providerKey, isPersistent);
if (result.Succeeded)
{
    // 登录成功
}