我已经找到了解决这个问题的方法。

基本上，我添加了具有管理员用户名的声明，如果该声明存在，我知道正在模拟。当管理员想要停止模拟时，系统会为声明检索原始用户名，删除旧的模拟Cookie并为管理员创建新的Cookie：

[AuthenticateAdmin] // <- make sure this endpoint is only available to admins
public async Task ImpersonateUserAsync(string userName)
{
    var context = HttpContext.Current;

    var originalUsername = context.User.Identity.Name;

    var impersonatedUser = await userManager.FindByNameAsync(userName);

    var impersonatedIdentity = await userManager.CreateIdentityAsync(impersonatedUser, DefaultAuthenticationTypes.ApplicationCookie);
    impersonatedIdentity.AddClaim(new Claim("UserImpersonation", "true"));
    impersonatedIdentity.AddClaim(new Claim("OriginalUsername", originalUsername));

    var authenticationManager = context.GetOwinContext().Authentication;
    authenticationManager.SignOut(DefaultAuthenticationTypes.ApplicationCookie);
    authenticationManager.SignIn(new AuthenticationProperties() { IsPersistent = false }, impersonatedIdentity);
}

有关更多信息，请参见我的博客文章：使用ASP.Net Identity 2模拟用户。

Asp.Net Core中的用户模拟

2017年7月更新：该主题非常受欢迎，因此我研究了Core中的用户模拟，并且更新的API的原理非常相似。冒充的方法如下：

    [Authorize(Roles = "Admin")] // <-- Make sure only admins can access this 
    public async Task<IActionResult> ImpersonateUser(String userId)
    {
        var currentUserId = User.GetUserId();

        var impersonatedUser = await _userManager.FindByIdAsync(userId);

        var userPrincipal = await _signInManager.CreateUserPrincipalAsync(impersonatedUser);

        userPrincipal.Identities.First().AddClaim(new Claim("OriginalUserId", currentUserId));
        userPrincipal.Identities.First().AddClaim(new Claim("IsImpersonating", "true"));

        // sign out the current user
        await _signInManager.SignOutAsync();

        // If you use asp.net core 1.0
        await HttpContext.Authentication.SignInAsync(cookieOptions.ApplicationCookieAuthenticationScheme, userPrincipal);
        // If you use asp.net core 2.0 (the line above is deprecated)
        await HttpContext.SignInAsync(cookieOptions.ApplicationCookieAuthenticationScheme, userPrincipal);

        return RedirectToAction("Index", "Home");
    }

这是停止模拟的方法：

    [Authorize(Roles = "Admin")] // <-- Make sure only admins can access this 
    public async Task<IActionResult> StopImpersonation()
    {
        if (!User.IsImpersonating())
        {
            throw new Exception("You are not impersonating now. Can't stop impersonation");
        }

        var originalUserId = User.FindFirst("OriginalUserId").Value;

        var originalUser = await _userManager.FindByIdAsync(originalUserId);

        await _signInManager.SignOutAsync();

        await _signInManager.SignInAsync(originalUser, isPersistent: true);

        return RedirectToAction("Index", "Home");
    }

我的博客中有完整说明：http : //tech.trailmax.info/2017/07/user-impersonation-in-asp-net-core/ GitHub上的完整代码示例：https : //github.com/trailmax/AspNetCoreImpersonation