能用C#写一段实现单点登录的代码吗

当然可以，以下是一个简单的示例代码，使用 ASP.NET Identity 实现单点登录：

1. 首先，需要在 Visual Studio 中创建一个 ASP.NET Web 应用程序，并将身份验证类型设置为“个人帐户”。

2. 在 Startup.cs 文件中，添加以下代码：

using Microsoft.AspNetCore.Authentication.Cookies;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Identity;
using Microsoft.Extensions.DependencyInjection;

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddDbContext<ApplicationDbContext>();
        services.AddIdentity<ApplicationUser, IdentityRole>()
            .AddEntityFrameworkStores<ApplicationDbContext>()
            .AddDefaultTokenProviders();

        services.ConfigureApplicationCookie(options =>
        {
            options.Cookie.Name = "MyAppCookie";
            options.LoginPath = "/Account/Login";
            options.LogoutPath = "/Account/Logout";
            options.AccessDeniedPath = "/Account/AccessDenied";
            options.SlidingExpiration = true;
        });
    }

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        app.UseAuthentication();
        app.UseAuthorization();
    }
}

3. 在 AccountController.cs 文件中，添加以下代码：

using Microsoft.AspNetCore.Authentication;
using Microsoft.AspNetCore.Authentication.Cookies;
using Microsoft.AspNetCore.Mvc;
using System.Collections.Generic;
using System.Security.Claims;
using System.Threading.Tasks;

public class AccountController : Controller
{
    private readonly UserManager<ApplicationUser> _userManager;
    private readonly SignInManager<ApplicationUser> _signInManager;

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

    [HttpGet]
    public IActionResult Login(string returnUrl = null)
    {
        ViewData["ReturnUrl"] = returnUrl;
        return View();
    }

    [HttpPost]
    public async Task<IActionResult> Login(LoginViewModel model, string returnUrl = null)
    {
        ViewData["ReturnUrl"] = returnUrl;
        if (ModelState.IsValid)
        {
            var result = await _signInManager.PasswordSignInAsync(model.Email, model.Password, model.RememberMe, lockoutOnFailure: false);
            if (result.Succeeded)
            {
                var user = await _userManager.FindByEmailAsync(model.Email);
                var claims = new List<Claim>
                {
                    new Claim(ClaimTypes.NameIdentifier, user.Id),
                    new Claim(ClaimTypes.Name, user.UserName)
                };
                var claimsIdentity = new ClaimsIdentity(claims, CookieAuthenticationDefaults.AuthenticationScheme);
                var authProperties = new AuthenticationProperties
                {
                    IsPersistent = model.RememberMe,
                    RedirectUri = returnUrl
                };
                await HttpContext.SignInAsync(CookieAuthenticationDefaults.AuthenticationScheme, new ClaimsPrincipal(claimsIdentity), authProperties);
                return RedirectToLocal(returnUrl);
            }
            ModelState.AddModelError(string.Empty, "Invalid login attempt.");
        }
        return View(model);
    }

    [HttpGet]
    public async Task<IActionResult> Logout()
    {
        await HttpContext.SignOutAsync(CookieAuthenticationDefaults.AuthenticationScheme);
        return RedirectToAction(nameof(HomeController.Index), "Home");
    }

    private IActionResult RedirectToLocal(string returnUrl)
    {
        if (Url.IsLocalUrl(returnUrl))
        {
            return Redirect(returnUrl);
        }
        else
        {
            return RedirectToAction(nameof(HomeController.Index), "Home");
        }
    }
}

4. 在 Login.cshtml 文件中，添加以下代码：

@model LoginViewModel
@{
    ViewData["Title"] = "Login";
}

<h2>@ViewData["Title"]</h2>

<div class="row">
    <div class="col-md-4">
        <form asp-controller="Account" asp-action="Login" asp-route-returnUrl="@ViewData["ReturnUrl"]" method="post" class="form-horizontal">
            <h4>Use a local account to log in.</h4>
            <hr />
            <div asp-validation-summary="All" class="text-danger"></div>
            <div class="form-group">
                <label asp-for="Email" class="col-md-2 control-label"></label>
                <div class="col-md-10">
                    <input asp-for="Email" class="form-control" />
                    <span asp-validation-for="Email" class="text-danger"></span>
                </div>
            </div>
            <div class="form-group">
                <label asp-for="Password" class="col-md-2 control-label"></label>
                <div class="col-md-10">
                    <input asp-for="Password" class="form-control" />
                    <span asp-validation-for="Password" class="text-danger"></span>
                </div>
            </div>
            <div class="form-group">
                <div class="col-md-offset-2 col-md-10">
                    <div class="checkbox">
                        <label>
                            <input asp-for="RememberMe" /> @Html.DisplayNameFor(m => m.RememberMe)
                        </label>
                    </div>
                </div>
            </div>
            <div class="form-group">
                <div class="col-md-offset-2 col-md-10">
                    <button type="submit" class="btn btn-default">Log in</button>
                </div>
            </div>
            <div class="form-group">
                <div class="col-md-offset-2 col-md-10">
                    <p>
                        <a asp-controller="Account" asp-action="ForgotPassword">Forgot your password?</a>
                    </p>
                    <p>
                        <a asp-controller="Account" asp-action="Register">Register as a new user</a>
                    </p>
                </div>
            </div>
        </form>
    </div>
</div>

这段代码实现了基本的单点登录功能，具体实现可能会根据你的实际需求而有所不同。