以下是解决ASP.NET Core MVC中Discord Oauth2请求中Oauth状态丢失或无效的示例代码：

1. 确保在Startup.cs文件中正确配置Discord Oauth2：

public void ConfigureServices(IServiceCollection services)
{
    // 添加Discord认证服务
    services.AddAuthentication(options =>
    {
        options.DefaultScheme = CookieAuthenticationDefaults.AuthenticationScheme;
        options.DefaultChallengeScheme = "Discord";
    })
    .AddCookie()
    .AddOAuth("Discord", options =>
    {
        options.ClientId = "YOUR_DISCORD_CLIENT_ID";
        options.ClientSecret = "YOUR_DISCORD_CLIENT_SECRET";
        options.CallbackPath = new PathString("/signin-discord");
        options.AuthorizationEndpoint = "https://discord.com/api/oauth2/authorize";
        options.TokenEndpoint = "https://discord.com/api/oauth2/token";
        options.UserInformationEndpoint = "https://discord.com/api/users/@me";
        options.ClaimActions.MapJsonKey(ClaimTypes.NameIdentifier, "id");
        options.ClaimActions.MapJsonKey(ClaimTypes.Name, "username");
        options.ClaimActions.MapJsonKey("urn:discord:avatar", "avatar");
        options.SaveTokens = true;
    });

    // 添加MVC服务
    services.AddMvc();
}

2. 在Controller中使用Discord Oauth2进行身份验证：

public class DiscordController : Controller
{
    [HttpGet]
    public IActionResult Login()
    {
        // 重定向到Discord Oauth2认证
        return Challenge(new AuthenticationProperties { RedirectUri = "/" }, "Discord");
    }

    [HttpGet]
    public async Task Callback()
    {
        // 获取Discord用户的身份信息
        var authenticateResult = await HttpContext.AuthenticateAsync("Discord");
        var userId = authenticateResult.Principal.FindFirst(ClaimTypes.NameIdentifier)?.Value;
        var username = authenticateResult.Principal.FindFirst(ClaimTypes.Name)?.Value;
        var avatarUrl = authenticateResult.Principal.FindFirst("urn:discord:avatar")?.Value;

        // 处理身份信息，例如保存到数据库或创建用户会话
        // ...

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

3. 添加Discord登录按钮到视图页面：

@using Microsoft.AspNetCore.Authentication

@if (User.Identity.IsAuthenticated)
{
    
Welcome, @User.Identity.Name!
}
else
{
    
        
    
}

通过以上代码示例，您可以在ASP.NET Core MVC应用程序中使用Discord Oauth2进行身份验证，并解决Oauth状态丢失或无效的问题。