看起来这段代码可以工作。它生成一个GraphServiceClient，该GraphServiceClient在每次调用时重用相同的令牌，而不是生成新的令牌。

 public GraphServiceClient GenerateGraphUserClient()
    {
        string userToken = GetUserAccessToken();
        GraphServiceClient client= new GraphServiceClient("https://graph.microsoft.com/v1.0", new DelegateAuthenticationProvider(async (requestMessage) =>
        {
            requestMessage.Headers.Authorization = new AuthenticationHeaderValue("bearer", userToken);
        }));
        return client;
    }

public string GetUserAccessToken()
    {
        string[] scopes = new string[] { "https://graph.microsoft.com/.default" };
        IPublicClientApplication publicClientApplication = PublicClientApplicationBuilder
        .Create(_clientId)
        .WithTenantId(_domain)
        .Build();
        var securePassword = new SecureString();
        foreach (char c in _password)
            securePassword.AppendChar(c);
        var result = publicClientApplication.AcquireTokenByUsernamePassword(scopes, _userName, securePassword).ExecuteAsync().Result;
        return result.AccessToken;
    }