How to log in only once using Graph API?

Question

I’m developing a small Windows form app to test Graph API functions. I have two functionalities in the application, user's log in and create a team. I created a class that contains a function for creating a GraphServiceClient. I call this function both when the users logs in and when I create a team. The problem is that the user login window is displayed both times, and I want that once the user logs in, to somehow save that GraphServiceClient instance so that the user does not have to double logs in. Here is the code:

 

public static class GraphManager
{
    public static GraphServiceClient graphClient;

    private static string[] scopes = new string[] { "user.read" };
    public static string TokenForUser = null;
    public static DateTimeOffset expiration;

    private const string ClientId = "599ed98d-4356-4a96-ad37-04391e9c48dc";

    private const string Tenant = "common"; // Alternatively "[Enter your tenant, as obtained from the Azure portal, e.g. kko365.onmicrosoft.com]"
    private const string Authority = "https://login.microsoftonline.com/" + Tenant;

    // The MSAL Public client app
    private static IPublicClientApplication PublicClientApp;

    private static string MSGraphURL = "https://graph.microsoft.com/beta/";
    private static AuthenticationResult authResult;

    public static GraphServiceClient GetGraphClient()
    {
        if(graphClient == null)
        {
            // Create Microsoft Graph client.
            try
            {
                graphClient = new GraphServiceClient(
                    "https://graph.microsoft.com/v1.0",
                    new DelegateAuthenticationProvider(
                        async (requestMessage) =>
                        {
                            var token = await GetTokenForUserAsync();
                            requestMessage.Headers.Authorization = new AuthenticationHeaderValue("bearer", token);
                            // This header has been added to identify our sample in the Microsoft Graph service.  If extracting this code for your project please remove.
                            requestMessage.Headers.Add("SampleID", "uwp-csharp-snippets-sample");

                        }));
                return graphClient;
            }

            catch (Exception ex)
            {
                Debug.WriteLine("Could not create a graph client: " + ex.Message);
            }
        }
        return graphClient;
    }

    public static async Task<string> GetTokenForUserAsync()
    {
        if (TokenForUser == null || expiration <= DateTimeOffset.UtcNow.AddMinutes(5))
        {
            PublicClientApp = PublicClientApplicationBuilder.Create(ClientId)
          .WithAuthority(Authority)
          .WithRedirectUri("https://login.microsoftonline.com/common/oauth2/nativeclient")
           .WithLogging((level, message, containsPii) =>
           {
               Debug.WriteLine($"MSAL: {level} {message} ");
           }, LogLevel.Warning, enablePiiLogging: false, enableDefaultPlatformLogging: true)
          .Build();

            // It's good practice to not do work on the UI thread, so use ConfigureAwait(false) whenever possible.
            IEnumerable<IAccount> accounts = await PublicClientApp.GetAccountsAsync().ConfigureAwait(false);
            IAccount firstAccount = accounts.FirstOrDefault();

            try
            {
                authResult = await PublicClientApp.AcquireTokenSilent(scopes, firstAccount)
                                                  .ExecuteAsync();
            }
            catch (MsalUiRequiredException ex)
            {
                // A MsalUiRequiredException happened on AcquireTokenSilentAsync. This indicates you need to call AcquireTokenAsync to acquire a token
                Debug.WriteLine($"MsalUiRequiredException: {ex.Message}");

                authResult = await PublicClientApp.AcquireTokenInteractive(scopes)
                                                  .ExecuteAsync()
                                                  .ConfigureAwait(false);
            }

            TokenForUser = authResult.AccessToken;
        }

        return TokenForUser;
    }

}

private async void button1_Click(object sender, EventArgs e)
    {

        GraphServiceClient graphClient = GraphManager.GetGraphClient();
        User graphUser = await graphClient.Me.Request().GetAsync();
        label2.Text = graphUser.DisplayName;
    }

 private async void button2_Click(object sender, EventArgs e)
    {
        GraphServiceClient graphClient = GraphManager.GetGraphClient();

        var team = new Team
            {
                DisplayName = "Test1Test2",
                Description = "My sample team's description.",
                AdditionalData = new Dictionary<string, object>()
            {
                {"[email protected]", "https://graph.microsoft.com/beta/teamsTemplates('educationClass')" }
            }
            };

            await graphClient.Teams.Request().AddAsync(team);
            MessageBox.Show("Successfully created team!");
    }