I want to create a service that consumes Microsoft Graph API with reading the Outlook Calendar with this code

public class AuthenticationHelper
{
    private static string _clientId = "xxx";
    private static string _tenantId = "xxx";
    private static string _clientSecret = "xxx";
    private static string[] _scopes = new string[] { "https://graph.microsoft.com/.default" };

    public static async Task<string> GetAccessTokenAsync()
    {
        IConfidentialClientApplication app = ConfidentialClientApplicationBuilder.Create(_clientId)
            .WithClientSecret(_clientSecret)
            .WithAuthority(new Uri($"https://login.microsoftonline.com/{_tenantId}"))
            .Build();

        AuthenticationResult result = await app.AcquireTokenForClient(_scopes).ExecuteAsync();
        return result.AccessToken;
    }
}

//Call Microsoft Graph API
public class CalendarHelper
{
    private static GraphServiceClient GetGraphClient(string accessToken)
    {
        return new GraphServiceClient(
            new DelegateAuthenticationProvider((requestMessage) =>
            {
                requestMessage.Headers.Authorization = new AuthenticationHeaderValue("Bearer", accessToken);
                return Task.CompletedTask;
            }));
    }

    public static async Task ListCalendarEventsAsync()
    {
        string accessToken = await AuthenticationHelper.GetAccessTokenAsync();
        var graphClient = GetGraphClient(accessToken);

        var events = await graphClient.Me.Calendar.GetAsync();

        Console.WriteLine(events);
    }
}type here

At the CalendarHelper class, the GraphServiceClient is unable to find the DelegateAuthenticationProvider of the GraphServiceClient method.

Does anyone can solve my code this pattern?