Not able to get access token using MSAL.Net in VSTO application

Question

I have created a VSTO Excel Add-in project (with C#, Target .Net Framework version 4.8, Visual Studio 2022, working on Windows 10 Enterprise (version 21H2)).

With this Microsoft Excel add-in, I want to do authentication for my APIs by getting the access token on behalf of user, for the users present in my Azure Active Directory, using MSAL.Net.

I did my App Registration in Azure and also configured the desired Redirect URLs, scopes.

I am facing some issues in getting access token from the VSTO application, but my same code is working fine in a .Net Console Application (target framework 4.8)

Below is the code (I have referred it from the Quick Start guides available on Azure portal for various platforms), the code is working fine with Console application, able to get the access token through method app.AcquireTokenInteractive(scopes).

But the same method is throwing System.Net.Http.HttpRequestException Exception in the VSTO application,
Exception thrown: 'System.Net.Http.HttpRequestException' in mscorlib.dll

Exception thrown: 'System.Net.Http.HttpRequestException' in mscorlib.dll

An exception of type 'System.Net.Http.HttpRequestException' occurred in mscorlib.dll but was not handled in user code

An error occurred while sending the request.

Here is the code which I am using in both type of projects:

using System;
using System.Threading.Tasks;
using Microsoft.Identity.Client;


public class MyWindowsAuthenticator
{
        private static string ClientId = "********my app's client id********"; // secret

        private static string Tenant = "organizations";
        private static string Instance = "https://login.microsoftonline.com/";
        private static IPublicClientApplication _clientApp;

        //Set the scope for API call to user.read
        string[] scopes = new string[] { "user.read" };

        public static IPublicClientApplication PublicClientApp { get { return _clientApp;}}


        public async Task AuthorizeAsync()
        {
            var builder = PublicClientApplicationBuilder.Create(ClientId)
                .WithAuthority($"{Instance}{Tenant}")
                .WithDefaultRedirectUri();

            WindowsBrokerOptions options = new WindowsBrokerOptions();

            builder.WithWindowsBrokerOptions(options);

            //builder.WithWindowsBroker(true);

            _clientApp = builder.Build();

            AuthenticationResult authResult = null;
            var app = PublicClientApp;

            IAccount firstAccount;
            firstAccount = PublicClientApplication.OperatingSystemAccount;

            try
            {

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

                try
                {
                    authResult = await app.AcquireTokenInteractive(scopes)
                        .WithAccount(firstAccount)
                        .WithPrompt(Prompt.SelectAccount)
                        .ExecuteAsync();
                }
                catch (MsalException msalex)
                {
                    var errMsg = $"Error Acquiring Token:{System.Environment.NewLine}{msalex}";
                    Console.WriteLine(errMsg);
                }
            }
            catch (Exception ex)
            {
                var errMsg = $"Error Acquiring Token Silently:{System.Environment.NewLine}{ex}";
                Console.WriteLine(errMsg);
                return;
            }
        }

        
    }

Please help me in understanding the cause for the HttpRequestException in the VSTO application.

Answer 1

Adding

ServicePointManager.Expect100Continue = true;
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;

before creating the app solved the issue for me.