AuthenticatingApp

This article assumes that application has been registered in Azure. See this link for more information -> Registering application

Once Azure Application has been setup and configured for consuming Exchange REST Api, easiest was to authenticate is by implementing IAuthorizationTokenProvider interface.

IAuthorizationTokenProvider has following definition:

/// <summary>
/// Authorization token provider.
/// </summary>
public interface IAuthorizationTokenProvider
{
    /// <summary>
    /// Construct authentication header.
    /// </summary>
    /// <returns></returns>
    AuthenticationHeaderValue GetAuthenticationHeader();
}

Simplest way to implement that interface is by consuming ADAL library -> https://www.nuget.org/packages/Microsoft.IdentityModel.Clients.ActiveDirectory/

Once nuget package has been installed in your project you can create custom IAuthorizationProvider for use against Exchange REST Api:

/// <summary>
/// Test authentication provider. 
/// </summary>
internal class TestAuthenticationProvider : IAuthorizationTokenProvider
{
    /// <summary>
    /// Create new instance of <see cref="TestAuthenticationProvider"/>
    /// </summary>
    internal TestAuthenticationProvider()
    {
        this.ResourceUri = "https://outlook.office365.com";
    }

    /// <summary>
    /// Resource uri.
    /// </summary>
    private string ResourceUri { get; }

    /// <summary>
    /// Retrieve token.
    /// </summary>
    /// <returns></returns>
    private string GetToken()
    {
        string authority = $"https://login.microsoftonline.com/{AppConfig.TenantId}";
        AuthenticationContext context = new AuthenticationContext(authority);
        
        X509Certificate2 certFromStore = null;
        using (X509Store store = new X509Store(StoreLocation.LocalMachine))
        {
            store.Open(OpenFlags.ReadOnly);
            X509Certificate2Collection collection = store.Certificates.Find(
                X509FindType.FindByThumbprint, 
                AppConfig.CertThumbprint,
                false);

            if (collection.Count == 1)
            {
                certFromStore = collection[0];
            }
        }

        if (certFromStore == null)
        {
            throw new ArgumentNullException("Certificate");
        }

        ClientAssertionCertificate cert = new ClientAssertionCertificate(
            AppConfig.ApplicationId.ToString(),
            certFromStore);

        AuthenticationResult token = context.AcquireTokenAsync(
            this.ResourceUri,
            cert).Result;

        return token.AccessToken;
    }

    /// <inheritdoc cref="IAuthorizationTokenProvider.GetAuthenticationHeader"/>
    public AuthenticationHeaderValue GetAuthenticationHeader()
    {
        string token = this.GetToken();
        return new AuthenticationHeaderValue(
            "Bearer", 
            token);
    }
}

These should be values for AppConfig keys:

1. AppConfig.TenantId -> Guid of your Office 365 tenant,
2. AppConfig.CertThumbprint -> Certificate thumbprint retrieved while configuring Application manifest in Azure,
3. AppConfig.ApplicationId -> Guid of your application on Azure (Application ID)

Once interface implemented it can be used to provide authorization to the service:

ExchangeService exchangeService = ExchangeService(new TestAuthenticationProvider(), "testmbx@domain.com");