Authenticating to Google API with OAuth2

I am trying out the sample code from Perform Google Apps Domain-wide Delegation of Authority using C# / .NET and, like in some other samples I have tried, the part of the code that creates an object that uses the auth variable says that they syntax is wrong. Here is the code I have:

using System;
using System.Security.Cryptography;
using System.Security.Cryptography.X509Certificates;
using DotNetOpenAuth.OAuth2;
using Google.Apis.Authentication.OAuth2;
using Google.Apis.Authentication.OAuth2.DotNetOpenAuth;
using Google.Apis.Drive.v2;
using Google.Apis.Util;

namespace GoogleAPIDemo
{
    class DriveServiceObject
    {
        private const string SERVICE_ACCOUNT_EMAIL = "<some-id>@developer.gserviceaccount.com";
        private const string SERVICE_ACCOUNT_PKCS12_FILE_PATH = @"\path\to\<public_key_fingerprint>-privatekey.p12";

        /// <summary>
        /// Build a Drive service object authorized with the service account
        /// that acts on behalf of the given user.
        /// </summary>
        /// @param userEmail The email of the user.
        /// <returns>Drive service object.</returns>
        static DriveService BuildService(String userEmail)
        {
            X509Certificate2 certificate = new X509Certificate2(SERVICE_ACCOUNT_PKCS12_FILE_PATH, "notasecret",
                X509KeyStorageFlags.Exportable);

            var provider = new AssertionFlowClient(GoogleAuthenticationServer.Description, certificate)
            {
                ServiceAccountId = SERVICE_ACCOUNT_EMAIL,
                Scope = DriveService.Scopes.Drive.GetStringValue(),
                ServiceAccountUser = userEmail,
            };
            var auth = new OAuth2Authenticator<AssertionFlowClient>(provider, AssertionFlowClient.GetState);

            return new DriveService(auth);
        }
    }
}

The error that I see is

(local variable) OAuth2Athenticator<AssertionFlowClient> auth

Error:
   The best overloaded method match for 'Google.Apis.DriveService.DriveService(Googel.Apis.Services.BaseClientService.Initializer)' has some invalid arguments

This is my first time writing an app that uses Google's APIs and any help getting this working would be greatly appreciated!

This one works:

        var provider = new AssertionFlowClient(
            GoogleAuthenticationServer.Description,
            new X509Certificate2(privateKeyPath, keyPassword, X509KeyStorageFlags.Exportable))
        {
            ServiceAccountId = serviceAccountEmail,
            Scope = DriveService.Scopes.Drive.GetStringValue(),
            ServiceAccountUser = driveHolderAccountEmail
        };
        var auth = new OAuth2Authenticator<AssertionFlowClient>(provider, AssertionFlowClient.GetState);

        m_service = new DriveService(new BaseClientService.Initializer()
        {
            Authenticator = auth
        });