Example of using Google Service accounts p12, Json and key. For the Google .net client library

Google Service Account

using Google.Apis.AnalyticsReporting.v4;
using Google.Apis.Auth.OAuth2;
using Google.Apis.Services;
using System;
using System.IO;
using System.Security.Cryptography.X509Certificates;

namespace GoogleAnaltyics.V4
{
    public class ServiceAccountJson
    {
        public string type { get; set; }
        public string project_id { get; set; }
        public string private_key_id { get; set; }
        public string private_key { get; set; }
        public string client_email { get; set; }
        public string client_id { get; set; }
        public string auth_uri { get; set; }
        public string token_uri { get; set; }
        public string auth_provider_x509_cert_url { get; set; }
        public string client_x509_cert_url { get; set; }
    }


    public class ServiceAccountAuthExample
    {

        /// <summary>
        /// Authenticating to Google using a Service account
        /// Documentation: https://developers.google.com/accounts/docs/OAuth2#serviceaccount
        /// </summary>
        /// <param name="serviceAccountEmail">From Google Developer console https://console.developers.google.com</param>
        /// <param name="serviceAccountCredentialFilePath">Location of the .p12 or Json Service account key file downloaded from Google Developer console https://console.developers.google.com</param>
        /// <returns>AnalyticsService used to make requests against the Analytics API</returns>
        public static AnalyticsReportingService AuthenticateServiceAccount(string serviceAccountEmail, string serviceAccountCredentialFilePath)
        {
            try
            {
                if (string.IsNullOrEmpty(serviceAccountCredentialFilePath))
                    throw new Exception("Path to the .p12 service account credentials file is required.");
                if (!File.Exists(serviceAccountCredentialFilePath))
                    throw new Exception("The service account credentials .p12 file does not exist at: " + serviceAccountCredentialFilePath);
                if (string.IsNullOrEmpty(serviceAccountEmail))
                    throw new Exception("ServiceAccountEmail is required.");

                // These are the scopes of permissions you need. It is best to request only what you need and not all of them
                string[] scopes = new string[] { AnalyticsReportingService.Scope.Analytics };             // View your Google Analytics data

                // For Json file
                if (Path.GetExtension(serviceAccountCredentialFilePath).ToLower() == ".json")
                {
                    GoogleCredential credential;
                    using (var stream = new FileStream(serviceAccountCredentialFilePath, FileMode.Open, FileAccess.Read))
                    {
                        credential = GoogleCredential.FromStream(stream)
                             .CreateScoped(scopes);
                    }

                    // Create the  Analytics service.
                    return new AnalyticsReportingService(new BaseClientService.Initializer()
                    {
                        HttpClientInitializer = credential,
                        ApplicationName = "Analytics Authentication Sample",
                    });
                }
                else if (Path.GetExtension(serviceAccountCredentialFilePath).ToLower() == ".p12")
                {   // If its a P12 file

                    var certificate = new X509Certificate2(serviceAccountCredentialFilePath, "notasecret", X509KeyStorageFlags.MachineKeySet | X509KeyStorageFlags.Exportable);
                    var credential = new ServiceAccountCredential(new ServiceAccountCredential.Initializer(serviceAccountEmail)
                    {
                        Scopes = scopes
                    }.FromCertificate(certificate));

                    // Create the  Analytics service.
                    return new AnalyticsReportingService(new BaseClientService.Initializer()
                    {
                        HttpClientInitializer = credential,
                        ApplicationName = "Analytics Authentication Sample",
                    });

                }
                else
                {
                    throw new Exception("Unsupported Service accounts credentials.");
                }

            }
            catch (Exception ex)
            {
                Console.WriteLine("Create service account AnalyticsService failed" + ex.Message);
                throw new Exception("CreateServiceAccountAnalyticsServiceFailed", ex);
            }
        }

        /// <summary>
        /// Authenticating to Google using a Service account
        /// Documentation: https://developers.google.com/accounts/docs/OAuth2#serviceaccount
        /// </summary>
        /// <param name="serviceAccountEmail">From Google Developer console https://console.developers.google.com</param>
        /// <param name="key">Key from with in json file   
        /// 
        /// Example:
        /// ServiceAccountAuth tmp = JsonConvert.DeserializeObject<ServiceAccountAuth>(File.ReadAllText(serviceAccountJsonPath));
        /// 
        /// </param>
        /// <returns>AnalyticsService used to make requests against the Analytics API</returns>
        public static AnalyticsReportingService AuthenticateServiceAccountFromKey(string serviceAccountEmail, string key)
        {
            try
            {
                if (string.IsNullOrEmpty(key))
                    throw new Exception("Key is required.");
                if (string.IsNullOrEmpty(serviceAccountEmail))
                    throw new Exception("ServiceAccountEmail is required.");

                // These are the scopes of permissions you need. It is best to request only what you need and not all of them
                string[] scopes = new string[] { AnalyticsReportingService.Scope.Analytics };             // View your Google Analytics data


                var credential = new ServiceAccountCredential(new ServiceAccountCredential.Initializer(serviceAccountEmail)
                {
                    Scopes = scopes
                }.FromPrivateKey(key));


                // Create the  Analytics service.
                return new AnalyticsReportingService(new BaseClientService.Initializer()
                {
                    HttpClientInitializer = credential,
                    ApplicationName = "Analytics Authentication Sample",
                });

            }
            catch (Exception ex)
            {
                Console.WriteLine("Create service account AnalyticsService failed" + ex.Message);
                throw new Exception("CreateServiceAccountAnalyticsServiceFailed", ex);
            }
        }

    }
}