Demonstrate how to use service account to access Google Sheets doc.

Program.cs

using Google.Apis.Auth.OAuth2;
using Google.Apis.Services;
using Google.Apis.Sheets.v4;
using Google.Apis.Sheets.v4.Data;
using System;
using System.Collections.Generic;
using System.IO;
using System.Text;

namespace SheetsQuickstart
{
    class Program
    {
        // If modifying these scopes, delete your previously saved credentials
        // at ~/.credentials/sheets.googleapis.com-dotnet-quickstart.json
        static string[] Scopes = { SheetsService.Scope.SpreadsheetsReadonly };
        static string ApplicationName = "SheetsLab";

        static void Main(string[] args)
        {
            GoogleCredential credential;


            // Put your credentials json file in the root of the solution and make sure copy to output dir property is set to always copy 
            using (var stream = new FileStream("credentials.json",
                FileMode.Open, FileAccess.Read))
            {
                credential = GoogleCredential.FromStream(stream).CreateScoped(Scopes);
            }

            // Create Google Sheets API service.
            var service = new SheetsService(new BaseClientService.Initializer()
            {
                HttpClientInitializer = credential,
                ApplicationName = ApplicationName
            });

            // Define request parameters.
            String spreadsheetId = "Your sheet id";
            String range = "Your sheet name with range";
            SpreadsheetsResource.ValuesResource.GetRequest request =
                    service.Spreadsheets.Values.Get(spreadsheetId, range);

            ValueRange response = request.Execute();
            IList<IList<Object>> values = response.Values;
            if (values != null && values.Count > 0)
            {
                StringBuilder builder = new StringBuilder();
                foreach (var row in values)
                {
                    var rowStr = string.Join(',', row);
                    builder.AppendLine(rowStr);
                }

                Console.WriteLine(builder.ToString());
            }
            else
            {
                Console.WriteLine("No data found.");
            }
            Console.Read();
        }
    }
}