public struct Optional<T>
    {
        public bool HasValue { get; }
        public T Value { get; }
    }

class ConsoleWriteLineWalker : CSharpSyntaxWalker
{
    public ConsoleWriteLineWalker()
    {
        Arguments = new List<ExpressionSyntax>();
    }
 
    public List<ExpressionSyntax> Arguments { get; }
 
    public override void VisitInvocationExpression(InvocationExpressionSyntax node)
    {
        var member = node.Expression as MemberAccessExpressionSyntax;
        var type = member?.Expression as IdentifierNameSyntax;
        if (type != null && type.Identifier.Text == "Console" && member.Name.Identifier.Text == "WriteLine")
        {
            if (node.ArgumentList.Arguments.Count == 1)
            {
                var arg = node.ArgumentList.Arguments.Single().Expression;
                Arguments.Add(arg);
                return;
            }
        }
 
        base.VisitInvocationExpression(node);
    }
}

static void getConstantValue()
{
    // Get the syntax tree.
    var code = @"
                using System;
 
                class Foo
                {
                    void Bar(int x)
                    {
                        Console.WriteLine(3.14);
                        Console.WriteLine(""qux"");
                        Console.WriteLine('c');
                        Console.WriteLine(null);
                        Console.WriteLine(x * 2 + 1);
                    }
                }
                ";
 
    var tree = CSharpSyntaxTree.ParseText(code);
    var root = tree.GetRoot();
 
    // Get the semantic model from the compilation.
    var mscorlib = MetadataReference.CreateFromFile(typeof(object).Assembly.Location);
    var comp = CSharpCompilation.Create("Demo").AddSyntaxTrees(tree).AddReferences(mscorlib);
    var model = comp.GetSemanticModel(tree);
 
    // Traverse the tree.
    var walker = new ConsoleWriteLineWalker();
    walker.Visit(root);
 
 
    // Analyze the constant argument (if any).
    foreach (var arg in walker.Arguments)
    {
        var val = model.GetConstantValue(arg);
        if (val.HasValue)
        {
            Console.WriteLine(arg + " has constant value " + (val.Value ?? "null") + " of type " + (val.Value?.GetType() ?? typeof(object)));
        }
        else
        {
            Console.WriteLine(arg + " has no constant value");
        }
    }
}

 3.14 has constant value 3.14 of type System.Double
"qux" has constant value qux of type System.String
'c' has constant value c of type System.Char
null has constant value null of type System.Object
x * 2 + 1 has no constant value

static void workingWithSymbols()
{
    // Get the syntax tree.
    var code = @"
                using System;
 
                class Foo
                {
                    void Bar(int x)
                    {
                        // #insideBar
                    }
                }
 
                class Qux
                {
                    protected int Baz { get; set; }
                }
                ";
 
    var tree = CSharpSyntaxTree.ParseText(code);
    var root = tree.GetRoot();
 
    // Get the semantic model from the compilation.
    var mscorlib = MetadataReference.CreateFromFile(typeof(object).Assembly.Location);
    var comp = CSharpCompilation.Create("Demo").AddSyntaxTrees(tree).AddReferences(mscorlib);
    var model = comp.GetSemanticModel(tree);
 
    // Traverse enclosing symbol hierarchy.
    var cursor = code.IndexOf("#insideBar");
    var barSymbol = model.GetEnclosingSymbol(cursor);
    for (var symbol = barSymbol; symbol != null; symbol = symbol.ContainingSymbol)
    {
        Console.WriteLine(symbol);
    }
 
    // Analyze accessibility of Baz inside Bar.
    var bazProp = ((CompilationUnitSyntax)root)
        .Members.OfType<ClassDeclarationSyntax>()
        .Single(m => m.Identifier.Text == "Qux")
        .Members.OfType<PropertyDeclarationSyntax>()
        .Single();
    var bazSymbol = model.GetDeclaredSymbol(bazProp);
    var canAccess = model.IsAccessible(cursor, bazSymbol);
}

 Foo.Bar(int)
Foo
<global namespace>
Demo.exe
Demo, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null

static void bindingSymbols()
{
    // Get the syntax tree.
    var code = @"
                using System;
 
                class Foo
                {
                    private int y;
 
                    void Bar(int x)
                    {
                        Console.WriteLine(x);
                        Console.WriteLine(y);
 
                        int z = 42;
                        Console.WriteLine(z);
 
                        Console.WriteLine(a);
                    }
                }";
 
    var tree = CSharpSyntaxTree.ParseText(code);
    var root = tree.GetRoot();
 
    // Get the semantic model from the compilation.
    var mscorlib = MetadataReference.CreateFromFile(typeof(object).Assembly.Location);
    var comp = CSharpCompilation.Create("Demo").AddSyntaxTrees(tree).AddReferences(mscorlib);
    var model = comp.GetSemanticModel(tree);
 
    // Traverse the tree.
    var walker = new ConsoleWriteLineWalker();
    walker.Visit(root);
 
    // Bind the arguments.
    foreach (var arg in walker.Arguments)
    {
        var symbol = model.GetSymbolInfo(arg);
        if (symbol.Symbol != null)
        {
            Console.WriteLine(arg + " is bound to " + symbol.Symbol + " of type " + symbol.Symbol.Kind);
        }
        else
        {
            Console.WriteLine(arg + " could not be bound");
        }
    }
}

 x is bound to int of type Parameter
y is bound to Foo.y of type Field
z is bound to z of type Local
a could not be bound

using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Logging.Console;

namespace EFLogging
{
    public class BloggingContextWithFiltering : DbContext
    {
        // It is very important that applications do not create a new ILoggerFactory instance for each context instance. 
        // Doing so will result in a memory leak and poor performance.
        public static readonly LoggerFactory MyLoggerFactory
            = new LoggerFactory(new[]
            {
                new ConsoleLoggerProvider((category, level)
                    => category == DbLoggerCategory.Database.Command.Name
                       && level == LogLevel.Information, true)
            });

        public DbSet<Blog> Blogs { get; set; }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
            => optionsBuilder
                .UseLoggerFactory(MyLoggerFactory) // Warning: Do not create a new ILoggerFactory instance each time
                .UseSqlServer(
                    @"Server=(localdb)\mssqllocaldb;Database=EFLogging;Trusted_Connection=True;ConnectRetryCount=0");
    }
}

using System;
using System.Diagnostics;
using System.IO;
using System.Management;
using Microsoft.Win32;

namespace PdfFilePrinter
{
    /// <summary>
    /// Executes the Adobe Reader and prints a file while suppressing the Acrobat print
    /// dialog box, then terminating the Reader.
    /// </summary>
    public class AcroPrint
    {
        /// <summary>
        /// The Adobe Reader or Adobe Acrobat path such as 'C:\Program Files\Adobe\Adobe Reader X\AcroRd32.exe'.
        /// If it's not specified, the InstalledAdobeReaderPath property value will be used.
        /// </summary>
        public string AdobeReaderPath { set; get; }

        /// <summary>
        /// Returns the default printer name.
        /// </summary>
        public string DefaultPrinterName
        {
            get
            {
                var query = new ObjectQuery("SELECT * FROM Win32_Printer");
                using (var searcher = new ManagementObjectSearcher(query))
                {
                    foreach (var mo in searcher.Get())
                    {
                        if (((bool?)mo["Default"]) ?? false)
                            return mo["Name"] as string;
                    }
                }
                return string.Empty;
            }
        }

        /// <summary>
        /// The name and path of the PDF file to print.
        /// </summary>
        public string PdfFilePath { set; get; }

        /// <summary>
        /// Name of the printer such as '\\PrintServer\HP LaserJet'.
        /// If it's not specified, the DefaultPrinterName property value will be used.
        /// </summary>
        public string PrinterName { set; get; }

        /// <summary>
        /// Returns the HKEY_CLASSES_ROOT\Software\Adobe\Acrobat\Exe value.
        /// If AcroRd32.exe does not exist, returns string.Empty
        /// </summary>
        public string InstalledAdobeReaderPath
        {
            get
            {
                var acroRd32Exe = Registry.ClassesRoot.OpenSubKey(@"Software\Adobe\Acrobat\Exe", writable: false);
                if (acroRd32Exe == null)
                    return string.Empty;

                var exePath = acroRd32Exe.GetValue(string.Empty) as string;
                if (string.IsNullOrEmpty(exePath))
                    return string.Empty;

                exePath = exePath.Trim(new[] { '"' });
                return File.Exists(exePath) ? exePath : string.Empty;
            }
        }

        /// <summary>
        /// Executes the Adobe Reader and prints a file while suppressing the Acrobat print
        /// dialog box, then terminating the Reader.
        /// </summary>
        /// <param name="timeout">The amount of time, in milliseconds, to wait for the associated process to exit. The maximum is the largest possible value of a 32-bit integer, which represents infinity to the operating system.</param>
        public void PrintPdfFile(int timeout = Int32.MaxValue)
        {
            if (!File.Exists(PdfFilePath))
                throw new ArgumentException(PdfFilePath + " does not exist.");

            var args = string.Format("/N /T \"{0}\" \"{1}\"", PdfFilePath, getPrinterName());
            var process = startAdobeProcess(args);
            if (!process.WaitForExit(timeout))
                process.Kill();
        }

        private Process startAdobeProcess(string arguments = "")
        {
            var startInfo = new ProcessStartInfo
                {
                    FileName = this.getExePath(),
                    Arguments = arguments,
                    CreateNoWindow = true,
                    ErrorDialog = false,
                    UseShellExecute = false,
                    Verb = "print"
                };

            return Process.Start(startInfo);
        }

        private string getPrinterName()
        {
            var printer = PrinterName;
            if (string.IsNullOrEmpty(printer))
                printer = DefaultPrinterName;

            if (string.IsNullOrEmpty(printer))
                throw new ArgumentException("Please set the PrinterName.");

            return printer;
        }

        private string getExePath()
        {
            var exePath = AdobeReaderPath;
            if (string.IsNullOrEmpty(exePath) || !File.Exists(exePath))
                exePath = InstalledAdobeReaderPath;

            if (string.IsNullOrEmpty(exePath))
                throw new ArgumentException("Please set the full path of the AcroRd32.exe or Acrobat.exe.");

            return exePath;
        }
    }
}

            new AcroPrint 
            {
                PdfFilePath = @"D:\path\test.pdf"
            }.PrintPdfFile();

public class Car {}
public class SportsCar : Car
{
   public string Color { get; set; }
}

public static void Run<T>(T car) where T : Car
{
   if (car is SportsCar sportsCar)
   {
   }

   switch (car)
   {
      case SportsCar sCar:
      break;
   }
}

 An expression of type "T" cannot be handled by a pattern of type "SportsCar"

public static void Run<T>(T car) where T : Car
{
   if ((object)car is SportsCar sportsCar)
   {
   }

   switch ((object)car)
   {
      case SportsCar sCar:
      break;
   }
}

Use natural language to execute SQL queries 

    public class SQLiteDbContext : DbContext, IUnitOfWork
    {
        public SQLiteDbContext(DbContextOptions options) : base(options)
        { }

        public virtual DbSet<User> Users { set; get; }

namespace EFCoreMultipleDb.DataLayer.SQLite.Context
{
    public class SQLiteDbContextFactory : IDesignTimeDbContextFactory<SQLiteDbContext>
    {
        public SQLiteDbContext CreateDbContext(string[] args)
        {
            var basePath = Directory.GetCurrentDirectory();
            Console.WriteLine($"Using `{basePath}` as the BasePath");
            var configuration = new ConfigurationBuilder()
                                    .SetBasePath(basePath)
                                    .AddJsonFile("appsettings.json")
                                    .Build();
            var builder = new DbContextOptionsBuilder<SQLiteDbContext>();
            var connectionString = configuration.GetConnectionString("SqliteConnection")
                                                .Replace("|DataDirectory|", Path.Combine(basePath, "wwwroot", "app_data"));
            builder.UseSqlite(connectionString);
            return new SQLiteDbContext(builder.Options);
        }
    }
}

{
  "Logging": {
    "LogLevel": {
      "Default": "Warning"
    }
  },
  "AllowedHosts": "*",
  "ConnectionStrings": {
    "SqlServerConnection": "Data Source=(LocalDB)\\MSSQLLocalDB;Initial Catalog=ASPNETCoreSqlDB;AttachDbFilename=|DataDirectory|\\ASPNETCoreSqlDB.mdf;Integrated Security=True;MultipleActiveResultSets=True;",
    "SqliteConnection": "Data Source=|DataDirectory|\\ASPNETCoreSqliteDB.sqlite",
    "InUseKey": "SqliteConnection"
  }
}

var connectionStringKey = Configuration.GetConnectionString("InUseKey");
var connectionString = Configuration.GetConnectionString(connectionStringKey)
                     .Replace("|DataDirectory|", Path.Combine(Directory.GetCurrentDirectory(), "wwwroot", "app_data"));

For /f "tokens=2-4 delims=/ " %%a in ('date /t') do (set mydate=%%c_%%a_%%b)
For /f "tokens=1-2 delims=/:" %%a in ("%TIME: =0%") do (set mytime=%%a%%b)
dotnet build
dotnet ef migrations --startup-project ../EFCoreMultipleDb.Web/ add V%mydate%_%mytime% --context SQLiteDbContext

dotnet build
dotnet ef --startup-project ../EFCoreMultipleDb.Web/ database update --context SQLServerDbContext

namespace EFCoreMultipleDb.Web
{
    public class Startup
    {
        public void Configure(IApplicationBuilder app, IHostingEnvironment env)
        {
            applyPendingMigrations(app);
// ...
        }

        private static void applyPendingMigrations(IApplicationBuilder app)
        {
            var scopeFactory = app.ApplicationServices.GetRequiredService<IServiceScopeFactory>();
            using (var scope = scopeFactory.CreateScope())
            {
                var uow = scope.ServiceProvider.GetService<IUnitOfWork>();
                uow.Migrate();
            }
        }
    }
}

namespace EFCoreMultipleDb.DataLayer.SQLite.Context
{
    public class SQLiteDbContext : DbContext, IUnitOfWork
    {
    // ... 

        public void Migrate()
        {
            this.Database.Migrate();
        }
    }
}

namespace EFCoreMultipleDb.Web
{
    public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddScoped<IUsersService, UsersService>();

            var connectionStringKey = Configuration.GetConnectionString("InUseKey");
            var connectionString = Configuration.GetConnectionString(connectionStringKey)
                     .Replace("|DataDirectory|", Path.Combine(Directory.GetCurrentDirectory(), "wwwroot", "app_data"));
            switch (connectionStringKey)
            {
                case "SqlServerConnection":
                    services.AddScoped<IUnitOfWork, SQLServerDbContext>();
                    services.AddDbContext<SQLServerDbContext>(options =>
                    {
                        options.UseSqlServer(
                            connectionString,
                            dbOptions =>
                                {
                                    var minutes = (int)TimeSpan.FromMinutes(3).TotalSeconds;
                                    dbOptions.CommandTimeout(minutes);
                                    dbOptions.EnableRetryOnFailure();
                                });
                    });
                    break;
                case "SqliteConnection":
                    services.AddScoped<IUnitOfWork, SQLiteDbContext>();
                    services.AddDbContext<SQLiteDbContext>(options =>
                    {
                        options.UseSqlite(
                            connectionString,
                            dbOptions =>
                                {
                                    var minutes = (int)TimeSpan.FromMinutes(3).TotalSeconds;
                                    dbOptions.CommandTimeout(minutes);
                                });
                    });
                    break;
                default:
                    throw new NotImplementedException($"`{connectionStringKey}` is not defined in `appsettings.json` file.");
            }

            services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_2);
        }

{
  "ConnectionStrings": {
    // …
    "InUseKey": "SqlServerConnection"
  }
}

namespace EFCoreMultipleDb.Services
{
    public interface IUsersService
    {
        Task<User> FindUserAsync(int userId);
    }

    public class UsersService : IUsersService
    {
        private readonly IUnitOfWork _uow;
        private readonly DbSet<User> _users;

        public UsersService(IUnitOfWork uow)
        {
            _uow = uow;
            _users = _uow.Set<User>();
        }

        public Task<User> FindUserAsync(int userId)
        {
            return _users.FindAsync(userId);
        }
    }
}