.NET Core 2.1.2 comprises:

• .NET Core Runtime 2.1.2 (release notes)
• .NET Core SDK 2.1.302

.NET Core 2.1.1 comprises:

• .NET Core Runtime 2.1.1
• .NET Core SDK 2.1.301

using System;
using System.ComponentModel.DataAnnotations;

namespace AngularTemplateDrivenFormsLab.Models
{
    public class Movie
    {
        public int Id { get; set; }

        [Required(ErrorMessage = "Movie Title is Required")]
        [MinLength(3, ErrorMessage = "Movie Title must be at least 3 characters")]
        public string Title { get; set; }

        [Required(ErrorMessage = "Movie Director is Required.")]
        public string Director { get; set; }

        [Range(0, 100, ErrorMessage = "Ticket price must be between 0 and 100.")]
        public decimal TicketPrice { get; set; }

        [Required(ErrorMessage = "Movie Release Date is required")]
        public DateTime ReleaseDate { get; set; }
    }
}

using AngularTemplateDrivenFormsLab.Models;
using Microsoft.AspNetCore.Mvc;

namespace AngularTemplateDrivenFormsLab.Controllers
{
    [Route("api/[controller]")]
    public class MoviesController : Controller
    {
        [HttpPost]
        public IActionResult Post([FromBody]Movie movie)
        {
            if (ModelState.IsValid)
            {
                // TODO: save ...
                return Ok(movie);
            }

            ModelState.AddModelError("", "This record already exists."); // a cross field validation
            return BadRequest(ModelState);
        }
    }
}

 {"":["This record already exists."],"TicketPrice":["Ticket price must be between 0 and 100."]}

  errors: string[] = [];

  processModelStateErrors(form: NgForm, responseError: HttpErrorResponse) {
    if (responseError.status === 400) {
      const modelStateErrors = responseError.error;
      for (const fieldName in modelStateErrors) {
        if (modelStateErrors.hasOwnProperty(fieldName)) {
          const modelStateError = modelStateErrors[fieldName];
          const control = form.controls[fieldName] || form.controls[this.lowerCaseFirstLetter(fieldName)];
          if (control) {
            // integrate into Angular's validation
            control.setErrors({
              modelStateError: { error: modelStateError }
            });
          } else {
            // for cross field validations -> show the validation error at the top of the screen
            this.errors.push(modelStateError);
          }
        }
      }
    } else {
      this.errors.push("something went wrong!");
    }
  }

  lowerCaseFirstLetter(data: string): string {
    return data.charAt(0).toLowerCase() + data.slice(1);
  }

 const control = form.controls[fieldName] || form.controls[this.lowerCaseFirstLetter(fieldName)];

  model = new Movie("", "", 0, "");
  successfulSave: boolean;
  errors: string[] = [];

  constructor(private movieService: MovieService) { }

  ngOnInit() {
  }

  submitForm(form: NgForm) {
    console.log(form);

    this.errors = [];
    this.movieService.postMovieForm(this.model).subscribe(
      (data: Movie) => {
        console.log("Saved data", data);
        this.successfulSave = true;
      },
      (responseError: HttpErrorResponse) => {
        this.successfulSave = false;
        console.log("Response Error", responseError);
        this.processModelStateErrors(form, responseError);
      });
  }

<form #form="ngForm" (submit)="submitForm(form)" novalidate>
  <div class="alert alert-danger" role="alert" *ngIf="errors.length > 0">
    <ul>
      <li *ngFor="let error of errors">
        {{ error }}
      </li>
    </ul>
  </div>
  <div class="alert alert-success" role="alert" *ngIf="successfulSave">
    Movie saved successfully!
  </div>

control.setErrors({
  modelStateError: { error: modelStateError }
});

<ng-template #validationErrorsTemplate let-ctrl="control">
  <div *ngIf="ctrl.invalid && ctrl.touched">
    <div class="alert alert-danger"  *ngIf="ctrl.errors.required">
      This field is required.
    </div>
    <div class="alert alert-danger"  *ngIf="ctrl.errors.minlength">
      This field should be minimum {{ctrl.errors.minlength.requiredLength}} characters.
    </div>
    <div class="alert alert-danger"  *ngIf="ctrl.errors.maxlength">
      This field should be max {{ctrl.errors.maxlength.requiredLength}} characters.
    </div>
    <div class="alert alert-danger"  *ngIf="ctrl.errors.pattern">
      This field's pattern: {{ctrl.errors.pattern.requiredPattern}}
    </div>
    <div class="alert alert-danger"  *ngIf="ctrl.errors.modelStateError">
      {{ctrl.errors.modelStateError.error}}
    </div>
  </div>
</ng-template>

  <div class="form-group" [class.has-error]="releaseDate.invalid && releaseDate.touched">
    <label class="control-label" for="releaseDate">Release Date</label>
    <input type="text" name="releaseDate" #releaseDate="ngModel"  class="form-control"
      required [(ngModel)]="model.releaseDate" />
    <ng-container *ngTemplateOutlet="validationErrorsTemplate; context:{ control: releaseDate }"></ng-container>
  </div>

// Cv2.ImRead
using (var src = new Mat(@"..\..\Images\Penguin.Png", LoadMode.AnyDepth | LoadMode.AnyColor))
using (var dst = new Mat())
{
    Cv2.CvtColor(src, dst, ColorConversion.BgrToGray);
 
    // How to export
    using (var bitmap = dst.ToBitmap()) // => OpenCvSharp.Extensions.BitmapConverter.ToBitmap(dst)
    {
        bitmap.Save("gray.png", ImageFormat.Png);
    }
 
    using (new Window("BgrToGray C++: src", image: src))
    using (new Window("BgrToGray C++: dst", image: dst))
    {
        Cv2.WaitKey();
    }
}

cv::Mat src = cv::imread ("foo.jpg");
cv::Mat dst;
cv::cvtColor (src, dst, CV_BGR2GRAY);

using (var src = new IplImage(@"..\..\Images\Penguin.Png", LoadMode.AnyDepth | LoadMode.AnyColor))
using (var dst = new IplImage(src.Size, src.Depth, src.NChannels))
{
    for (var y = 0; y < src.Height; y++)
    {
        for (var x = 0; x < src.Width; x++)
        {
            CvColor pixel = src[y, x];
            dst[y, x] = new CvColor
            {
                B = (byte)(255 - pixel.B),
                G = (byte)(255 - pixel.G),
                R = (byte)(255 - pixel.R)
            };
        }
    }
 
    // [C] Accessing Pixel
    // https://github.com/shimat/opencvsharp/wiki/%5BC%5D-Accessing-Pixel
 
    using (new CvWindow("C Interface: Src", image: src))
    using (new CvWindow("C Interface: Dst", image: dst))
    {
        Cv.WaitKey(0);
    }
}

// Cv2.ImRead
using (var src = new Mat(@"..\..\Images\Penguin.Png", LoadMode.AnyDepth | LoadMode.AnyColor))
using (var dst = new Mat())
{
    src.CopyTo(dst);
 
    for (var y = 0; y < src.Height; y++)
    {
        for (var x = 0; x < src.Width; x++)
        {
            var pixel = src.Get<Vec3b>(y, x);
            var newPixel = new Vec3b
            {
                Item0 = (byte)(255 - pixel.Item0), // B
                Item1 = (byte)(255 - pixel.Item1), // G
                Item2 = (byte)(255 - pixel.Item2) // R
            };
            dst.Set(y, x, newPixel);
        }
    }
 
    // [Cpp] Accessing Pixel
    // https://github.com/shimat/opencvsharp/wiki/%5BCpp%5D-Accessing-Pixel
 
    //Cv2.NamedWindow();
    //Cv2.ImShow();
    using (new Window("C++ Interface: Src", image: src))
    using (new Window("C++ Interface: Dst", image: dst))
    {
        Cv2.WaitKey(0);
    }
}

using Microsoft.AspNetCore.Mvc;

namespace Core3xWebApi.Controllers
{
    [ApiController]
    [Route("[controller]")]
    public class HomeController : ControllerBase
    {
        [HttpGet]
        public ActionResult<string> Get()
        {
            return null;
        }
    }
}

namespace Core3xWebApi
{
    public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddControllers(options =>
            {
                // remove formatter that turns nulls into 204 - No Content responses
                // this formatter breaks SPA's Http response JSON parsing
                options.OutputFormatters.RemoveType<HttpNoContentOutputFormatter>();
                options.OutputFormatters.Insert(0, new HttpNoContentOutputFormatter
                {
                    TreatNullValueAsNoContent = false
                });
            });
        }

public static int Add(in int number1, in int number2)
{
   number1 = 5; // Cannot assign to variable 'in int' because it is a readonly variable
   return number1 + number2;
}

<ItemGroup>
     <PackageReference Include="BenchmarkDotNet" Version="0.10.12" />
</ItemGroup>

using BenchmarkDotNet.Attributes;

namespace CS72Tests
{
    public struct Input
    {
        public decimal Number1;
        public decimal Number2;
    }

    [MemoryDiagnoser]
    public class InBenchmarking
    {
        const int loops = 50000000;
        Input inputInstance = new Input();

        [Benchmark(Baseline = true)]
        public decimal RunNormalLoop_Pass_By_Value()
        {
            decimal result = 0M;
            for (int i = 0; i < loops; i++)
            {
                result = Run(inputInstance);
            }
            return result;
        }

        [Benchmark]
        public decimal RunInLoop_Pass_By_Reference()
        {
            decimal result = 0M;
            for (int i = 0; i < loops; i++)
            {
                result = RunIn(in inputInstance);
            }
            return result;
        }

        public decimal Run(Input input)
        {
            return input.Number1;
        }

        public decimal RunIn(in Input input)
        {
            return input.Number1;
        }
    }
}

static void Main(string[] args)
{
    var summary = BenchmarkRunner.Run<InBenchmarking>();

                      Method |      Mean |    Error |   StdDev | Scaled | Allocated |
---------------------------- |----------:|---------:|---------:|-------:|----------:|
 RunNormalLoop_Pass_By_Value | 280.04 ms | 2.219 ms | 1.733 ms |   1.00 |       0 B |
 RunInLoop_Pass_By_Reference |  91.75 ms | 1.733 ms | 1.780 ms |   0.33 |       0 B |

    public static class Factorial
    {
        public static int Calculate(in this int num)
        {
            int result = 1;
            for (int i = num; i > 1; i--)
                result *= i;

            return result;
        }
    }

int num = 3;
Console.WriteLine($"(in num) -> {Factorial.Calculate(in num)}");
Console.WriteLine($"(num) -> {Factorial.Calculate(num)}");
Console.WriteLine($"num. -> {num.Calculate()}");

(in num) -> 6
(num) -> 6
num. -> 6

    public class CX
    {
        public void A(Input a)
        {
            Console.WriteLine("int a");
        }

        public void A(in Input a)
        {
            Console.WriteLine("in int a");
        }
    }

    public class Y
    {
        public void Test()
        {
            var inputInstance = new Input();
            var cx = new CX();
            cx.A(inputInstance); // The call is ambiguous between the following methods or properties: 'CX.A(Input)' and 'CX.A(in Input)'
        }
    }

public IEnumerable<int> B(in int a) // Iterators cannot have ref or out parameters
{
   Console.WriteLine("in int a");
   yield return 1;
}

public async Task C(in int a) // Async methods cannot have ref or out parameters
{
   await Task.Delay(1000);
}

using System;

namespace CS72Tests
{
    struct MyStruct
    {
        public int MyValue { get; set; }

        public void UpdateMyValue(int value)
        {
            MyValue = value;
        }
    }

    public static class TestInStructs
    {
        public static void Run()
        {
            var myStruct = new MyStruct();
            myStruct.UpdateMyValue(1);

            UpdateMyValue(myStruct);

            Console.WriteLine(myStruct.MyValue);
        }

        static void UpdateMyValue(in MyStruct myStruct)
        {
            myStruct.UpdateMyValue(5);
        }
    }
}

static void UpdateMyValue(in MyStruct myStruct)
{
   myStruct.MyValue = 5; // Cannot assign to a member of variable 'in MyStruct' because it is a readonly variable
   myStruct.UpdateMyValue(5);
}

namespace ASPNETCoreIdentitySample.DataLayer.Mappings
{
    public class RoleConfiguration : IEntityTypeConfiguration<Role>
    {
        private readonly SiteSettings _siteSettings;
        private readonly ILookupNormalizer _keyNormalizer;

        public RoleConfiguration(SiteSettings siteSettings, ILookupNormalizer keyNormalizer)
        {
            _siteSettings = siteSettings ?? throw new ArgumentNullException(nameof(siteSettings));
            _keyNormalizer = keyNormalizer ?? throw new ArgumentNullException(nameof(keyNormalizer));
        }

        public void Configure(EntityTypeBuilder<Role> builder)
        {
            builder.ToTable("AppRoles");

            var adminUserSeed = _siteSettings.AdminUserSeed;
            builder.HasData(
                new Role
                {
                    Id = 1,
                    Name = adminUserSeed.RoleName,
                    NormalizedName = _keyNormalizer.NormalizeName(adminUserSeed.RoleName),
                    ConcurrencyStamp = Guid.NewGuid().ToString()
                });
        }
    }
}

    public class UserConfiguration : IEntityTypeConfiguration<User>
    {
        private readonly SiteSettings _siteSettings;
        private readonly ILookupNormalizer _keyNormalizer;
        private readonly IPasswordHasher<User> _passwordHasher;

        public UserConfiguration(
            SiteSettings siteSettings,
            ILookupNormalizer keyNormalizer,
            IPasswordHasher<User> passwordHasher)
        {
            _siteSettings = siteSettings ?? throw new ArgumentNullException(nameof(siteSettings));
            _keyNormalizer = keyNormalizer ?? throw new ArgumentNullException(nameof(keyNormalizer));
            _passwordHasher = passwordHasher ?? throw new ArgumentNullException(nameof(passwordHasher));
        }

        public void Configure(EntityTypeBuilder<User> builder)
        {
            builder.ToTable("AppUsers");

            var adminUserSeed = _siteSettings.AdminUserSeed;
            builder.HasData(
                new User
                {
                    Id = 1,
                    UserName = adminUserSeed.Username,
                    NormalizedUserName = _keyNormalizer.NormalizeName(adminUserSeed.Username),
                    Email = adminUserSeed.Email,
                    NormalizedEmail = _keyNormalizer.NormalizeEmail(adminUserSeed.Email),
                    EmailConfirmed = true,
                    IsEmailPublic = true,
                    LockoutEnabled = true,
                    TwoFactorEnabled = false,
                    PasswordHash = _passwordHasher.HashPassword(null, adminUserSeed.Password),
                    ConcurrencyStamp = Guid.NewGuid().ToString(),
                    SecurityStamp = string.Empty,
                    IsActive = true
                });
        }
    }

    public class UserRoleConfiguration : IEntityTypeConfiguration<UserRole>
    {
        public void Configure(EntityTypeBuilder<UserRole> builder)
        {
            builder.HasOne(userRole => userRole.Role)
                   .WithMany(role => role.Users)
                   .HasForeignKey(userRole => userRole.RoleId);

            builder.HasOne(userRole => userRole.User)
                   .WithMany(user => user.Roles)
                   .HasForeignKey(userRole => userRole.UserId);

            builder.ToTable("AppUserRoles");

            builder.HasData(
                new UserRole
                {
                    UserId = 1,
                    RoleId = 1
                });
        }
    }

    public class ApplicationDbContextFactory : IDesignTimeDbContextFactory<ApplicationDbContext>
    {
        public ApplicationDbContext CreateDbContext(string[] args)
        {
            // ....
            services.TryAddScoped<ILookupNormalizer, UpperInvariantLookupNormalizer>();
            services.TryAddScoped<IPasswordHasher<User>, PasswordHasher<User>>();

using Microsoft.EntityFrameworkCore.Infrastructure;

namespace ASPNETCoreIdentitySample.DataLayer.Context
{
    public class ApplicationDbContext :
        IdentityDbContext<User, Role, int, UserClaim, UserRole, UserLogin, RoleClaim, UserToken>,
        IUnitOfWork
    {
        public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options)
            : base(options) { }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            base.OnModelCreating(modelBuilder);

            var siteSettings = this.GetService<IOptionsSnapshot<SiteSettings>>()?.Value;
            var lookupNormalizer = this.GetService<ILookupNormalizer>();
            var passwordHasher =  this.GetService<IPasswordHasher<User>>()

            modelBuilder.ApplyConfiguration(new RoleConfiguration(siteSettings, lookupNormalizer));
            modelBuilder.ApplyConfiguration(new UserConfiguration(siteSettings, lookupNormalizer, passwordHasher));
            modelBuilder.ApplyConfiguration(new UserUsedPasswordConfiguration(siteSettings, passwordHasher));
// ...

dotnet ef migrations --startup-project ../ASPNETCoreIdentitySample/ add V1

services.AddDbContext<BloggingContext>(
   options => options.UseSqlServer(connectionString));

services.AddDbContextPool<BloggingContext>(
   options => options.UseSqlServer(connectionString));

public class BloggingContext : DbContext
{
   public BloggingContext(DbContextOptions<BloggingContext> options) : base(options){}

        public static void RunWithoutContextPooling()
        {
            Console.WriteLine("\nRun Without ContextPooling");
            var serviceProvider = new ServiceCollection()
                .AddEntityFrameworkSqlServer()
                .AddDbContext<BloggingContext>(
                    c => c.UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=Demo.ContextPooling;Trusted_Connection=True;ConnectRetryCount=0;"))
                .BuildServiceProvider();

            new RunTests().Start(serviceProvider);
        }

        public static void RunWithContextPooling()
        {
            Console.WriteLine("\nRun With ContextPooling");
            var serviceProvider = new ServiceCollection()
                .AddEntityFrameworkSqlServer()
                .AddDbContextPool<BloggingContext>(
                    c => c.UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=Demo.ContextPooling;Trusted_Connection=True;ConnectRetryCount=0;"),
                    poolSize: 16)
                .BuildServiceProvider();

            new RunTests().Start(serviceProvider);
        }

Run Without ContextPooling
[10:49:30.728] Context creations: 637 | Requests per second: 597
[10:49:31.746] Context creations: 1069 | Requests per second: 1050
[10:49:32.765] Context creations: 1088 | Requests per second: 1067
[10:49:33.784] Context creations: 1139 | Requests per second: 1119
[10:49:34.802] Context creations: 1138 | Requests per second: 1117
[10:49:35.831] Context creations: 1153 | Requests per second: 1120
[10:49:36.845] Context creations: 1126 | Requests per second: 1111
[10:49:37.873] Context creations: 1014 | Requests per second: 987
[10:49:38.898] Context creations: 1139 | Requests per second: 1111
[10:49:39.918] Context creations: 1086 | Requests per second: 1065

Total context creations: 10592
Requests per second:     1034

Run With ContextPooling
[10:49:40.982] Context creations: 32 | Requests per second: 1388
[10:49:41.991] Context creations: 0 | Requests per second: 1691
[10:49:43.014] Context creations: 0 | Requests per second: 1684
[10:49:44.031] Context creations: 0 | Requests per second: 1702
[10:49:45.049] Context creations: 0 | Requests per second: 1694
[10:49:46.067] Context creations: 0 | Requests per second: 1401
[10:49:47.075] Context creations: 0 | Requests per second: 1510
[10:49:48.107] Context creations: 0 | Requests per second: 1669
[10:49:49.127] Context creations: 0 | Requests per second: 1679
[10:49:50.147] Context creations: 0 | Requests per second: 1688

Total context creations: 32
Requests per second:     1610

  var manager = new breeze.EntityManager({  
  dataService: dataService,          
  metadataStore: metadataStore,                     
  saveOptions: new breeze.SaveOptions({    allowConcurrentSaves: true, tag: [{}] })   
                 });

var dataService = new breeze.DataService({  
serviceName: "/breeze/"+ "Automobile",             
hasServerMetadata: false,
namingConvention: breeze.NamingConvention.camelCase        
});
var metadataStore = new breeze.MetadataStore({});

[BreezeController]
public class AutomobileController : ApiController
    {
        readonly EFContextProvider<ApplicationDbContext> _contextProvider =
        new EFContextProvider<ApplicationDbContext>();
        [HttpGet]
        public string Metadata()
        {
            return _contextProvider.Metadata();
        }
        [HttpGet]
        public IQueryable<Customer> Customers() {
           return _contextProvider.Context.Customers;
        }

        [System.Web.Http.HttpPost]
        public SaveResult SaveChanges(JObject saveBundle)
        {
           _contextProvider.BeforeSaveEntitiesDelegate = BeforeSaveEntities;
           _contextProvider.AfterSaveEntitiesDelegate = afterSaveEntities;
            return _contextProvider.SaveChanges(saveBundle);
        }
protected Dictionary<Type, List<EntityInfo>> BeforeSaveEntities(Dictionary<Type, List<EntityInfo>> saveMap)
        {
        }
private void afterSaveEntities(Dictionary<Type, List<EntityInfo>> saveMap, List<KeyMapping> keyMappings)
        {
        }
    }