The Tiered Compilation feature changes the premise by allowing .NET to have multiple compilations for the same method that can be hot-swapped at runtime.  

Issues Fixed in 15.9.19

• Fixed an issue in C++ optimizer where the impact of writing to unknown memory inside a call wasn’t properly accounted for in the caller.

Security Advisory Notices

• ASP.NET Core Denial of Service Vulnerability
• ASP.NET Core Remote Code Execution Vulnerability

Here are a couple of the most relevant improvements you may want to verify:

• Work on the EF Core in-memory provider was finished and most query features should now be working (the majority of it went into RC1)
• EF Core’s compilation performance was improved significantly for complex queries 

Issues Fixed in 15.9.1

These are the issues addressed in 15.9.1:

• Fixed a bug where Visual Studio would fail to build projects using the Microsoft Xbox One XDK.

Details of What's New in 15.9.1

Universal Windows Platform Development SDK

The Windows 10 October 2018 Update SDK (build 17763) is now the default selected SDK for the Universal Windows Platform development workload.

 Unable to create a map expression fro

m Question.Weight (System.Int32) to QuestionViewModel.Weight (System.Byte)
Unable to create a map expression from Question.Weight (System.Int32) to QuestionViewModel.Weight (System.Byte)
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: AutoMapper.AutoMapperMappingException: Unable to create a map expression from Question.Weight (System.Int32) to QuestionViewModel.Weight (System.Byte)
Unable to create a map expression from Question.Weight (System.Int32) to QuestionViewModel.Weight (System.Byte)

Microsoft's New Free SQL Server 2019 Training Materials — SQL Server 2019 Ground to Cloud covers, unsurprisingly, how to get started with SQL Server whether on-prem or on Azure. There’s also a 219 page PDF you can download and work through. 

Safari, Firefox, Edge and Chrome are removing support for TLS 1.0 and 1.1 in March of 2020. If you manage websites, this means there’s less than a year to enable TLS 1.2 (and, ideally, 1.3) on your servers, otherwise all major browsers will display error pages, rather than the content your users were expecting to find. 

If Not a And b Then
  ...
Else
  ...
EndIf

if(!(a) && b)
{
...
}
else
{
}

public interface ICommand
{
}

public class Command1 : ICommand
{
}

public class Command2 : ICommand
{
}

ICommand command = new Command1();
if (!(command is Command2))
{

}

if (command is not Command2)
{

}

if ((command is ICommand) && !(command is Command2))
{

}

if (command is ICommand and not Command2)
{

}

if (command is Command1 or Command2)
{

}

var number = new Random().Next(1, 10);
if (number > 2 && number < 8)
{

}

if (number is > 2 and < 8)
{

}

public static bool IsLetterOrSeparator(char c) =>
   (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || c == '.' || c == ',';

public static bool IsLetterOrSeparator(char c) =>
   c is (>= 'a' and <= 'z') or (>= 'A' and <= 'Z') or '.' or ',';

 var number = new Random().Next(1, 10);
 if (number <= 0)
 {
     Console.WriteLine("Less than or equal to 0");
 }
 else if (number > 0 && number <= 10)
 {
     Console.WriteLine("More than 0 but less than or equal to 10");
 }
 else
 {
     Console.WriteLine("More than 10");
 }

// C#9.0
 switch (number)
 {
     case <= 0:
         Console.WriteLine("Less than or equal to 0");
         break;
     case > 0 and <= 10:
         Console.WriteLine("More than 0 but less than or equal to 10");
         break;
     default:
         Console.WriteLine("More than 10");
         break;
 }

// C#7.0
 switch (number)
 {
     case int value when value <= 0:
         Console.WriteLine("Less than or equal to 0");
         break;
     case int value when value > 0 && value <= 10:
         Console.WriteLine("More than 0 but less than or equal to 10");
         break;
     default:
         Console.WriteLine("More than 10");
         break;
 }

 var message = number switch
 {
     <= 0 => "Less than or equal to 0",
     > 0 and <= 10 => "More than 0 but less than or equal to 10",
     _ => "More than 10"
 };

private static int GetDiscount(Product p) => p switch
        {
            Food => 0, // Food _ => 0 before C# 9
            Book b => 75, // Book b _ => 75 before C# 9
            _ => 25
        };

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 ef --configuration Release --startup-project ../ProjName/ migrations add V%mydate%_%mytime%

What’s new?
Reloadable endpoints via configuration for Kestrel

Kestrel now has the ability to observe changes to configuration passed to KestrelServerOptions.Configure and unbind from existing endpoints and bind to new endpoints without requiring you to restart your application.