Guide to the Language

 In Parameters
Similar to the ref parameter modifier, C# 7.2 added the in modifier, which 
provides the ability to pass an argument as a readonly reference. Any code 
in the method that attempts to modify an in parameter (or its members in 
the case of a struct) will fail at compile time and so the parameter must be 
initialized prior to the method call.
class MyApp
{
static void Test(in int num)
{
// num = 5; // error: readonly parameter
}
static void Main()
{
int i = 10;
Test(i); // passed by readonly reference
Test(2); // allowed, temporary variable created
}
}
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175
Like the ref modifier, the in modifier prevents unnecessary copies 
from being made of value types. This is useful for performance reasons, 
particularly when passing a large struct object to a method that’s called 
multiple times.
 Constant Guideline
In general, it is a good idea to always declare variables as const or 
readonly if they do not need to be reassigned. This ensures that the 
variables will not be changed anywhere in the program by mistake, which 
in turn helps to prevent bugs. It also clearly conveys to other developers 
when a variable is intended not to be modified.
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© Mikael Olsson 2020 
M. Olsson, C# 8 Quick Syntax Reference, 
https://doi.org/10.1007/978-1-4842-5577-3_30
CHAPTER 30
Asynchronous 
Methods
An asynchronous method is a method that can return before it has finished 
executing. Any method that performs a potentially long-running task, such 
as accessing a web resource or reading a file, can be made asynchronous 
to improve the responsiveness of the program. This is especially important 
in graphical applications, because any method that takes a long time 
to execute on the user interface thread will cause the program to be 
unresponsive while waiting for that method to complete.