Guide to the Language

 Diagnostic Directives
There are two diagnostic directives: #error and #warning. The #error 
directive is used to abort a compilation by generating a compilation error. 
This directive can optionally take a parameter that provides an error 
description.
#if Professional && Enterprise
#error Build cannot be both Professional and Enterprise
#endif
Similar to error, the #warning directive generates a compilation 
warning message. This directive will not stop the compilation.
#if !Professional && !Enterprise
#warning Build should be Professional or Enterprise
#endif
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 Line Directive
Another directive that affects the compiler’s output is #line. This directive 
is used to change the line number and optionally the source filename that 
is displayed when an error or warning occurs during compilation. This is 
mainly useful when using a program that combines the source files into an 
intermediate file, which is then compiled.
#line 500 "MyFile"
#error MyError // MyError on line 500
 Region Directives
The last two directives are #region and #endregion. They delimit a section 
of code that can be expanded or collapsed using the outlining feature of 
Visual Studio.
#region MyRegion
#endregion
Just as the conditional directives, regions can be nested any number of 
levels deep.
#region MyRegion
#region MySubRegion
#endregion
#endregion
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© Mikael Olsson 2020 
M. Olsson, C# 8 Quick Syntax Reference, 
https://doi.org/10.1007/978-1-4842-5577-3_26
CHAPTER 26
Delegates
A delegate is a type used to reference a method. This allows methods to be 
assigned to variables and passed as arguments. The delegate’s declaration 
specifies the method signature to which objects of the delegate type 
can refer. Delegates are by convention named with each word initially 
capitalized, followed by Delegate at the end of the name.
delegate void MyDelegate(string str);
A method that matches the delegate’s signature can be assigned to a 
delegate object of this type.
class MyClass
{
static void Print(string s)
{
System.Console.WriteLine(s);
}
static void Main()
{
MyDelegate d = Print;
}
}

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This delegate object will behave as if it were the method itself, 
regardless of whether it refers to a static or an instance method. A method 
call on the object will be forwarded by the delegate to the method, and any 
return value will be passed back through the delegate.
MyDelegate d = Print;
d("Hello"); // "Hello"
The syntax used here to instantiate the delegate is actually a simplified 
notation that was introduced in C# 2.0. The backward compatible way 
to instantiate a delegate is to use the regular reference type initialization 
syntax.
MyDelegate d = new MyDelegate(Print);