C# 10 what's new;Note=Erxin.txt

C# 10 what's new;Note=Erxin

# Reference 
- https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-10




# New features 
- improved structure types 

using the record struct or readonly record struct declarations. You can now clarify that a record is a reference type with the record class

- interpolated string handler 

create a type that builds the resulting string from an interpolated string expression. The .NET libraries use this feature in many APIs

- global using directives 

the global modifier to any using directive to instruct the compiler that the directive applies to all source files in the compilation
 
- file scoped namespace delcaration 

namespace MyNamespace;

- extended property patterns 

	+ before 
{ Prop1: { Prop2: pattern } }
	
	+ after 
{ Prop1.Prop2: pattern }

- C# 10 includes many improvements to how lambda expressions

can have a natural type 

can declare a return type 

can apply attributes 

- const strings may be initialized using string interpolatio

- record types can seal ToString

- assignment and declaration in same deconstruction 

(x, int y) = point;

- improved definite assignment, no error

// Or, using ??
if (c?.GetDependentValue(out object obj) ?? false)
{
   representation = obj.ToString(); // undesired error
}

- In C# 10 and later, you can specify a different async method builder for a single method

- You can use the System.Runtime.CompilerServices.CallerArgumentExpressionAttribute to specify a parameter that the compiler replaces with the text representation of another argument.

public static void Validate(bool condition, [CallerArgumentExpression("condition")] string? message=null)
{
    if (!condition)
    {
        throw new InvalidOperationException($"Argument failed validation: <{message}>");
    }
}

- C# 10 supports a new format for the #line pragma. You likely won't use the new format, but you'll see its effects.