Community Tech Days C# 4.0

C# 4.0 Language Enhancements
Sankarsan Bose
11th October 2009

Agenda

Journey of C#
Current Trends
C# 4.0 Features
Dynamic Objects
Named And Optional Parameters
Covariance And Contravariance
Improved COM InterOp

Journey of C#
???
Functional + LINQ
Generics
Static Language + Managed Code

Current Trends
C#

C# 4.0 Features

Dynamic Programming
Optional and Named Parameters
Co- and Contra-variance
Improved COM Interoperability

Dynamic Programming
Ruby Code Sample

No type is defined
Stores string value
“+” operator concatenates
Assigned int values
“+” operator adds

Type Determined At Runtime Based On Type Of Value Stored

Dynamic Programming
Ruby Code Sample

Both classes define “print”
Exactly same signature
No type defined for parameter “p”
Any object supporting method “print” is a valid input
This is called “Duck Typing”

Method To Be Invoked Is Decided At Runtime

Dynamic Programming
Dynamic Method Dispatch in C# (Before 4.0)
Both are staticallytyped as IOfficeDocument

Dynamic Programming
Dynamically Typed Objects in C# 4.0

New keyword “dynamic”
Statically Typed as “dynamic”
Actual type determined at runtime

Dynamic Method Invocation

Dynamic Programming
Pros

No risk of brittle class hierarchy
Need not implement all methods of interface
More flexible , highly productive

Cons

Developer needs thorough understanding of codebase
Misuse might lead to poor maintainability

Dynamic Programming
Demo

Dynamic Programming
C# is a statically typedlanguage
How we are making it dynamic ???
Here Comes DLR
Dynamic Language Runtime

Dynamic Programming
DLR is additional libraries to fit the dynamic languages into the CLR

Store program/code as data
Caching mechanism for dynamic call
Provide language specific support

Dynamic Programming
Class Generated Under The Hood

CallSite is created before method call
Actual method call is delegated to CallSite
Compiler Generated Class
Reference to CallSite

Dynamic Programming
CLR – DLR Interaction
Runtime
Compile Time
CLR
Compiler injects call to DLR CallSite
C# Source Code
MSIL Code
Call is delegated to DLR Call Site Cache
DLR
CallSite Cache
Determines type Determines method
Returns Expression Tree
Call Runtime Binder for Cache miss
C# Runtime Binder

Optional & Named Parameters
“b” & “c” are nullable & optional
Calling code has to pass null value
This is an issue for large no of parameters

Solution Using Overload in C# (Before 4.0)
One overload accepts only “a”
Another accepts “a” & “b”
This is not possible
Signature clash with the previous method

Solution Using C# 4.0
Optional Parameter with Default Value
Optional Parameter omitted in the call
Method call with parameter names
Position reversed

Demo

Covariance & Contravariance
Subclass
Ellipse
Circle
As per Liskov Substitution Principle(LSP):
Any program which expects an object of Ellipse
Will function without any change with an object of Circle
Covariance – Type substitution as per LSP
Contravariance – Type substitution reverse to LSP

Delegate Covariance in C# (Already Present)
Func<Base> is a delegate which returns type “Base”
Func<Base> can be replaced by function with return type “Derived”

Delegate Contravariance in C# (Already Present)
Func<Derived , Object> is a delegate which accepts type “Derived”
Func<Base> can be replaced by function which accepts type “Base”

Covariance in Arrays (Existing in C# )
This covariance is allowed
This code compiles
But throws Runtime Exception

Invariance in Generic Types(Existing in C# )
This covariance is not allowed
This code will not compile
Before C# 4.0 Generic Types are Invariant

Covariance in Generic Types of C# 4.0
This covariance is allowed
public interface IEnumerable<out T> : IEnumerable{IEnumerator<T> GetEnumerator();}

Contravariance in Generic Types of C# 4.0
This Contravariance is allowed
public interface IComparer<in T>{ public int Compare(T left, T right);}

Demo

Optional “ref” modifier
Optional and named parameters
Dynamic objects
No additional type casts needed
InterOp type embedding (“No PIA”)

Pass Parameters without using “ref”

Take advantage of named parameters & optional arguments

Demo

Journey of C# - Revisited
Dynamic
Functional + LINQ
Generics
Static Language + Managed Code

Thank You

References

http://channel9.msdn.com/shows/Going+Deep/Inside-C-40-dynamic-type-optional-parameters-more-COM-friendly/
http://channel9.msdn.com/posts/Charles/C-40-Meet-the-Design-Team/
http://channel9.msdn.com/shows/Going+Deep/Expert-to-Expert-Anders-Hejlsberg-The-Future-of-C/
http://channel9.msdn.com/pdc2008/TL16/