David Evans cs.virginia/evans

David Evanshttp://www.cs.virginia.edu/evans

CS201j: Engineering SoftwareUniversity of VirginiaComputer Science

Lecture 22: C#: “Sharp”, “Hash” or “Pound”?

26 November 2002 CS 201J Fall 2002 2

Menu• Db and C#

• CLU

• What does the “J” in CS201J really stand for?

Today’s notes: web onlyLots of links to Java history and C# info


Java History

• 1991: Green Project starts, developing software for interactive TV boxes

• 1992: Develop a language, “Oak” for programming them

• Renamed “Java” (by Kim Polese) in a meeting in a coffee shop

• March 1995: Posted on the Sun web site• May 1995: Sun announces Java, HotJava

browser, Netscape licenses



Microsoft and JavaTM

• Dec 1995: Microsoft licenses Java• Microsoft replaces Sun’s JNI native interface

with their own “improved”, incompatible version• Oct 1997: Sun sues Microsoft – breach of

contract to provide “Java compatible” products• Microsoft countersues Sun for failing to deliver

an implementation that passes Sun’s test suite and failing to provide public test cases

• Jan 2001: Sun wins lawsuit (Microsoft pays $20M, accepts termination of Java license, and agrees not to us Java trademarks)


Exam Question 10:

“…attempt to implement a new language that will offer the performance advantages of C with the safety and ease of development advantages of Java…”


C#

C# is a simple, modern, object oriented, and type-safe programming language derived from C and C++. It will immediately be familiar to C and C++ programmers. C# aims to combine the high productivity of Visual Basic and the raw power of C++.C# Language Specification (p. 15 of 403)



“C# aims to combine the high productivity of Visual Basic (Java) and

the raw power of C++.”Why is this hard?

• Garbage collection depends on:– Knowing which values are addresses– Knowing that objects without references cannot

be reached

• If your language allows direct manipulation of addresses these are impossible


Possible Solution• Require type safety

– No unchecked conversions between types, no conversions between numeric and pointer types

• Restrict what can be done with addresses: statically check that only in-object manipulations are permitted

• Microsoft’s solution:– Give up! Let programmers make sections of

code “unsafe” then they can


Unsafe in C#While practically every pointer type construct in C or C++ has a reference type counterpart in C#, there are nonetheless situations where access to pointer types becomes a necessity. For example, interfacing with the underlying operating system, …, or implementing a time-critical algorithm may not be possible or practical without access to pointers. To address this need, C# provides the ability to write unsafe code.

In unsafe code it is possible to declare and operate on pointers, to perform conversions between pointers and integral types, to take the address of variables, and so forth. In a sense, writing unsafe code is much like writing C code within a C# program.

Unsafe code is in fact a “safe” feature from the perspective of both developers and users. Unsafe code must be clearly marked with the modifier unsafe, so developers can’t possibly use unsafe features accidentally, and the execution engine works to ensure that unsafe code cannot be executed in an untrusted environment.

C# Language Specification, Appendix B


Unsafestatic unsafe void copy (byte[] src, byte [] dst, int count) { fixed (byte* pSrc = src, pDst = dst) { byte *ps = pSrc; byte *pd = pDst; for (int n = count; n != 0; n--) { *pd = *ps; pd++; ps++; } }}

Within an unsafe block, we can manipulate pointers similarly C.

What if stop-and-copy garbage collector runs while inside the loop?


Fixedstatic unsafe void copy (byte[] src, byte [] dst, int count) { fixed (byte* pSrc = src, pDst = dst) { byte *ps = pSrc; byte *pd = pDst; for (int n = count; n != 0; n--) { *pd = *ps; pd++; ps++; } }}

Fixed pins the object where it is; within the fixed block, garbage collector may not move src or dst. C# compiler will disallow assignments to pointers without the fixed.


C# - Java• Java compiles to Java

byte codes (JVML)

• Java VM runs JVML code

• Java VM (bytecode verifier) verifies safety properties of JVML code

• Designed around Internet

• C# compiles to Microsoft Intermediate Language (MSIL)

• Microsoft Common Language Runtime (CLR) runs MSIL code

• CLR verifies safety properties of MSIL

• Designed around .NET


Types• In Java:

– Primitive Types: int, char, etc. (on the stack)– Object Types: objects, arrays (on the heap)

• In C#:– Value Types: int, char, struct, etc. (on the

stack)– Reference Types: objects, arrays (on the

heap)– All types are subtypes of object (including

value types)


struct types

• Programmers can define their own types that will be stored on the stack

public struct Point { public int x, y; }

Point points[1000];In fact, the built-in primitive types (e.g., int) in C# are just struct types!


Boxing and Unboxing

• Value types need to be “boxed” before they can be used as subtypes of object:

int i = 123;object box = i; int j = (int) box;

int i = 123;object box = new int_Box(i);

int i = 123;Object box = new Integer (i); Boxing makes a copy of

a value type on the heap.Cast “unboxes”, can fail run-time type check.


Implicit Boxing and Unboxing

Vector v;int i = 23;

v.add (i); int el = (int) v.elementAt (0);

Implicit boxing to put i value in heap object


C# Examplestatic void Main(string[] args){ int i = 20; object o = i; MyInt mi = new MyInt (20); MyInt mi2 = mi; i++; mi++; Console.WriteLine (“Values: " + i + " / " + o + " / " + mi + " / " + mi2);}Values: 21 / 20 / 21 / 21

int may be a subtype of object, but assignment means something different for objects an ints!


MyInt Classclass MyInt { private int val; public MyInt (int value) { val = value; } public static MyInt operator++ (MyInt mi) { mi.val++; return mi; }

public override string ToString () { return val.ToString (); }}

You can overload operators (thisis why mi++ works)

You need the override keyword to indicatewhen a method is overridden.


Without the override

ConsoleApplication1.MyInt20

class MyInt { private int val; public MyInt (int value) { val = value; } public static MyInt operator++ (MyInt mi) { mi.val++; return mi; } public string ToString () { return val.ToString (); }}static void Main(string[] args) { MyInt mi = new MyInt (20); Console.WriteLine (mi); string s = mi.ToString (); Console.WriteLine (s);}

Calls object.ToString ()Calls MyInt.ToString ()

class1.cs(10,38): warning CS0114: 'ConsoleApplication1.MyInt.ToString()' hides inherited member 'object.ToString()'. To make the current member override that implementation, add the override keyword. Otherwise add the new keyword.


Overriding• Java

– Overriding is automatic: lots of confusion between overriding and overloading

• e.g., public boolean equals (Cell c)

– Methods declared with final cannot be overrode

• C#– Methods declared with virtual can be overridden,

must explicitly use override keyword in method header

– Methods declared without virtual can be overriden but overriding method must use the new keyword in method header

– Methods declared with sealed cannot be overriden


C# Example

static void Main(string[] args){ int i = 20; object o = i; MyInt mi = new MyInt (20); MyInt mi2 = mi; i++; mi++; Console.WriteLine (“Values: " + i + " / " + o + " / " + mi + " / " + mi2);}Values: 21 / 20 / 21 / 20

MyInt is a struct instead of a class, so it is now stored on the stack, and assignment means copying!

struct MyInt { private int val; public MyInt (int value) { val = value; } public static MyInt operator++ (MyInt mi) { mi.val++; return mi; }

public override string ToString () { return val.ToString (); }}


Other Differences• synchronized → lock

• Meaning is identical (lock is a better name)

• import → using• Meaning slightly different (more like C++ namespaces

than Java packages)

• extends → : implements → :• Exceptions

• C# can have catch without exception type (catches any exception like: catch (Exception e) …

– C# has no throws clauses in declaration, and will compile code without catch clauses


Iteration Abstraction

Vector v; //@v.elementType == \type(String)…for (Enumeration e = v.elements() ; e.hasMoreElements() ;) { System.out.println ((String) e.nextElement());}

foreach (string s in namesList) Console.WriteLine(s);

In Java:

In C#:


Getters and Setters

• In Java:– Need to define methods like getId (), getScore

(), etc. by hand

• In C#:public int Score { get { return score; } set { score = value; } }

In client code: x.score = x.score + 1; x.setScore (x.getScore () + 1);

Syntactic sugar for this in CLU.


“Sharp”, “Hash” or “Pound”?• Microsoft says: “pronounced C sharp”

– But, it is hard to call any language with unsafe and fixed “sharp”

• C# is a “hash” of C, Java and C++– But, people might think they made a “hash” of it

• The C# in a nutshell book weighs a few “pound”s – 856 pages, Java 4th edition = 992 pages, Stephen

Hawking’s “Universe in a Nutshell” = 224 pages– But, it will be awkward to rename the second edition

“Ckg in a Nutshell”

• Db = a half-tone short of the successor to C?


CLU(Liskov et. al., 1975)


CLU• Language designed by Liskov’s group in

the 1970s, focused on providing abstraction mechanisms

• Your textbook was originally called “Abstraction and Specification in Program Development” and used CLU as the main language

• CLU died due to lack of commercial compiler support– Only 2½ CLU programmers left


CLU Datatype Implementationset = cluster [t: type] is create, insert, elements rep = array[t]

create = proc () returns (set) return up (rep$new ()) end create

insert = proc (s: set, el: t) rep$addh (down (s), el) end insert

elements = iter (s: set) yields (t) for el: t in rep$elements (down (s)) do yield el end end elementsend set

No this object, need to explicitly pass in set.

Datatype implementation can have type parameters! Can’t do this in Java or C# (in C++, templates provide similar functionality very awkwardly)

Explict conversions between rep and abstract type (up and down)

Simple and elegant way to define iteration abstractions. Java has nothing (enumerations), C# has foreach for builtin arrays and ArrayList type, but you can’t define your own.


Good News / Bad News• Good News

– You now know enough to list that you know Java, C, C# and CLU on your resume!

• Bad News– No one is hiring CLU programmers– You shouldn’t want to work for anyone too

easily impressed by you knowing the othersApplicants must also have extensive knowledge of Unix, although they should have sufficiently good programming taste to not consider this an achievement.

Hal Abelson, MIT job advertisement

26 November 2002 CS 201tJ Fall 2002 32

What is the “J” for?


“Jeffersonian”


Snakes Tournament• Thursday’s class will meet in Olsson 001 (usual

time): Open to anyone, bring your cheering supporters to intimidate your opponents

• Qualification requirements:– Pass basic functionality tests– ESC/Java (don’t need to be warning-free to qualify, but

must have annotated important, checkable invariants)

• Tournaments:– Human driven snakes– Automatic snakes


Returning Exam 2

• You have a circled star on your exam: read the explanation on the exam comments carefully to interpret it!– Final cannot count against you, but it is more

“optional” for some of you than others!

• Final will be handed out Thursday, and due Tuesday, 10 December– A somewhat Jeffersonian essay question

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David Evans cs.virginia/evans