(2) c sharp introduction_basics_part_i

Editor's Notes

• #2: Introductory words: We write programs to solve problems. Now let's inspect a problem that we want to solve...
• #4: The user story describes the problem to be solved in everyday language. Now let's inspect how we can solve this problem with a program. - We are going to discuss solutions in some high level language (HLL). What is an HLL? Allows to express solutions in a more human readable way. Hardware near languages (e.g. assembly) are hardware specific and not standardized, HLLs are in opposite hardware independent and standardized (so is C#). What is a low level language? A low level language is a machine near language that works only on one type of machine (assembly), so they are often not portable.
• #5: A dialect of the Beginner's All-purpose Symbolic Instruction Code (BASIC). - The reputation-problem of BASIC programmers: the word "Beginner" in the name of the language! VB is a proprietary programming language designed by Microsoft. VB 6 was a very productive language (it provided very good means of integration on the Windows platform) and its success has been continued with VB running on .Net. The VB languages are very popular in the US. Its relative, VBA, is an approachable language for end-users and is used to automate windows applications like MS Excel. This approachability is also due to the fact that VB is case-insensitive. - Esp. beginners have often to cope with casing of variable and function names, and exactly this problem is not present in VB. VB's code is rather noisy as there exist many keywords. Typically VB-only programmers step into the "4GL-trap": "IDE, technology, framework and language is all VB!" - 4GL-developers can't see a difference here, everything is integrated.
• #6: C++/CLI is an ECMA standard (ECMA-372). C++/CLI is the only ".Net language", in which native applications can be compiled in Visual Studio. ECMA: "European Computer Manufacturers Association" in past. But meanwhile only the acronym "ECMA" or "ECMA International" is used to make the international (and no longer european) focus clear. C++/CLI (Common Language Infrastructure) is Microsoft's C++ variant (extension with some syntactical stuff) for .Net programming. BeforeC++/CLI was present, there was "Managed C++" for .Net, which is meanwhile deprecated. For the Windows Runtime (WinRT) there exists another variant of C++, which is primarily targeted to be used with COM to program apps for Windows 8. - It is called C++/CX (Component Extensions). The syntax of C++/CX is similar to the syntax of C++/CLI, but not all syntactic similar idioms are equivalent.
• #8: This programming language has a completely different but very compact syntax (in this example), in comparison to the other languages being presented. Often examples in F# are "head blowing" to show the compactest syntax (this was also done deliberately in this example). We can also code F# programs to solve real life problems with well-known control flow constructs (like if-branches and for-loops). In F# whitespaces are significant (off-side rule), if the #light compiler directive is set. Otherwise a more verbose, but whitespace-agnostic syntax has to be used (featuring keywords like begin and end). F# is a so-called functional programming language. - Programming constructs don't work like a bunch of statements, rather do they describe the result of the operation, w/o any side effect. In F# all symbols (they are even called values) are immutable by default! F# is also special as is it is very typesafe. For almost all kinds of conversions explicit conversions or casts are required. - On the other hand this is rarely required, as F# has great support for type inference. Such languages are often used in Artificial Intelligence (AI) and the programming of interpreters and compilers, for what F# provides special tools. F# is open source. F# can also be used as scripting language.
• #9: With this programming language we'll deal in this workshop. Created by Microsoft and mainly by Anders Hejlsberg in 2000. Interestingly Hejlsberg worked for Borland until he went to Microsoft in 1996. At Borland he created Turbo Pascal and Delphi. C# is an ECMA standard (ECMA-334). Meanings of the name "C sharp": a "sharp" kind of the C programming language or C with four pluses. It's the main programming language of the "language diverse" .Net platform. - Microsoft does its new stuff first in C#. Modern multi-paradigm language, a mixture of Java and C++ in syntax. C-style syntax, simple to use. Application areas: All kinds of application systems. Good acceptance, because there is a good documentation available as well as some height quality IDEs allowing Rapid Application Development (RAD). (RAD was already present in Visual Basic (VB) and Delphi, its success is continuing in C#.) MS Visual Studio (the Express edition is free) Borland Developer Studio #develop (free)
• #11: IL is the very low level language specified by the CLI. This piece of IL was generated from C# 5 code. It was generated in debug mode, which generates nop op-codes, which makes debugging easier and line numbers. The nops have been removed. IL is a platform independent, object oriented and stack-based assembly language. Translation: E.g. a C# compiler will generate IL from a piece of C# code. Then, as another part of the building procedure, the IL code will be assembled into so-called bytecode. The result is a CLI assembly. During run time the bytecode will be translated into native machine code (by a JIT compiler of the native runtime) and then it will be executed.
• #12: ECMA: "European Computer Manufacturers Association" in past. But meanwhile only the acronym "ECMA" or "ECMA International" is used to make the international (and no longer european) focus clear. ISO/IEC: International Standards Organization, International Electrotechnical Commission. There also exists a port to unix-like OS' (Unix/Linux/Mac OS) that is called "Mono".
• #13: Functional, oo and imperative. But mainly imperative (i.e. from top to bottom). We can use pointers to memory: To access Win32 APIs (written in C). Only in unsafe contexts. The syntax is similar to C: Blocks (i.e. {}). Statements, delimited by semicolons and expressions. Operators and comments Imperative execution of a program. CTS is present for programming. int, bool and string are present as keywords. Good acceptance, because there is a good documentation available as well as some IDEs. MS Visual Studio (the Express edition is free) #develop (free)
• #14: What we called member function in C++ is called method in .Net/C#. The using directive tells the compiler to use the specified namespace. The types, which are used by the program, are defined in the namespace System and the using directive makes that types (e.g. Console) known to the compiler. The namespace System contains the core .Net types. Referenced libraries (i.e. assemblies) must be added into the project. A type can reside in a namespace that makes the type unique, so that it can be qualified. The type. All assemblies have to define types in general. Here the class Program defines the type Program. Types define code that resides in methods. Methods are functions, which are presented by a type. The execution of a C# program starts in Main(). I.e. each C# application must provide a Main() method. An array of strings is passed to Main(); Main() does not return a value to the caller. What is an array? What data could be contained in that array? What data could be returned by Main()? Main() must be called w/o having an object of the type Program, therefor it is declared as static method. (Non static methods will be introduced later.) The code of methods contains statements and expressions. What's the difference? A statement consists of expressions; expressions have a type; statements end with semicolons. There also exist empty statements (a standalone semicolon). In methods' code typically new objects will be created and methods will be called on them. In this Main() method a message will be output to the console. How does console input and output work?
• #16: What keywords do you know from any programming languages? Syntax versus semantics; what's that? Grammar/keywords versus their meaning. In programming errors can occur on different "times". Can you explain this statement? The time during programming is often called "design time". (In C/C++ there also exits the link time, during which errors can happen.)
• #17: In C# multiple variables of the same type can be defined/declared in one statement.
• #18: What are primitive types? The text "Hello World" is not stored like today's date or the number 42. To work efficiently with data C# offers primitive data types, which are constrained to a specific type of information. These types are integrated into C# (as keywords), the compiler can directly deal with them, e.g. it knows how to compare them (not for string, as the string's methods need to be called). Also the CLR has integrated support for primitive types. What is a literal? A literal is a value of specific type that can be written out in source code directly. In a sense a literal is the opposite of a symbol. The type unsigned int is not CLS compliant => Do use the type int instead of unsigned int primarily. Literals? C# does not define octal literals (C/C++ and Java do). The float datatypes can't be compared with ordinary operators, because of different accuracies. Use double.Epsilon for comparisons (double.Epsilon > Math.Abs(value1 - value2)). The type double should be used primarily instead of float. Literals? In C# we can use the D/d suffix for literals of type double (not in C/C++ or Java). The bool type describes logical values. The expressions used in control structures are of type bool. Literals? The type char is an integral type. A string is composed of multiple chars. Important: string objects are immutable, instead of C++' std::strings, which are modifiable! string-operations create new string objects. C# knows some escape characters and provides verbatim strings. strings and chars are unicode based (having a size of 2B). For us as learners of the C# language the usage of the keywords var and dynamic is forbidden. Type conversion: A type conversion from a smaller type to a greater type works implicitly, if the types are "related" (char -> int). A type conversion from a greater type to a smaller type must be done explicitly, and is only allowed, if the types are "related" (int -> char). This casting must be done explicitly, because there is the chance that data will be lost on that conversion from a greater to a smaller type. The class Convert provides some methods, with which some conversions of non-related types can be performed. The Parse() and ToString() methods of the primitive types can be used to convert numeric or logical values from or to strings. The checked context allows to check an overflow during the conversion of values. Why is it needed to check for overflow? Why is it dangerous? In C# arithmetic operations are only possible with 32b and 64b values. byte b = 100; b = (byte) (b + 200); The variable b and the value 200 will be converted to 32b (int) values each, then the addition will be executed and then that result must be explicitly cast to a byte. The sum (the value 300) of 32b will not fit into byte, so a part of 300 will overflow (the most significant bits of the result will be discarded) and 44 will make up the result of b. With the expression checked((byte)(b + 200)), this overflow wouldn't happen, instead an OverflowException would be thrown! Often overflows are errors (but sometimes it is part of the algorithm, e.g. for hash calculation). The keyword checked will replace overflowing operators with throwing ones (add -> add.ovf in this example (only for add, mul, sub and conv)), nothing else. We can activate checking for arithmetic overflow and underflow for the whole VS project w/o using the checked keyword (then checked is the default for arithmetic operations). - This option is not activated when we create a new VS project. Only integral and the decimal will overflow, other floating point types can handle infinity.
• #20: What are binary operators? These operators have an arity of two, i.e. they have two operands. The operator is written in between the operands (infix notation). Which other binary operators do you know? What are unary operators? These operators have one operand. There exist prefix and postfix operators. Increment and decrement operators -> useful in loops! Apple's Swift programming language guide does explicitly suggest to use prefix increment/decrement by default, unless the specific behavior of the postfix variants is required. (This could be understood like Apple counts prefix increment/decrement as "primary operator".) What are ternary operators? The decision operator ?: (sometimes we call it "Elvis operator").
• #22: What does that mean "integral division has an integer result"? I.e. the results are no floating point values! Integral division: How to do that in a correct way? Use casts or the correct literals on/for any of the operands! The division by 0 is not "not allowed" in maths, instead it is "just" undefined. What does the operator % do? This operator doesn't calculate the modulus, instead it calculates the remainder! Why do we need bitwise operators?
• #23: What does it mean: "Strings are immutable"? C# doesn't define a comma-operator. As we see, C# doesn't provide a delete operator! - Heap memory is managed automatically by a garbage collector! Many operators can be overloaded for user defined types in C#. We can't define new operators. We can't modify arity, placement, precedence or associativity of operators. The programming language Haskell allows defining new operators and it allows modifying the precedence and associativity (together called "fixity" in Haskell) of present operators. Opinion [NLu]: don't overload operators: it is never required in C# and it is often done wrong, or at least in a questionable way (In C++, overloading of canonical operators is often required for UDTs. - In C# it is only syntactic sugar and not required for a canonical form of a type.). E.g. the +-operator is overloaded for string to do string-concatenation. - On the other hand the +-operator means addition, which is commutative (one of the minimum requirements of addition in maths), but string-concatenation is never commutative! (The ,-operator (as sequence operator) would make more sense for string-concatenation.)
• #24: What is short circuit evaluation?
• #25: What is precedence? What is associativity? Why is it relevant? E.g. if we have a couple of method calls that we combine with the + operator, we'll have to know which method will be called first, because the methods could have side effects! Rules of the thumb: Binary operators are generally left associative. Unary operators, the ternary operator and the assignment operators are right associative. What is order of execution? Why is it relevant? E.g. if we have an expression like h(g(), f()) or d() * s() how do we know, which functions are being called first? In C# we know that d() is called before s() is called (left -> right), in the other expression first g() and f() are called and then h() is called (inner -> outer). It is relevant to know that, because the function calls can have side effects! The order of execution is undefined in C++!