Chapter 3, Lesson 2: Encoding, decoding.

When is manual control of encoding required:

• When interoperating with legacy/UNIX sys.
• r/w files in other languages.
• ASCII - 0-127 - 7-bit byte.
• for languagues other than English - 128-255 - 8-bit byte.
• code pages - ASCII for 0-127, and language specific values for 128-255.
• code page - a list of character codes (code points) in a certain order.
• code pages - support specific languages.
• Windows code pages - 256 code points + zero-based.
• Unicode - a massive code page with 10000+ characters
• stds for encoding Unicode - UTF-16, UTF-8,...
• overloaded Stream constructor - new StreamWriter("abc.txt", false, Encoding.Unicode);

Ch3, lesson 1 - Forming Regular Expressions.

• Text processing - pattern matching, (sub)string extraction, (sub)string replacement.

Pattern Matching

• namespace - System.Text.RegularExpressions.
• statc mthd - System.Text.RegularExpressions.Regex.IsMatch(,);
• ^,$ - begin, end.
• \b - word boundary.
• \B - match NOT on a \b boundary.
• @ - backslashes are treated literally.
• * >=0
• + >=1
• {n} - repeat previous char n times.
• {n,m} - repeat prev char bet n & m times.
• {n,} - repeat previous char min n times.
• ? - prev. char optional.
• . - single char.
• [] - character class/range - e.g. [0-9] = \d
• \d - numeric digit, \D - non-numeric char - [^0-9]
• \s - white-space char, \S - non-white space char
• \w - word char = [a-zA-Z0-9_], \W - [^a-zA-Z0-9_]
• () - to match group of chars
• (?pattern) - name a group - to refer to matched data later.
• backreferencing - to find repeating groups of characters.
• (?\w)\k - finds doubled word characters.
• \1 - 1st backreference in a reg expr, \2 - 2nd backref in regex,...
• extract matched data - access elts of Match>Groups array.
• Replace substrings using reg expr...

Ch2, lesson 4 - Working with Isolated Storage

• Bad idea to give pgs unfettered access to a computer. THE SANDBOX OF LIMITED SECURITY.
• Q) Where do pgs that need to persist state go?
• A) Isolated Storage (IS). Now pgs can run as least-priveleged users + persist state without having to test whether app has enough rights to save data to the hard drive.
• Main benefit of isolated storage - app will run regardless of whether runing under partial, limited or full-trust.

How to Create a Store

• Before saving to isolated storage, one must determine how to scope the data in the store:
• mthd 1 - Assembly/Machine - creates a store that is specific to the calling assembly & lcl machine. Useful for application-level data.
• mthd 2 - Assembly/User - creates a store that is specific to the calling assembly & current user. Useful for user-level data.
• create assembly/machine store - IsolatedStorageFile machineStore = IsolatedStorageFile.GetMachineStoreForAssembly(); This IS is specific to the assembly that is calling it.

IsolateStorageFileStream

• encapsulates stream to read, write and create files in IS.

R/W

• r/w -> IS like r/w any other data into the file system.
• 1. Create a Store - IsolatedStorageFile userStore = IsolatedStorageFile.GetUserStoreForAssembly();
• 2. Create a Stream - IsolatedStorageFileStream userStream = new IsolatedStorageFileStream("UserSettings.set", FileMode.Create, userStore);

Ch2, lesson3 - Compressing Streams.

• save space or bandwidth by compressing data.
• I/O system - 2 mthds for compressing data: GZIP, DEFLATE.
• both mthds - lmtd to compress 4GB of uncompressed data.
• GZipStream, DeflateStream classes.
• GZipStream Vs DeflateStream - GZIP has header. Header might be useful when decrompressing with gzip tool. If internal - DeflateStream file slightly smaller. If external - gzip, as may need header.
• CompressionStreams - write to another stream, not a resource (e.g. a file or memory).
• CompressionStream - wraps the stream that contains (or will contain) compressed data.
• 1. Open file to be compressed + file to be written to - FileStream sourceFile = File.OpenRead(inFileName);
• 2. wrap outgoing stream - GZipStream compStream = new GZipStream(destFile, CompressionMode.Compress);
• 3. read data from source & feed -> compression stream -
• int theByte = sourceFile.ReadByte();
• while (theByte != -1)
• {
• compStream.WriteByte.((byte)theByte);
• theByte = sourceFile.ReadByte();
• }

Ch2, lesson2 - Reading & Writing Files.

• All stream classes derive from abstract class Stream.
• File class - static mthds for opening (r/w) & creating files.
• File class - can return FileStream obj or StreamReader, StreamWriter.
• FileStream - basic func for opening file streams for reading & writing.
• StreamReader/StreamWriter - easier. Reads stream as string.
• File - Open(), OpenText(), Create(), CreateText(). ...Text() ret StreamReader.
• StringReader/Writer - read to-fro in-memory strings.
• BinaryReader/Writer - handle binary data to/fro streams.
• MemoryStream - to create in-memory streams. Used where need to create a stream before really need to store it.
• MemoryStream - can wrap MemoryStream obj in StreamWriter to write to MemoryStream.

Ch2, Lesson1 - Navigating the File System.

What are File System Classes?

• System.IO - Set of classes to manipulate files, directories & drives.
• file sys classes - informational & utility.
• Informational - derive from FileSystemInfo base class. -> FileInfo, DirectoryInfo.
• DriveInfo - not derived from FileSystemInfo.
• Utility classes - provide static mthds to perform ops - incl File, Directory & Path classes.
• Path - provides static mthds for manipulating a file system path.
• FileSystemWatcher - provides mthds for monitoring file system directories for changes.
• FileSystemWatcher - poss. to get more events generated than can be handled. FileSystemWatcher then throws Error event, which can be captured (by registering for it + turning events on).

• 
  

Lesson 4: Converting bet. types.

Conversion in C#

• narrowing conversion - prohibited.
• widening conversion - allowed.

Ways to Perform Explicit Conversion

• System.Convert - bet. types that impl. IConvertible interface.
• (type) - cast operator - bet. types that define conversion operators.

Boxing/Unboxing

• boxing - converts a value type to ref type. (e.g. convert int -> Object)
• unboxing - ref -> value.
• boxing/unboxing - incur overheads.
• boxing - occurs on calling virtual mthds that structure inherits from System.Object - e.g. ToString().
• Best Practices:
  1. Impl. overloads for mthds that accept mult. value types. Better to create mult. overloads than 1 proc taking Object.
• 2. Use generics instead of accpeting Object args.
• 3. Override ToString, Equals, GetHash virtual members when defining structures.

How to Implement Conversion in Custom Types

• conversion operators - for narrowing ( k'word explicit)/widening (k'word implicit) conversions bet. numeric types.
• Override ToString() to provide conversion to strings, and override Parse() to provide conversion from strings.

(Real) Lesson 3: Constructing Classes

What is Inheritance?

• Inheritance allows one to create new classes from existing ones, and use said derived instance as if one were using the base instance.
• Derived classes can be used interchangeably. i.e. where a method takes a paramter of a given type, an object of any of its derived types can be passed in.

What is an interface?

• Interface - aka 'contract'.
• Defines a common set of 'members' that all classes that implement said interface, must provide.
• Within the interface declaration, define the members. e.g IDisposable provides the Dispose() mthd, to free up resources. ICloneable - supports copying an object.
• Classes can implement multiple interfaces.
• Visual Studio has a shortcut to extract an interface - Refactor -> Extract Interface.

What are Partial Classes?

• Allows one to split a class definition across multiple source files.
• Benefit - hides detail. Allows derived class writers to focus on that which is significant.

What are Generics?

• Part of the .NF type system
• Enables type-definition while leaving some details unspecified.
• Instead of specifying the types of parameters or member classes, you allow code that uses your type to specify it. Thus consumer code can tailor a type to its own needs.
• .NF 2.0 - System.Collections.Generic namespace - incl. Dictionary, Queue, SortedDictionary, SortedList...
• Above classes have non-generic counterparts in System.Collection - but they offer improved performance and type safety.

Why use Generics?

• In .NF 1.1 - developers used Object class as generic param, and then cast other classes to/fro Object. Generics offer 2 advanatges over this approach:
• 1. reduced runtime errors - compiler cannot catch types errors resulting from casting to/fro Object.
• 2. Improved performance - casting requires 'boxing & unboxing' which slows performance. Generics doesn't require casting or boxing.

How to Create a Generic Type

class Gen

{

public T t;

public U u;

public Gen(T _t, U _u)

{

t = _t;

u = _u;

}

}

• The consuming code will determine the types of T & U.
• Limitation in creating a generic type/class - code valid only if it will compile for every possible constructed instance. Thus you are limited to the capabilities of the base Object class.

How to Consume a Generic Type

• When you consume, you must specify the types for any generics used.

Constraints

• Generics would be v.limited if one could only write code that would compile for any consuming class, because we would then be limited to the capabilities of base Object class.
• Constraints - overcome this limitiation.
• Constraints - place requirements on the types that consuming code can sub for generic.
• 4 types of constraint:

1. Interface - Only allow types that impl. specific interface(s) to use generic.

2. Base class - Only allow types that match/inherit from a specific base class to use gen.

3. Constructor - Requesires types that use gen. to implement a parameterless constructor.

4. Ref. or value type - requires types that use gen. to be either a ref. or value type.

class CompGen

where T : IComparable

{

.

.

.

public T Max()

{

if( t2.CompareTo()t1) <>

return t1;

}

}

Events - what is an event?

• Event - a msg sent by a object to signla the occurance of some action.
• event sender - obj that raises the event.
• event receiver - obj that captures/responds to the event.

What is a Delegate?

• Acts as an intermediary bet event sender class & receiving obj.
• delegate - a class that holds a ref to a mthd.
• delegate class - unlike other classes, it has a signature. And it holds refs to mthds that match its signature.
• delegate decl. - supplies signature of the delegate.
• std signature of an event handler delegate - no ret val., 1st param type Object - refers to instance that raises event. 2nd param derived from EventArgs - holds event data.
• public delegate void AlarmEventHandler(object sender, EventArgs e);
• If event generates data - 2nd param custom type derived from EventArgs.
• associate event with mthd that will handle event - add instance of the delegate to the event.

How to respond to an Event

• 1. Create mthd to respond to event. mthd must match delegate signature.
• 2. Add event handler - to indicate which mthd should receive events.

How to raise an Event

• 1. create a delegate - public delegate void MyEventHandler(object sender, EventArgs e);
• 2. create an event member - public event MyEventHandler MyEvent;
• 3. Invoke the delegate within a mthd when need to raise event:

What are attributes?

• Attributes describe a type, method or property in a way that can be programmatically queried ~ 'reflection'. Use attributes to:
• 1. specify security priveleges for a class - e.g. needing to read a particular file. Runtime will thus throw an exception <>
• Describe an assembly - title, description...

•