.NET Fundamentals

November 13, 2003

Week 2

Class Agenda – November 13, 2003

Questions / Homework?

C#

Windows Forms

Simple Calculator Class Exercise (part 1)

More Windows Controls

Clock Class Exercise

MessageBox / Exceptions in .NET

Simple Calculator Class Exercise (part 2)

Homework Assignment

Questions?

• Common Email List

• Homework Packaging / winzip project

Homework

• Add name near or at the top of your code

your name – Homework Week 1 – date

• Zip entire project

• I will acknowledge receipt of homework

Class Goals

Class Goal #1

Provide an overview of the .NET Architecture and it’s Major Components– Programming Languages– ADO.NET– ASP.NET– Web Services– XML Integration

Class Goal #2

Understand the .NET Frameworks as an Object Oriented, Strongly Typed, Computing Environment

Class Goals #3

Work with various .NET Framework components– Common Language Runtime (CLR)– .NET Framework Class Library (FCL)– Assemblies– Strong Names– The Global Assembly Cache (GAC)– ADO.NET– ASP.NET– Web Services

Class Goals #4

Develop a basic fluency programming in C#

Course Schedule

Week Topics

1 Introduction to .NET Framework, C#

2 C#, Windows Forms Programming, Exceptions

3 Introduction to Framework Class Library

Types in .NET

Objects and Object Construction

Boxing and Unboxing

Interfaces

Week Topics

4 ADO.NET (managed data access)

- Connections

- Adapters

- Data Sets

- Grid Control

5 Bootstrapping the Common Language Resource (CLR)

Modules and Assemblies

.Net Packaging

Public and Private Keys

Shared Assemblies

The Global Assemble Cache (GAC)

Strong Names

Week Topics

6 ASP.NET

Introduction to Web Services

Web Security

Advanced Concepts:

- Serialization

- Reflection

Web Services Class Project

7 Web Services Class Project (continued)

8 Web Services Class Project (continued)

C#

Data Types

A word on types

• All types in .NET derive from System.Object• They all provide implementations of ToString()

and GetType()• To get a string with the type of any variable, you

can call <var>.GetType()• Whenever you call Console.WriteLine(obj) the

ToString() method on obj is implicitly called. The default ToString implementation for classes simply returns the name of the class.

Primitive Types

C# Type .NET Framework type

bool System.Boolean

byte System.Byte

sbyte System.Sbyte

char System.Char

decimal System.Decimal

double System.Double

float System.Single

Primitive Types (continued)

C# Type .NET Framework type

int System.Int32

uint System.UInt32

long System.Int64

ulong System.UInt64

object System.Object

short System.Int16

ushort System.UInt16

string System.String

Boolean

bool bTmp;

bTmp = True;

bool bTmp = False;

Boolean variables are stored as 16-bit (2-byte) numbers, but they can only be True or False. Use the keywords True and False to assign one of the two states to Boolean variables.

Integer Data Types

Integer variables are stored as signed 32-bit (4-byte) integers ranging in value from -2,147,483,648 through 2,147,483,647.

The Integer data type provides optimal performance on a 32-bit processor, as the smaller integral types are slower to load and store from and to memory. (Int32)

Short variables are stored as signed 16-bit (2-byte) integers ranging in value from -32,768 through 32,767. (Int16)

Long variables are stored as signed 64-bit (8-byte) integers ranging in value from -9,223,372,036,854,775,808 through 9,223,372,036,854,775,807. (Int64)

Defining Integers

int i;

int i, j, k;

int i = 12;

j = i; j is now equal to 12

i = 15;

k = i + j; k is equal to 27

To write an Integer, convert it to a String using:

k.ToString();

Floating Point Data Types

Double variables are stored as signed IEEE 64-bit (8-byte) double-precision floating-point numbers ranging in value from -1.79769313486231570E+308 through -4.94065645841246544E-324 for negative values and from 4.94065645841246544E-324 through 1.79769313486231570E+308 for positive values.

Single variables are stored as signed IEEE 32-bit (4-byte) single-precision floating-point numbers ranging in value from -3.4028235E+38 through -1.401298E-45 for negative values and from 1.401298E-45 through 3.4028235E+38 for positive values. Single-precision numbers store an approximation of a real number.

Double.01, 12345.424, -10.0, 1235000.9999, 125.75

double dTmp;dTmp = 12.625;

double dTmp = 12.625;

Double variables are stored as signed IEEE 64-bit (8-byte) double-precision floating-point numbers ranging in value from -1.79769313486231570E+308 through -4.94065645841246544E-324 for negative values and from 4.94065645841246544E-324 through 1.79769313486231570E+308 for positive values.

Number Formatting

Returns a string formatted according to instructions contained in a format String

expression.

double aDbl;

aDbl.ToString("..."); << insert formating

Sample Double Formatting Code

double aDbl = 1234.567;

MyStr = aDbl("##,##0.000") ' Returns "1,234.567".

MyStr = aDbl("###0.00") ' Returns "1234.57".

Strings

Fundamentals of Strings

Strings– A series of characters treated as a single unit– String literals “How are you?”– Objects of class String

How do we define strings?

string strTmp;

strTmp = “time will tell”;

string strTmp = “time will tell”;

strTmp = Console.ReadLine();

string strTmp2;

strTmp2 = strTmp;

strTmp2 “time will tell”

Concatenating Strings

string strCity = “Calais”;

string strState = “ME”;

string strZip = “04270”;

string strLoc;

strLoc = strCity + “, ” + strState + “ ” + strZip;

strLoc “Calais, ME 04270”

String Functions

string strTmp;

strTmp.Trim(); – removes leading and trailing spaces

strTmp.ToUpper(); – converts string to all upper case

strTmp.ToLower(); – converts string to all lower case

strTmp.Length; – returns string length as an integer

strTmp.SubString() – extracts a substring

String Function Examples

string strTmp = “ Hello World ”;

strTmp.Trim();

strTmp “Hello World”

string strTmp = “Hello World”;

strTmp.ToLower(); “hello world”

strTmp.ToUpper(); “HELLO WORLD”

String.Length Function

string strTmp;

strTmp = “in the beginning”;

The value of strTmp.Length is 16.

int i;

i = strTmp.Length;

The value of i is 16.

String.SubString() FunctionString.Substring(startIndex , length );

Parameters (are Integers)

startIndex – Where the substring starts. startIndex is zero-based.

length – The number of characters in the substring.

Substring Examplesstring strTmp;

strTmp = “around the world”;

strTmp.Substring(0,6); “around”

strTmp.Substring(11,5); “world”

strTmp.Substring(0,strTmp.Length);

“around the world”

Strings and StringBuilder Class

Strings and StringBuilder Class

• String processing– Useful in a variety of applications– String classes (System)

• General string processing, storage

– StringBuilder class (System.Text)• Facilitates efficient construction of strings

Using the StringBuilder Class 

The String object is immutable. Every time you use one of the methods in the System.String class, you create a new string object in memory, which requires a new allocation of space for that new object. In situations where you need to perform repeated modifications to a string, the overhead associated with creating a new String object can be costly. The System.Text.StringBuilder class can be used when you want to modify a string without creating a new object. For example, using the StringBuilder class can boost performance when concatenating many strings together in a loop.

Create a new instance of the StringBuilder class

You can create a new instance of the StringBuilder class by initializing your variable with one of the overloaded constructor methods, as illustrated in the following example.

StringBuilder MyStringBuilder = new

StringBuilder("Hello World!");

Setting the Capacity and Length

Although the StringBuilder is a dynamic object that allows you to expand the number of characters in the string that it encapsulates, you can specify a value for the maximum number of characters that it can hold. This value is called the capacity of the object and should not be confused with the length of the string that the current StringBuilder holds. For example, you might create a new instance of the StringBuilder class with the string "Hello", which has a length of 5, and you might specify that the object has a maximum capacity of 25. When you modify the StringBuilder, it does not reallocate size for itself until the capacity is reached. When this occurs, the new space is allocated automatically and the capacity is doubled. You can specify the capacity of the StringBuilder class using one of the overloaded constructors. The following example specifies that the MyStringBuilder object can be expanded to a maximum of 25 spaces.

StringBuilder MyStringBuilder = new StringBuilder("Hello World!",5);

Additionally, you can use the read/write Capacity property to set the maximum length of your object. The following example uses the Capacity property to define the maximum object length. MyStringBuilder.Capacity = 5;

The EnsureCapacity method can be used to check the capacity of the current StringBuilder. If the capacity is greater than the passed value, no change is made; however, if the capacity is smaller than the passed value, the current capacity is changed to match the passed value.

The Length property can also be viewed or set. If you set the Length property to a value that is greater than the Capacity property, the Capacity property is automatically changed to the same value as the Length property. Setting the Length property to a value that is less than the length of the string within the current StringBuilder shortens the string.

StringBuilder.methods

Method name Use

StringBuilder.AppendAppends information to the end of the current StringBuilder.

StringBuilder.AppendFormatReplaces a format specifier passed in a string with formatted text.

StringBuilder.InsertInserts a string or object into the specified index of the current StringBuilder.

StringBuilder.RemoveRemoves a specified number of characters from the current StringBuilder.

StringBuilder.ReplaceReplaces a specified character at a specified index.

Append

•The Append method can be used to add text or a string representation of an object to the end of a string represented by the current StringBuilder. The following example initializes a StringBuilder to "Hello World" and then appends some text to the end of the object. Space is allocated automatically as needed.

StringBuilder MyStringBuilder = new StringBuilder("Hello World!");

MyStringBuilder.Append(" What a beautiful day.");Console.WriteLine(MyStringBuilder);

This example displays Hello World! What a beautiful day to the console.

AppendFormat • The AppendFormat method adds text to the end of the StringBuilder,

but also implements the IFormattable interface and therefore accepts the standard format strings described in the formatting section. You can use this method to customize the format of variables and append those values to a StringBuilder. The following example uses the AppendFormat method to place an integer value formatted as a currency value at the end of a StringBuilder.

int MyInt = 25;

StringBuilder MyStringBuilder = New StringBuilder("Your total is ");

MyStringBuilder.AppendFormat("{0:C} ", MyInt);

Console.WriteLine(MyStringBuilder);

This example displays Your total is $25.00 to the console.

Insert

•The Insert method adds a string or object to a specified position in the current StringBuilder. The following example uses this method to insert a word into the sixth position of a StringBuilder.

StringBuilderMyStringBuilder = New StringBuilder("Hello World!");

MyStringBuilder.Insert(6, "Beautiful ");Console.WriteLine(MyStringBuilder);

This example displays Hello Beautiful World! to the console.

Remove

You can use the Remove method to remove a specified number of characters from the current StringBuilder, beginning at a specified zero-based index. The following example uses the Remove method to shorten a StringBuilder.

StringBuilder MyStringBuilder = New StringBuilder("Hello World!");

MyStringBuilder.Remove(5, 7);Console.WriteLine(MyStringBuilder);

This example displays Hello to the console.

Replace The Replace method can be used to replace characters within the StringBuilder object with another specified character. The following example uses the Replace method to search a StringBuilder object for all instances of the exclamation point character (!) and replace them with the question mark character (?).

StringBuilder MyStringBuilder = New StringBuilder("Hello World!");

MyStringBuilder.Replace("!"c, "?"c);Console.WriteLine(MyStringBuilder);

This example displays Hello World? to the console.

Iteration Statements

Iteration statements• For loops

– for ([initializers]; [expression]; [iterators]) statement

e.g.for (int i = 1; i <= 5; i++)

{ … }

• While loops– while (expression) statement

e.g. while (i<=5) {…}

• expression has to evaluate to boolean.• Other iteration statements: do, foreach

Selection Statements

C# - Selection Statements

• If statementif (expression) statement1 [else statement2]

– Both statement1 and statement2 themselves can be if statementsif (a <b) // {} has to be used for multiline

// statements{

Console.WriteLine(“Less Than”);… //do something else

} else

Console.WriteLine(“Greater Than”);

if / else if / else

if (some condition){

do something… }else if (some other condition){

do something… } else{

do something…

}

Jump Statements

• break

• continue

• default

• goto

• return

break Statements

• The break statement terminates the closest enclosing loop or switch statement in which it appears. Control is passed to the statement that follows the terminated statement, if any. This statement takes the following form:

• break;

continue Statements

• The continue statement passes control to the next iteration of the enclosing iteration statement in which it appears. It takes the following form:

• continue;

default Statement

switch (expression) { case constant-expression:

Statementjump-statement

[default: statement jump-statement]

}

goto Statements

• The goto statement transfers the program control directly to a labeled statement. It takes one of the following forms:

• goto identifier; goto case constant-expression; goto default; where:

• identifier • A label.

• constant-expression • A switch-case label.

return Statements

• The return statement terminates execution of the method in which it appears and returns control to the calling method. It can also return the value of the optional expression. If the method is of the type void, the return statement can be omitted. This statement takes the following form:

• return [expression];

switch Statements• More Elegant Than the If Else Construct• Begin with the words Switch followed by the

name of the target variable• For each value or range of values for the target

variable a separate case in listed.• Each case begins with the word case followed by a

specific value or range of values for that case.• Following each case statement is one of more lines

of code to be executed if that case is true.• An default case is often provided to catch any

cases that do not match the target variable.

C# - switch Statements• Switch statementswitch (expression) {

case constant-expression: statement jump-statement

[default: statement jump-statement]

}

• Unlike C++ and C, a jump statement is required after a statement – fall through not allowed.– Jump statement: break, continue, goto, return

C# - switch Statements

• This is allowed: switch(n)

{ case 0: case 1: cost += 25; break; …..

}

C# - switch Statements• But not this ! switch(n) { case 0:

Console.WriteLine(“Falling through 0”);

case 1: cost += 25; break; …

}

• Can switch on strings as well: case(“a”): …

C# Operators

C# Operators

• Usual slew of operators– +, -, *, /, +=, -= ……

• Interesting operators:– new– is, as– sizeof, typeof– checked, unchecked

Creating new objectsA call to new is required to access any non-static public member.

namespace HelloCall {class HelloClass {

static void Main(string[] args) {Console.WriteLine("In Hello Main");HelloClass foo = new HelloClass();foo.OtherCall();

}void OtherCall() {

Console.WriteLine("In OtherCall");}

}}

methods

methods

• Provide the ability to transfer control within a program.

• Simplify code sharing, providing an excellent way to eliminate redundant code.

• methods maybe called with one or more arguments or with no arguments at all

• methods maybe called with a return type or a return type of void to indicate no return type.

methods

Defining

private void aMethod()

{

insert code here;

}

Calling

aMethod();

methods with Arguments

private void aMethod(string strTmp)

{ … }

aMethod(“hi there”);

private void aMethod(string strTmp, int iTmp)

aMethod(“hi there”, 4);

methods with Arguments (continued)

private void aMethod(string strTmp, int iTmp)

{ … }

string strTmp = “Hello World”;

int iTmp = 14;

aMethod(strTmp, iTmp);

methods with return values

private bool AnyMethod(string strTmp, int iTmp)

{ … }

string strTmp = “Hello World”;

int iTmp = 14;

bool bRet;

bRet = AnyMethod(strTmp, iTmp);

Scope

Scope

When a variable is defined the computer assigns memory to that variable. When the variable is no longer needed the computer deletes the variable to free up the memory. Scope defines the time a variable is available to a program.

Scope

A variable defined in a method or in a for statement will be deleted when the method or for statement is exited.

Variables that need to persist longer than the life of the method need to be defined outside the method , at the class or module level.

Windows Forms

Introduction to Windows Forms

• Windows Form

• Using ToolBox to add Controls to Form

• Label Control

• Button

• if else (else if)

• Using F1 or Search for Help

What Are Controls?

• Controls are things like TextBoxes, Buttons, and Labels that you use to build a form.

• When you create Windows Form project the IDE provides a blank form.

• Controls are added to the form to provide functionality

What are properties?

• Forms Have Properties

• Controls Have Properties

• Properties are used to specify controls.

• Text Boxes have Font properties

• Fonts have size and color and bold and …

• Labels have a Text property to describe the label

What About Events?

• When you click on a button the IDE automatically creates a subroutine where you can write the code you wish executed when the button is pushed. This event is called an Click event.

private void button8_Click(...)

Windows Form

A Couple of Properties• ActiveForm points to current form• BackColor Property sets background color

To set a BackColor:this.BackColor = Color.Red;

To Test For a BackColor:if (this.BackColor.Equals(Color.Red)) { ... }

Use ToolBox to add Controls to Form

• Showing the ToolBox

• Selecting a Control

• Configuring the Control

Label Control

Windows Forms Label controls are used to display text or images that cannot be edited by the user, but can be edited dynamically by the programmer.

–BackColor defaults to Form–ForeColor

Label1.ForeColor = Color.White;

Note: Font changes must be made directly to the control on the form.

Button Control

The Windows Forms Button control allows the user to click it to perform an action.

button1.Text = “Push Me”;

button1.BackColor = Color.Blue;

button1.ForeColor = Color.Yellow;

Using F1 or Search for Help

• To Use Visual Studio.NET Help function, you can either:– Place the cursor on the control or command in

question and type an F1

or– Bring up the Help Search Window

Help Search– Enter the desired topic

Simple Calculator (Part 1)

RadioButton and GroupBox Control

RadioButtons in a GroupBox

RadioButton

Windows Forms RadioButton controls present a set of two or more mutually exclusive choices to the user. While radio buttons and check boxes may appear to function similarly, there is an important difference: when a user selects a radio button, the other radio buttons in the same group cannot be selected as well. In contrast, any number of check boxes can be selected. Defining a radio button group tells the user, "Here is a set of choices from which you can choose one and only one."

When a RadioButton control is clicked, its Checked property is set to true and the Click event handler is called. The CheckedChanged event is raised when the value of the Checked property changes. If the AutoCheck property is set to true (the default), when the radio button is selected all others in the group are automatically cleared. This property is usually only set to false when validation code is used to make sure the radio button selected is an allowable option. The text displayed within the control is set with the Text property, which can contain access key shortcuts. An access key allows a user to "click" the control by pressing the ALT key with the access key.

GroupBox Control

Windows Forms GroupBox controls are used to provide an identifiable grouping for other controls. Typically, you use group boxes to subdivide a form by function. For example, you may have an order form that specifies mailing options such as which overnight carrier to use. Grouping all options in a group box gives the user a logical visual cue, and at design time all the controls can be moved easily — when you move the single GroupBox control, all its contained controls move, too.

Sample Code

if (RadioButton1.Checked)

{ ... }

else if (RadioButton2.Checked)

{ ... }

else if (RadioButton3.Checked)

{ ... }

else if (RadioButton4.Checked)

{ ... }

Else

{

MessageBox.Show("No Radio Button Checked!", "Missing Radio Button", MessageBoxButtons.OK, MessageBoxIcon.Warning);

}

ComboBox

ComboBox

• ComboBox– Combines TextBox features with drop-down

list– Drop-down list

• Contains a list from which a value can be selected

• Scrollbar appears if necessary

ComboBox events and properties Description / Delegate and Event Arguments Common Properties DropDownStyle Determines the type of combo box. Value Simple means that

the text portion is editable and the list portion is always visible. Value DropDown (the default) means that the text portion is editable, but the user must click an arrow button to see the list portion. Value DropDownList means that the text portion is not editable and the user must click the arrow button to see the list portion.

Items The collection of items in the ComboBox control. MaxDropDownItems Specifies the maximum number of items (between 1 and 100)

that the drop-down list can display. If the number of items exceeds the maximum number of items to display, a scrollbar appears.

SelectedIndex Returns the index of the selected item. If there is no selected item, -1 is returned.

SelectedItem Returns a reference to the selected item. Sorted Indicates whether items are sorted alphabetically. Setting this

property’s value to True sorts the items. Default is False. Text Value currently displayed in the ComboBox. Common Event (Delegate EventHandler, event arguments EventArgs) SelectedIndexChanged Generated when the selected index changes (such as when a

different item is selected). This is the default event when control is double-clicked in designer.

Sample Code To Access Value

If no value is selected from a ComboBox the value of ComboBox.Text = ""

Otherwise the selected value is in ComboBox.Text

Sample Code – To Insert

int i;

bool bFound = false;

iAdded = Label5.Text.Length / 2;

For (int i = 0; i < ComboBox1.Items.Count - 1; i++) {

If (ComboBox1.Items(i) == iAdded.ToString) {

bFound = True;

break;

}

}

if (!bFound) {

ComboBox1.Items.Add(iAdded.ToString);

}

'ComboBox1.Sorted = true;

Sample Code – To Remove

for (int i = 0; i<ComboBox1.Items.Count; i++) {

if (ComboBox1.Items(i) == iAdded.ToString) {

ComboBox1.Items.RemoveAt(i);

break;

}

}

iAdded = 0;

The Use of Date and Time

• DateTime Data Type

• DateTimePicker and Timer Controls

• Format and DateDiff Functions

DateTime Data Type

• DateTime Structure

• Represents an instant in time, typically expressed as a date and time of day.

DateTime Member Functions

• NowDim today As DateTime = Now;

• Year, Month, Day, Hour, Minute, SecondDim Year As Integer = today.Year;Dim Minute As Integer = DateTime.Now.Minute

• DateTime.Now.ToLongDateStringLabel5.Text = DateTime.Now.ToLongDateString();

• DateTime.Now.ToLongTimeStringLabel5.Text = DateTime.Now.ToLongTimeString();

DateTimePicker Control

• Tool for Selecting a Date

• Returns Date Selected as a DateTime structure

DateTimePicker Control

The Windows Forms DateTimePicker control allows the user to select a single item from a list of dates or times. It appears in two parts: a drop-down list with a date or time represented in text, and a grid that appears when you click on the down-arrow next to the list. The grid looks like the MonthCalendar control, which can be used for selecting multiple dates. It returns a DateTime structure.

DateTimePicker Sample Code

DateTime aDT = DateTimePicker1.Value;

Timer Control

The Windows Forms Timer is a component that raises an event at regular intervals. This component is designed for a Windows Forms environment.

Timer Control Sample Code

Timer Properties

– Enabled True or False

– Interval The number of milliseconds between each timer tick. The value is not less than one. To get the number of seconds in the interval, divide this number by 1,000.

private void Timer1_Tick(object sender, System.EventArgs e)

{

Label3.Text = DateTime.Now.ToLongTimeString();

Label5.Text = DateTime.Now.ToLongDateString();

}

TimeSpan Structure

TimeSpan ts = DateTimePicker2.Value - DateTimePicker1.Value;

difHours = ts.TotalHours;

Clock Class Exercise

MessageBox

MessageBox Class

Displays a Dialog box that can contain text, buttons, and symbols to inform and instruct the

user.

Displays a MessageBox using the Question icon and specifying the No button as the default.

Result = MessageBox.Show(

this, _ Active Form {Optional}

Message, _ Message to display

Caption, Caption for MessageBox

MessageBoxButtons.YesNo, _ Buttons MessageBoxIcon.Question Icon);

Displays a message box with specified text, caption, buttons, and icon.

public DialogResult MessageBox.Show(string text, string caption, MessageBoxButtons buttons, MessageBoxIcon icon);Parameters

text - The text to display in the message box.

caption - The text to display in the title bar of the message box.

buttons - One of the MessageBoxButtons values that specifies which buttons to display in the message box.

icon - One of the MessageBoxIcon values that specifies which icon to display in the message box.

Return Value

One of the DialogResult values.

MessageBox Buttons

MessageBoxButton constants Description MessageBoxButton.OK OK button. Allows the user to acknowledge a

message. Included by default. MessageBoxButton.OKCancel OK and Cancel buttons. Allow the user to either

continue or cancel an operation.

MessageBoxButton.YesNo Yes and No buttons. Allow the user to respond to a question

MessageBoxButton.YesNoCancel Yes, No and Cancel buttons. Allow the user to respond to a question or cancel an operation.

MessageBoxButton.RetryCancel Retry and Cancel buttons. Typically used to allow the user to either retry or cancel an operation that has failed.

MessageBoxButton.AbortRetry-Ignore Abort, Retry and Ignore buttons. When one of a series of operations has failed, these butons allow the user to abort the entire sequence, retry the failed operation or ignore the failed operation and continue..

MessageBox IconsMessageBoxIcon Constants Icon Description

MessageBoxIcon.Exclamation Icon containing an exclamation point. Typically used to caution the user against potential problems.

MessageBoxIcon.Information Icon containing the letter "i." Typically used to display information about the state of the application.

MessageBoxIcon.Question Icon containing a question mark. Typically used to ask the user a question.

MessageBoxIcon.Error Icon containing an in a red circle. Typically used to alert the user of errors or critical situations.

Abort - The return value is Abort (usually sent from a button labeled Abort).

Cancel - The return value is Cancel (usually sent from a button labeled Cancel).

Ignore - The return value is Ignore (usually sent from a button labeled Ignore).

No - The return value is No (usually sent from a button labeled No).

None - Nothing is returned from the dialog box. This means that the modal dialog continues running.

OK - The return value is OK (usually sent from a button labeled OK).

Retry - The return value is Retry (usually sent from a button labeled Retry).

Yes - The return value is Yes (usually sent from a button labeled Yes).

DialogResult - Specifies identifiers to indicate the return value of a dialog box.

MessageBox Sample Code string strMessage = "No server name. Cancel operation?“; string strCaption = "No Server Name Specified“; int iBttns = MessageBoxButtons.YesNo; int iIcon = MessageBoxIcon.Question; DialogResult Result; 'Displays a MessageBox using the Question icon and specifying the No button as the default. Result = MessageBox.Show(strMessage, strCaption, iBttns , iIcon); ' Gets the result of the MessageBox display.  if (Result = DialogResult.Yes) { this.Close(); ' Closes the parent form.  }

Exceptions

Exceptions in .NET

• .NET relies heavily on exception handling.• In fact all error notification/handling by the

framework is in the form of exceptions.• Returning error values by your custom code is still

possible, but not encouraged.• The move from error codes to exceptions will

have an impact on how your code is structured and designed.

• Since exceptions are part of the framework, they are consistent across all languages.

Benefits of exception handling• Cleanup code is kept in a localized spot, and we can be assured that

the code will run.– This implies your logic isn’t scattered with a ton of cleanup code,

nor do you have to jump to cleanup blocks. (On error goto…)• Similarly, all code to handle exceptional situations in kept in a

central place.– Once this is done, you write the rest of your code as if the errors

can never occur.– Examples of exceptional situations are arithmetic overflows, out-

of-memory conditions, attempts to access resources after they have been closed, etc.

• When exceptions do occur, the availability of information that will help us locate and fix the error.– You typically get call stack information when unhandled

exceptions occur.

Evolution of exception handling• With Win32, you used error codes and GetLastError

HANDLE h = CreateFile(………);if (h == INVALID_FILE_HANDLE)

{DWORD d = GetLastError();FormatMessage(…d, …);

}

• With COM, you used HRESULTS HRESULT hr = MyCOMObject.DoSomething();if (FAILED(hr)){ // handle error}

The problem with error codes• What you get back is a 32 bit number, with no

additional information.• FormatMessage normally gave you very generic

error descriptions. (“Invalid parameter”), with no indication of where the problem took place.

• In order to make sense of error codes, you had to usually include header files. (So what do you do for other languages ?).

• In the case of “smart” error codes like HRESULTS, you were provided with macros that could be used to decipher the code.

Error codes (contd.)• Defining your own error codes was a very loose process.

– Pick a set of values and put them in a header file.– Take care that the codes do not conflict with other codes. Usually

achieved by starting your codes at some offset from the well-known set of codes.

– Provide some mechanism for callers to translate your codes into meaningful messages.

• After doing all this, you could still do the following: CreateFile(………); // ignore all errorsOR

MyCOMObject.DoSomething(); //ignore HRESULT

Exceptions• The last example shows how error codes allow you to opt-

in – you can opt to handle the error, but you ignore it by default.

• Exceptions change this to the opt-out model – you can explicitly choose to ignore the exception, but you must handle it by default.

– If you don’t handle it, your caller gets handed the exception, and so on up the chain.

• When exception handling was added to C++, it had a reputation for being slow. Part of the reason was that a fairly large burden was placed on the compiler to make sure cleanup (destruction) code was put in place whenever an exception could be thrown.

Exceptions• In .NET, since all languages are built on the runtime,

the runtime implements exception handling.• Unlike error codes, exceptions are objects that

represent exceptional conditions, and can carry a lot of information.

• In C#, the following keywords are used with exceptions.– try– catch– finally– throw

The try block• The try block is where you put sections of code

that may potentially throw exceptions.• It also contains code that requires common

cleanup or exception recovery operations.object o2 = new object(); try

{ //other code you may have …

int i2 = (int) o2; // Error }

• A try-block by itself is no good – it must be associated with at least one catch or finally block.

The catch block• A catch block contains code to execute in response to an

exception.• A try block can have zero or more catch blocks associated

with it.

object o2 = new object(); try{

//other code you may have …int i2 = (int) o2; // Error

}catch (InvalidCastException e) {

// exception handling code Console.WriteLine(“Invalid cast exception”); }

The catch block

• If the code in the try block doesn’t cause an exception to be thrown, the CLR never executes any code contained within the catch blocks.– The thread skips over the catch blocks and executes

code in the finally block (if one exists.)

– Any statements following the catch/finally blocks are executed

• The expression in the parentheses after the catch keyword is called an exception filter. In C# the exception filter has to be an object of the type System.Exception or a derivative.

The catch block

• You can place multiple catch blocks after a try block.

• The catch blocks are searched from top to bottom.– Important: Place the more specific exceptions

at the top, in the order you would like them caught.

– The C# compiler does generate a compile error if more general catch blocks are placed ahead of the more specific ones.

The catch block• If none of the catch blocks match the exception that

occurred, the CLR continues up the call stack looking for one.

• If none are found after reaching the top of the call stack, an unhandled exception is reported.

• Within a catch block, after any handler code you write, you can– Rethrow the same exception.– Throw a different exception.– Just fall out of the bottom.

• Code in a finally block (if present) is always executed.

The finally block• A finally block contains code that is guaranteed to

execute.• Typically contains cleanup code required by the actions

taken in the try block.• A finally block is associated with a try block. (I.e. no catch

block is required).

try {StreamWriter testOut = new

StreamWriter(@"output.txt",true);// …

}finally {

if (testOut != null) testOut.Close()}

The System.Exception Class• This class is the base class for all exceptions.• Interesting members:

– Message : a string that contains a description of the exception.

– Source : the name of the application or object that caused the error

– StackTrace : a string representation of the stack trace at the time the exception was thrown.

Throwing an exception• Within a catch block, we can choose to rethrow

the same exception, or throw a new one.try{

//other code you may have …int i2 = (int) o2; // Error

}catch (InvalidCastException e1) {

Console.WriteLine("Invalid cast exception"); throw; // rethrow the exception

}catch (NullReferenceException e1) {

Console.WriteLine("Null reference exception"); throw new Exception("New exception"); //throw new

ex.}

Throwing an ApplicationExceptiontry

{

if ((textBox1.Text == "") || (textBox2.Text == ""))

{

throw new System.ApplicationException("Values must be

numeric.");

}

}

catch (ApplicationException ex)

{

MessageBox.Show(ex.Message);

}

catch (Exception ex)

{

MessageBox.Show(ex.Message);}

Guidelines for exception handling

• Catch the most specific exception possible– Otherwise, you may end up handling exceptions you did

not intend to.• Catch the exception only if you are absolutely sure how to

handle it.• Order your exception blocks carefully. Remember, catch

blocks are executed sequentially.• Finally blocks are a good place to put cleanup code. Try not to

put code that may throw exceptions in the finally block. (If it does, the application will not crash, the exception will just be passed up the call chain.)

• Return values still have their place – do not use exceptions to return ‘normal’ error conditions.

Wrapping an exception• A lot of times, you just want to add more descriptive

information to the exception you are handling. In this case, you can wrap the exception that was just caught. InnerException will contain the original exception.

object o2 = new object(); try{ int i2 = (int) o2; // Error }catch (InvalidCastException e1) { Console.WriteLine("Invalid cast exception"); throw new InvalidCastException("My own info",

e1);}

A common mistake

• Folks commonly put this catch block in their code:catch (Exception e1)

{ // Do stuff

}

• Understand that this assumes you know how to handle every kind of exception

Another common mistake

• In one of the earlier slides, we did this:catch (NullReferenceException e1) {

Console.WriteLine("Null reference exception"); throw new Exception("New exception"); //throw new ex.

}

• This is generally a bad idea since we caught a more specific exception and replaced it with one that has less information.

Simple Calculator (Part 2)with

Exception Handling

Tic Tac Toe Program

Homework

Due

November 20, 2003