Chapter 6: Using VB .NET
Supplied Classes
Visual Basic .NET Programming:
From Problem Analysis to Program Design
Objectives
• Learn more about the Framework Class Library
• Invoke methods in the String class
• Display message boxes
• Format numeric output
• Work with dates
• Read and write sequential files
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Introducing the Framework
Class Library
• Assembly
– File with .dll suffix
– Contains Intermediate Language (IL)
• Framework Class Library
– Consists of approximately 100 assemblies
• Each containing one or more classes
– Organized logically into namespaces
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Introducing the Framework
Class Library (continued)
• Namespace
– Can contain both classes and other namespaces
– System is root of all other namespaces
• Framework compilers do not automatically search
all namespaces for classes used by code
– Except System namespace
– For all other namespaces
• Use Imports keyword
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Invoking Methods in the String
Class
• String data
– Collection of characters
• String class
– Member of System namespace.
– Stored in reference variable
• Property
– Special kind of VB .NET method
– Access like public variable
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Invoking Methods in the String
Class (continued)
• String instances in VB .NET are immutable
– Cannot be changed
– Methods create and return new String
• Each character in String instance has index
– Indicates position
– Index for first character is 0
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Using the Length Property
• Length property
– Contains number of characters in String
• To access length for String s1:
– s1.length
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Using the Copy Method
• Creates copy of String instance
• Returns reference to new instance
• Example:
– s2 = String.Copy(s1)
• Class method
– Invoke using class name
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Using the Chars Method
• Returns character located at specified index
• Example:
– s1.Chars(6)
• Instance method
– Invoke using reference variable name
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Using the Equals Method
• See if two String instances contain same data
• Example:
– s1.Equals(s2)
• Not same as =
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Using the Substring Method
• Extract one or more characters from a String
• Return new String containing extracted characters
• Arguments:
– Index of first character
– Number of characters
• Example:
– s2 = s1.Substring(0, 5)
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Using the Replace Method
• Replace one or more characters in String
– With one or more other characters
• Arguments:
– String containing character(s) to be replaced
– String containing replacement character(s)
• Example:
– s2 = s1.Replace(“Hello”, “Hi”)
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Using the Insert Method
• Add one or more characters into existing String
– Beginning at specified index
• Example:
– s2 = s1.Insert(6, “There “)
• Arguments:
– Index where to begin insertion
– Characters to be inserted
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Using the StartsWith and
EndsWith Methods
• StartsWith
– Compares String with beginning character(s) of
another String
– Returns True or False
• Depending on whether there is a match
• EndsWith
– Compares ending characters
• Use both methods to search for text containing
specified characters
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Using the StartsWith and
EndsWith Methods (continued)
• StartsWith example:
– s1.StartsWith(“Hi”)
• EndsWith example:
– s1.EndsWith(s3)
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Using the ToUpper, ToLower,
IndexOf, and ToString
Methods
• Change case of String value to uppercase or
lowercase
– ToUpper
• Uppercase
– ToLower
• Lowercase
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Using the ToUpper, ToLower,
IndexOf, and ToString
Methods (continued)
• IndexOf method
– Search String instance for specific value
– Returns index of first character of value
– Or -1
• If no matching value is found
• ToString
– Convert numeric value to String
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Displaying Message Boxes
• Use message box to
– Display message
– Get response
• MessageBox class
– Member of System.Windows.Forms namespace
– Single method named Show
• Creates instance of MessageBox
• Makes it visible
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Displaying Message Boxes
(continued)
• Show() method arguments
– Message to display
• Can string literal or variable
– Caption to display
– Buttons to display
– Type of icon to show
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Example 6-12: Displaying
message boxes
1. ' display a message
2. MessageBox.Show(“Hello Again”)
3. 'display a message and a caption
4. MessageBox.Show(“Hello Again”, “MessageBox
Demo”)
5. 'display message, caption, Yes/No/Cancel buttons
6. MessageBox.Show(“Hello Again”, “MessageBox
Demo”, MessageBoxButtons.YesNoCancel)
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Example 6-12: Displaying
message boxes (continued)
7. 'display message, caption, Yes/No/Cancel buttons,
and Icon
8. MessageBox.Show(“Hello Again”, “MessageBox
Demo”, MessageBoxButtons.YesNoCancel,
MessageBoxIcon.Information)
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Displaying Message Boxes
(continued)
• Button combinations
– Automatically displayed by IntelliSense
• After you type class name MessageBoxButtons
– Select specific buttons to be displayed from list
• Icons
– Displayed by IntelliSense when typing
MessageBoxIcon
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Displaying Message Boxes
(continued)
• show() method return value
– Data type DialogResult
– Compare to specific values using If statement
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Formatting Numeric Output
• Format numeric output to make it:
– More attractive
– Easier to read
• Use ToString method to format numeric data
– Argument types:
• Format codes
• Format mask
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Formatting Numeric Output
(continued)
• Format mask examples:
– s = phoneNo.ToString(“(###) ###-####”)
– s = d.ToString(“$#,##0.00”)
– ssNo.ToString(“###-##-####”)
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Working With Dates
• FCL structures:
– DateTime
• Contains actual date value
– TimeSpan
• Contains computed difference between two dates
– Belong to System namespace
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Working With Dates
(continued)
• Today
– Property of DateTime
– Gets system date
– Returns DateTime instance.
– Example:
• todaysDate = DateTime.Today
• Now property
– Captures current time
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Working With Dates
(continued)
• Format date display
– Invoke ToString method
– Pass arguments that describe desired format
• Creating DateTime instance example:
– eleanorsBirthday = New DateTime(1998, 12, 15)
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Comparing Dates
• Subtract() method
– DateTime class
– Compute number of days between DateTimes
– Returns instance of TimeSpan class
• TotalDays() method
– TimeSpan class
– Obtain number of days in TimeSpan
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Example 6-18: Computing the
Difference between Dates
1. ' compute the difference between two dates
2. Dim daysDifference As Double, ageDifference As
TimeSpan
3. ageDifference =
emilysBirthday.Subtract(eleanorsBirthday)
4. daysDifference = ageDifference.TotalDays()
5. Console.WriteLine(“The age difference is “ &
daysDifference)
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Comparing Dates (continued)
• Compare() method
– DateTime class
– Compares two DateTime instances
– Returns either:
• -1
•0
• +1
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Performing Arithmetic with
Dates
• DateTime class
– Methods add a value to month, day, or year
– Named AddMonths, AddDays, AddYears, etc.
– Example:
• todaysDate.AddMonths(1)
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Reading and Writing
Sequential Files
• Classes typically employed in sequential file
processing:
– StreamWriter
– StreamReader
• Database
– Data organized into one or more tables
– Tables can be related
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Reading and Writing
Sequential Files (continued)
• Files
– Contain data
– Organized into fields and records
• Field
– Individual data item
– Can be contained in a primitive variable or String
instance
• Record
– Consists of one or more related fields
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Reading and Writing
Sequential Files (continued)
• File
– Contains one or more related records
• Sequential file
– Data organized with one data item following
another
– System.IO namespace classes:
• StreamWriter
• StreamReader.
• Must use Imports statement
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Reading and Writing
Sequential Files (continued)
• StreamWriter class
– WriteLine() method
– Can overwrite or append file
• StreamReader class
–
–
–
–
ReadLine() method
Write loop when reading from sequential file
Check for existence of data before reading data
Use Peek() method
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Example 6-22: Appending to a
Sequential File
1. Dim customerFile As New
StreamWriter("C:\Customers.txt", True)
2. customerFile.WriteLine("Graham")
3. customerFile.WriteLine("Marietta")
4. customerFile.WriteLine("467-1234")
5. customerFile.Close()
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Programming Example:
Employee Report
• Input
– Sequential file named C:\Employee.txt containing
employee information
• Output
– Employee report containing:
•
•
•
•
1. Social Security number formatted nnn-nn-nnnn
2. Date employed formatted as monthname dd, yyyy
3. Hourly pay rate formatted as currency
4. Formatted employee name
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Summary
• Visual Basic .NET stores string data in instances
of String class
– Provides several useful methods and properties
– Immutable
• MessageBox
– Used to display message and get response
• Invoke ToString method in primitive structures to
format numeric data
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Summary (continued)
• Work with dates using:
– DateTime
– TimeSpan
• VB .NET uses two classes in System.IO
namespace to work with sequential files:
– StreamWriter
– StreamReader
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