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C#-2 Notes

Review Questions

1. What is wrong with this statement?
   this.Main( );

2. How do you check whether a string represents a number?

3. More than one control can be attached to an event handler. How can you get the VS IDE to do this?

4. What are the default events for these controls
    
   
   Button   TextBox   ListBox   RadioButtonList
   
   
5. What are the Enter and Leave events?

6. How do you determine the order that controls on an ASP.Net web page receive the focus when the tab key is repeatedly pressed?

7. How are each of these properties used in ASP.Net projects?
   AutoPostBack  CssClass  ImageUrl  IsPostBack  SelectedIndex

8. On an .aspx page, how do you avoid using delegates to connect events with their event handlers?

9. What is wrong with this web control tag?
   <TextBox Name="txtCount" Value="0" />

10. How can you avoid the need to upload the DLL for the code behind to the server for an .aspx page?

Examples

1. Controls as Classes   Events.WinformsEvents Example

2. Exercise:  Create an ASP.Net example with a page that contains these controls: LinkButton and Calendar

Part B: The String and StringBuilder Classes

1. String Manipulation Methods   Strings Example
   • string objects are immutable. They cannot be modified after they are created.

   • Fixed size strings are not allowed in C#.

   • string Properties: Chars, Length

   • string Methods: Compare, CompareTo, Concat, Copy, CopyTo, EndsWith, Equals, Format, IndexOf, IndexOfAny, LastIndexOf, LastIndexOfAny, Insert, PadLeft, PadRight, Remove, Replace, Split, StartsWith, Substring, ToCharArray, ToLower, ToString, ToUpper, Trim, TrimEnd, TrimStart.

2. The StringBuilder Object   SSBComparison Example
   • StringBuilder objects can be modified after they are created.

   • StringBuilder Properties: Capacity, Chars, Length, MaxCapacity

   • StringBuilder Methods: Append, AppendFormat, EnsureCapacity, Equals, Insert, Remove, Replace ToString.

Part C: Class Diagrams

• A Visual Studio class diagram shows this information:
  Class name

  Fields, also called instance variables

  Properties

  Methods

• Class diagrams enable round trip engineering, that is:
  The class diagram is created from the code,

  The code skeleton can be created from the class diagram.

• Here are some examples of class diagrams.

• Inheritance relationships between classes are indicated using arrows. Here is a class diagram showing an inheritance hierarchy. The classes are object, Person, Employee, Executive.

Part D: Using Classes and Objects

1. User Defined Classes
   Version 1   Vehicle1 Example
   • Properties are implemented using global variables.

   • A default constructor is supplied if no constructors are defined in the class. It performs these initializations:

     Datatype	Initial Value
     Any numeric type	0
     Char	'\0'
     Boolean	false
     DateTime	1/1/1 12:00:00 AM
     Any reference type	null

   • For reference type objects, the assignment operator = in the statement y = x; makes both x and y refer to the same object.

   • To compare the addresses in reference variables, use the ReferenceEquals operator, not the == operator:
     if (object.ReferenceEquals(x, y))
         Console.WriteLine("x and y are the same object.")
    

   Version 2   Vehicle2 Example

   • Properties can be implemented using Java-style accessor methods (getters) and mutator methods (setters).
    

   Version 3   Vehicle3 Example

   • Preferred implementation: use C# properties with get and get parts.
     public int Age
     {
         get { return mvarAge; }
         set { mvarAge = value; }
     }

   • Use .Net properties to participate fully in the .Net machinery.

   • The get and set parts can have different accessibilities.
     public int Age
     {
         get { return mvarAge; }
         private set { mvarAge = value; }
     }
    

   Version 4   C#-1.Structure and Vehicle4 Examples

   • Small objects with few properties can be more efficiently implemented as a value type structure instead of a reference type object.

   • Structures can have instance variables, constructors, methods and properties.

   • No user defined constructor with empty signature is allowed.

   • Structures cannot participate in inheritance.

   • For value type objects, y = x; copies the data from x into y. x and y remain distinct value type objects.

    

2. Constructors   Vector1 Example
   • Constructors can have parameters.

   • A constructor with empty signature must be called with empty parentheses:
     x = new Vehicle( );

   • More than one constructor can be provided, if the signatures are different.

   • A constructor can be called with an existing reference variable:
     Vehicle x;
     x = new Vehicle( );
     or initialized at declaration:
     Vehicle x = new Vehicle( );

   • Use the this keyword internally in a class for the reference varable referring to the current object.

   • Use this in a constructor to call another constructor of that class.

Part E:   File Input and Output.

1. Using the System.IO Methods   FileIO1, EventDriven.ShowPoem, Misc.CurrRateFromWeb Examples
   • Objects:   StreamReader, StreamWriter, BinaryReader.

   • Filename with absolute path:
     StreamReader sr = new StreamReader(@"c:\data.txt");

   • Filename with relative path:
     StreamReader sr = new StreamReader("data.txt");

   • A relative path is relative to the current folder, initially set to the folder containing the .exe file of the executing assembly.

   • If the current folder has been changed, use the following code to find the full path name of the executable file:
     fullPathName = System.Reflection.Assembly.
         GetExecutingAssembly.Location;

   • This shorter code can be used in a Windows application to find the folder that contains the executable file:
     fullPathName = Application.StartUpPath;

   • The ReadLine method of StreamReader returns null when it tries to read past the end of file.

   • The WriteLine method does not print anything when attempting to print "" or null.

   • The Peek method of StreamReader returns the Unicode code of the next unread character. It returns -1 if there are no more unread characters.

   • Always close a file when finished using it.

   • Closing a file flushes the output buffer to ensure that no data is missing from the file.

   • To read a file from the web, use the WebRequest, WebResponse, and Stream objects as shown in the CurrRateFromWeb example.

Part F:   Collections of Objects

1. ArrayList Collections   ArrayListCol Example
   • When using an ArrayList collection, or any other .Net collection, put this statement at the top of the Form1.cs file:
     using System.Collections;

   • Declaring and instantiating an ArrayList object:
     ArrayList a = new ArrayList( );

   • Adding a new element:
     col.Add(newMember);

   • Removing an element of a specified index:
     col.RemoveAt(memberIndex);

   • Accessing an element in a collection:
     col[memberIndex]

   • Iterating over all elements in a collection:
     foreach (object x in col)
     {
         // Statements using x.
     }

2. Advantages of an ArrayList Collection over an Array
   • Its size need not be specified in advance.

   • Its capacity can grow as needed.

   • New memory is allocated when a new object is inserted.

   • The hole in closes up when an object is removed.

3. An Array of Objects   VehicleArray Example
   • The VehicleArray Example inserts objects into an array and prints only the objects in the array that are not null.

4. A Collection of Objects   VehicleCol Example
   • An ArrayList collection contains objects of type object.

   • A value type object is boxed (stored on the heap) when it is added to the collection.

   • An object is unboxed when it is cast back and assigned to a value type variable.

   • Here is simpler example of boxing. Memory for the object y is allocated on the heap, but y is tagged as a value type object of type int.
     int x = 5;
     object y = x;

   • Now let's unbox y. Memory for z is allocated on the stack.
     int z = (int) y;

   • Extensive use of boxing and unboxing can cause performance problems.

   • Use generic collections to avoid boxing and unboxing.

Part G:   Inheritance.

1. Inheritance in Classes   Employee Example
   • Reserved words supporting inheritance in C#:
     public   private   protected   internal
     virtual   override   abstract   sealed

   • A method declared as internal can be accessed from anywhere in the project that contains it, but not from a different project in the solution.

   • Controls are often declared to have internal accessibility.

   • Here is the UML diagram of the Employee inheritance hierarchy.

   • To check whether inheritance makes sense, use the is-a test.

2. Inheritance in Arrays   Virtual Example
   • When a method is declared as virtual, the method called, out of a choice of overridden methods, depends not on the type of the reference but of the type of the object.

3. Abstract Classes   Abstract, Brushes Examples
   • An abstract class contains at least one method that must be overridden.

   • Such a method is declared with the abstract keyword and has an empty body.

4. Exception Handling   TryCatch Example
   • Use the try..catch construction to handle exceptions in a manner that prevents the application from crashing:
     try
     {
         // Code that might cause an exception.
     }
     catch(Exception1 ex2)
     {
         // Code that handles exception Exception1.
     }
     catch(Exception2 ex2)
     {
         // Code that handles exception Exception2.
     }
     finally
     {
         // Code that runs whether or not an
         // an exception occurs.
     }

   • Make sure that the code in the finally block cannot cause an exception.

   • The official list of .Net Exceptions.

5. Interfaces   Intro.Multiclass Example
   • An interface enforces a sort of uniformity among seemingly disparate classes.

   • An interface is a set of specifications to which a class must adhere if it implements the interface.

   • An interface can be thought of as a contract:
     The user of the interface agrees to implement methods according to the specifications.

     The writer of the interface agrees not to change the interface after it is published.

   • See the document .Net Interfaces.

Part H:   Exercise

1. Create an ASP.Net project that inputs the values Model, Year, and Miles creates a Vehicle object, and stores this object in an ArrayList collection. When a save button is clicked, the application writes the contents of this object to a CSV textfile.

2. Question: where are you going to keep the collection?

Part I:   Two More Interfaces

1. The IClonable Interface   C#-3.IClonable Example
   • A shallow copy of an object means simply copying the data members of the original object to the new object.

   • A deep copy of an object means
     copying the value type data members of the original object to the new object,

     allocating new objects for any reference types and copying the data from the objects of the reference types into the new objects.

   • Write a Clone method (required by the ICloneable interface) to make a deep copy of an object.

2. Implementing a Custom Enumerator   C#-3.CustomEnumerator and C#-3.CustomCollection Examples
   • If the IEnumerable interface is implemented in a class, its objects can participate in the For..Each construction.

   • If an object implements the IEnumerator interface, the enumerator of the object can be passed on to a collection class containing the object.