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C#-3 Notes

Review Questions

1. How do you insure that only one RadioButton web control can be selected in a group of radio buttons?

2. What does the keyword this mean?

3. What does this symbol mean?   :

4. What is inheritance and when should you use it?

5. What is the is-a test?

6. Within a constructor, how do you call an overloaded constructor from the same class? a constructor from the base class?

7. In a class, how do you invoke a method from the base class?

8. Here are some C# keywords that support inheritance. What do they mean?
   

   public  private  protected  internal
   virtual  override  abstract  sealed
   

9. In the Employee Example, all of the fields are private within the Person class. How do Employee and Executive objects access the contents of these fields?

10. In which order do these constructors file: derived class constructor, base class constructor?

11. True or False. Constructors can be inherited from the base class

12. Write an ASP.Net example that inputs the make, year, and miles in textboxes, creates a Vehicle object, and adds the vehicle object to an ArrayList collection. Use a Session variable to hold the collection so that it maintains its contents across postbacks.

13. Look at the Misc.CurrRateFromWeb Example.

Examples

Calendar  CurrRateFromWeb  Chess  VehicleWebpage  EmployeeComboBox1  EmployeeWebPage  BinaryFormatter
 
Example files: misc.zip and misc.htm.

Part A: Using the Debugger

1. Breakpoints   Event.Beverage Example

2. Step Into, Step Over, Step Out

3. Value Tooltips

4. Autos Window

5. Locals Window

6. Watch Window

7. Output Windows

8. Call Stack

Part B: Inheritance

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

2. Abstract Classes   C#-2.Abstract, C#-2.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.

3. 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.

   • Practice Problem:   Add try/catch blocks to the C#-1.CommandLineArgs Example to handle cases of missing or incorrect command line arguments.

4. Interfaces   Intro.Multiclass, Intro.Multiproject, Interface1 and Vector3 Examples
   • An interface inforces 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.

   • Interfaces are the Java and C# substitute for multiple inheritance.

   • See the document .Net Interfaces.

   • For more information on .Net interfaces, including the required methods for each, use Dynamic Help.

   • Objects from a class that implements the IComparable interface can be compared and sorted.

   • The IComparable interface requires the CompareTo method.

   • CompareTo returns
     positive if this > other

     negative if this < other

     zero if this == other

Part C: Generic Collections

1. Boxing and Unboxing   BoxUnbox Example
   • Boxing a variable means create a wrapper class for a value type object so that it can be copied to the heap.

   • Boxing occurs
     1. when a variable is converted from a value type to type object:
        int n = 5;
        object x = n;

     2. when an object is passed to a method with parameter type object:
        a.F(x);

     3. Boxing occurs when value type objects are added to an ArrayList or Hashtable collection.
        col.Add(x);

   • Unboxing copy the value of a boxed object to a value type object.

   • Unboxing occurs
     1. When an object of object type is assigned to a value type variable:
        object x = 5;
        int n = (int) x;

     2. When an object in an ArrayList or Hashtable is cast back to a value type and assigned to a value type variable.

   • Use the generic collections List and Dictionary to avoid the boxing and unboxing penalties that are incurred with ArrayList and Hashtable.

   • Boxing and unboxing are used instead of using wrapper classes for primitive types as Java does.

2. Using Generic Collections   Generic1 Example
   • A generic class requires a one or more class parameters in the class definition header. Example:
     public class A<T>
     {
         ...
     }

   • The actual class name or type is passed in during the constructor call that creates an object of the class. Example:
     x = new A<Person>< );

3. Practice Problem
   1. In a Console application, declare and instantiate an ArrayList collection; populate it with five int values.

   2. In a Console application, declare and instantiate a Hashtable collection; populate it with five int values, using these keys: "A", "B", "C", "D", and "E".

   3. Convert the answers of Practice Problems 1 and 2 to ones that use the generic collections List and Dictionary.

Part D: Miscellaneous Examples

1. Calendar   A web page that contains the Calendar and LinkButton web controls.

2. VehicleWebPage   From Vehicle model fields entered from a web page, create a collection of Vehicle objects, that can be written to disk.

3. EmployeeComboBox1   A desktop application that displays objects from the Person/Employee/Executive inheritance hierarchy.

4. EmployeeWebPage   Same as the EmployeeComboBox1 application, but display information on a web page. Complete the application by adding code to add new items.

5. BinaryFormatter   Show how to serialize a Dictionary collection of Employee objects using the BinaryFormatter class.

Part E: More C# Topics

1. Here is the Operator Precedence Table.

2. Operator Overloading   Vector2
   • Use the operator keyword to overload operators as in the Vector2 Example:
     public static Vector operator +(Vector v, Vector w)
     {
         return new Vector(v.x + w.x, v.y + w.y);
     }

   • Here is a list of the operators that can be overloaded:

     Unary:   +   -   !   ~   ++   --   false   true

     Binary:   +   -   *   /   %   &   |   ^   <<   >>  
            ==   !=   <   <=   >   >=
     

3. Checking for Integer Overflow
   • Try out this source code:
     

     int n = 3;
     for(int i = 1; i <= 10; i++)
     {
        Console.WriteLine(n);
        n *= 3;
     }
     

     Why is it outputing negative numbers?

   • The default compiler setting is not to check for integer overflow.

   • To set the VS compiler to check for integer overflow: On the project properties pages >> Build Tab >> Advanced Button >> Check for arithmetic overflow/underflow >> OK.

   • Even if the compiler is set to check for integer overflow, turn off checking for integer overflow by putting code withing an unchecked block:
     unchecked
     {
         // Code that could cause integer overflow.
     }

Part F:    Topics Related to Garbage Collection

1. The GarbageCollector   GarbageCollector Example
   • The memory of an object that is no longer referenced by any reference variable is garbage.

   • When more free memory is needed, garbage is collected from the managed heap by the garbage collector.

   • To force the .NET garbage collector to run, use System.GC.Collect( );

   • The Finalize method is called by .NET when the garbage collector reclaims the object's memory. The exact time when the garbage collector will run and when Finalize is called is unpredictable.

   • In VB, the Finalize method of the Object class can be overridden.

   • In C#, the Finalize method cannot be overridden. A class destructor is provided instead; it executes the code in the destructor, then executes the base class Finalize method.

   • VB6 had the event handler Class_Terminate, which ran immediately after the reference count of the object dropped to zero. VB.Net does not keep reference counts in objects.

   • The managed heap is divided into three generations 0, 1, and 2. New objects are stored in generation 0. When garbage is collected, objects that survive are promoted to generation generation 1 and objects that survive from generation 1 are promoted to generation 2.

2. IDisposable Interface   IDisposable Example
   • Required Method: Dispose
      
   • Managed resources consist of memory that can be freed by the garbage collector.

   • Unmanaged resources consist of resources that the garbage collector does not know about such as file handles, database connections and resources like bitmaps.

   • One never knows exactly when the garbage collector will run; don't wait for the Finalize method to free resources.
      
     
   • An object's Dispose method is similar to the delete operator for freeing memory in C++.

   • Dispose can be called by the user or by the runtime within the Finalize method.

   • Component objects maintain have a list of resources. Current practice is to have the component's Dispose method call call the Dispose method of each resource in the list.

   • Current practice is to define an overloaded Dispose method with parameter isDisposing.
     Pass in true if Dispose is being called by the user.

     Pass in false if Dispose is being called within Finalize.

   • An object that has a Dispose method should implement the IDisposable interface.

   • The Component class implements IDisposable and therefore has a Dispose method. Every control also has a Dispose method because Component is the parent class of Control.

   • Here the list of .Net Classes that Implement IDisposable.

3. The IClonable Interface   ICloneable 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.

     • allocating resources to match those of the original object.

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

Part G:    Unsafe Code

1. Pointer Types   Unsafe Example
   • Unsafe code is useful for porting C++ code to C#.

   • In addition to the Value and Reference types, the Pointer type from C++ is also available.

   • A pointer contains the address of a class or structure.

   • Important:   All code containing pointer variables must be placed within an unsafe block:
     unsafe
     {
         // Code containing pointer variables.
     }

   • The VS compiler must be set to allow unsafe code. Go to the project properties pages >> Build Tab >> Check the Allow Unsafe Code checkbox.

   • The declaration of a pointer variable p that contains the address of an int:
     int* p;

   • Dereferencing a pointer variable:
     x = *p;

   • Assigning the address of a variable to a pointer:
     p = &s;

   • Selecting a member from a class:
     a = p -> Age;
     p is a pointer to a Person object.

   • Pointer arithmetic is allowed.

   • These operations move a pointer address forward or backward in memory:
     p++;    // Point to next structure in memory.
     p--;    // Point to previous structure in memory.
     p += 5; // Move forward 5 structures.
     p -= 7; // Move backward 7 structures.

   • Pointer variables and operations are only required when porting C++ code to C#.

1. Using .Net System.IO Classes   C#-2.FileIO1 and Intro.Unicode Examples.
   • Objects:   StreamReader, StreamWriter, BinaryReader.

   • Use the following line to find the full path name of the executable file.
     fullPathName = System.Reflection.Assembly.
         GetExecutingAssembly.Location;

   • 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 "" or null is printed.

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

   • Some overloaded constructors for creating a StreamReader object:
     Simplest version to read straight ASCII text files:
     StreamReader sr = new StreamReader(fileName);

     true means "detect encoding from byte order marks:"
     StreamReader sr = new StreamReader(fileName, true);

     Use the UTF-8 encoding system for Unicode:
     StreamReader sr = new StreamReader(fileName,
         Encoding.UTF8);

   • Here is a brief overview of .NET Encodings, including UTF-8, UTF-16 and UTF-32.

   • See this article by Joel Spolsky for more details:
     The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets (No Excuses!)

2. The File Class
   • The File class provides static methods for creating, copying, deleting, moving, and opening files:
     File Methods: AppendAllText, AppendText, Copy, Create, CreateText, Delete, Decrypt, Encrypt, Exists, Move, Open, OpenRead, OpenText, OpenWrite, ReadAllBytes, ReadAllLines, ReadAllText, Replace, WriteAllBytes, WriteAllLines, WriteAllText.

   • To open an ASCII encoded file for reading:
     StreamReader sr = File.OpenFile(fileName);

   • To open an UTF-8 encoded file for reading:
     StreamReader sr = File.OpenText(fileName);

3. The FileInfo, DirectoryInfo and DriveInfo Classes
   • The FileInfo, DirectoryInfo and DriveInfo classes are used to obtain information about a file, directory or drive, respectively.
     FileInfo Properties: Attributes, CreationTime, Exists, FullName, IsReadOnly, LastAccessTime, LastWriteTime, Length, Name

     FileInfo Methods: AppendText, CopyTo, Create, Delete, Encrypt, Decrypt, MoveTo, Open, OpenRead, OpenText, OpenWrite, Replace

     DirectoryInfo Properties: Attributes, CreationTime, Directory, Exists, FullName, IsReadOnly, LastAccessTime, LastWriteTime, Length, Name, Parent

     DirectoryInfo Methods: Create, CreateSubdirectory, GetDirectories, GetFiles, MoveTo

     DriveInfo Properties: AvailableFreeSpace, DriveFormat, DriveType, IsReady, Name, RootDirectory, TotalFreeSpace, TotalSize, VolumeLabel

     DriveInfo Methods: GetDrives

Part J: Delegates and Events

1. Creating and Using Delegates   Delegates Example
   • A delegate is a type-safe function pointer.

   • Use a delegate in cases where a function is chosen, but invoked at a later time. For example, an event handler is associated with an event, but the event occurs at a later time.

   • Declaring a delegate:
     public delegate ReturnType DelegateType(type2 param2);

   • Declaring Delegate Variable:
     DelegateType delegateVariable;

   • Assigning a method to a delegate (The signatures of the method and delegate must match):
     delegateVariable = methodName;
     or
     delegateVariable = new DelegateType(methodName);

2. User Defined Events   RaiseEvent Example
   • Declaring an Event:
     public event DelegateType EventName(type1 param1,
         type2 param2);

   • Raising the Event:
     EventName(type1 arg1, type2 arg1);

   • Attaching an Event Handler to an Event:
     objectName.EventName +=
         new DelegateName(EventHandlerName);

   • User defined events are an alternative to exceptions for communicating abnormal situations.

3. Anonymous Delegates
   • Event handlers can be attached directly to a control's event using an anonymous delegate like this:

     button1.Click += delegate(object source, EventArgs e)
     {
         textBox1.Text = "I\'ve been clicked.";
     };