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==Demonstrate as==

<source lang="csharp">
/*
C#: The Complete Reference
by Herbert Schildt
Publisher: Osborne/McGraw-Hill (March 8, 2002)
ISBN: 0072134852
*/

// Demonstrate as.

using System;

class A {}
class B : A {}

public class CheckCast1 {
public static void Main() {
A a = new A();
B b = new B();

b = a as B; // cast, if possible

if(b==null)
Console.WriteLine("Cast b = (B) a is NOT allowed.");
else
Console.WriteLine("Cast b = (B) a is allowed");
}
}

</source>

==Demonstrate is==

<source lang="csharp">
/*
C#: The Complete Reference
by Herbert Schildt
Publisher: Osborne/McGraw-Hill (March 8, 2002)
ISBN: 0072134852
*/

// Demonstrate is.

using System;

class A {}
class B : A {}

public class UseIs {
public static void Main() {
A a = new A();
B b = new B();

if(a is A) Console.WriteLine("a is an A");
if(b is A)
Console.WriteLine("b is an A because it is derived from A");
if(a is B)
Console.WriteLine("This won"t display -- a not derived from B");

if(b is B) Console.WriteLine("B is a B");
if(a is object) Console.WriteLine("a is an Object");
}
}

</source>

==Illustrates the use of the is operator==

<source lang="csharp">
/*
Mastering Visual C# .NET
by Jason Price, Mike Gunderloy
Publisher: Sybex;
ISBN: 0782129110
*/
/*
Example3_9.cs illustrates the use of
the is operator
*/
public class Example3_9
{
public static void Main()
{
int myInt = 0;
bool compatible = myInt is int;
System.Console.WriteLine("myInt is int = " + compatible);
compatible = myInt is long;
System.Console.WriteLine("myInt is long = " + compatible);
compatible = myInt is float;
System.Console.WriteLine("myInt is float = " + compatible);
}
}

</source>

==Interfaces:The As Operator==

<source lang="csharp">
/*
A Programmer"s Introduction to C# (Second Edition)
by Eric Gunnerson
Publisher: Apress L.P.
ISBN: 1-893115-62-3
*/
// 10 - Interfaces\The As Operator
// copyright 2000 Eric Gunnerson
using System;
public class TheAsOperator
{
public static void Main()
{
DiagramObject[] dArray = new DiagramObject[100];

dArray[0] = new DiagramObject();
dArray[1] = new TextObject("Text Dude");
dArray[2] = new TextObject("Text Backup");

// array gets initialized here, with classes that
// derive from DiagramObject. Some of them implement
// IScalable.

foreach (DiagramObject d in dArray)
{
IScalable scalable = d as IScalable;
if (scalable != null)
{
scalable.ScaleX(0.1F);
scalable.ScaleY(10.0F);
}
}
}
}
interface IScalable
{
void ScaleX(float factor);
void ScaleY(float factor);
}
public class DiagramObject
{
public DiagramObject() {}
}
public class TextObject: DiagramObject, IScalable
{
public TextObject(string text)
{
this.text = text;
}
// implementing IScalable.ScaleX()
public void ScaleX(float factor)
{
Console.WriteLine("ScaleX: {0} {1}", text, factor);
// scale the object here.
}

// implementing IScalable.ScaleY()
public void ScaleY(float factor)
{
Console.WriteLine("ScaleY: {0} {1}", text, factor);
// scale the object here.
}

private string text;
}

</source>

==Operators and Expressions:Type operators:Is==

<source lang="csharp">
/*
A Programmer"s Introduction to C# (Second Edition)
by Eric Gunnerson
Publisher: Apress L.P.
ISBN: 1-893115-62-3
*/
// 14 - Operators and Expressions\Type operators\Is
// copyright 2000 Eric Gunnerson
using System;
interface IAnnoy
{
void PokeSister(string name);
}
class Brother: IAnnoy
{
public void PokeSister(string name)
{
Console.WriteLine("Poking {0}", name);
}
}
class BabyBrother
{
}
public class TypeoperatorsIs
{
public static void AnnoyHer(string sister, params object[] annoyers)
{
foreach (object o in annoyers)
{
if (o is IAnnoy)
{
IAnnoy annoyer = (IAnnoy) o;
annoyer.PokeSister(sister);
}
}
}
public static void Main()
{
TestoperatorsIs.AnnoyHer("Jane", new Brother(), new BabyBrother());
}
}

</source>

==Test is and as==

<source lang="csharp">
/*
Learning C#
by Jesse Liberty
Publisher: O"Reilly
ISBN: 0596003765
*/
using System;
namespace InterfaceDemo
{
interface IStorable
{
void Read();
void Write(object obj);
int Status { get; set; }
}
// the Compressible interface is now the
// base for ILoggedCompressible
interface ICompressible
{
void Compress();
void Decompress();
}
// extend ICompressible to log the bytes saved
interface ILoggedCompressible : ICompressible
{
void LogSavedBytes();
}

// Document implements both interfaces
class Document : IStorable, ILoggedCompressible
{
// the document constructor
public Document(string s)
{
Console.WriteLine("Creating document with: {0}", s);
}
// implement IStorable
public void Read()
{
Console.WriteLine(
"Implementing the Read Method for IStorable");
}
public void Write(object o)
{
Console.WriteLine(
"Implementing the Write Method for IStorable");
}
public int Status
{
get { return status; }
set { status = value; }
}
// implement ICompressible
public void Compress()
{
Console.WriteLine("Implementing Compress");
}
public void Decompress()
{
Console.WriteLine("Implementing Decompress");
}
// implement ILoggedCompressible
public void LogSavedBytes()
{
Console.WriteLine("Implementing LogSavedBytes");
}
// hold the data for IStorable"s Status property
private int status = 0;
}

public class TesterISAS
{
public void Run()
{
Document doc = new Document("Test Document");
// cast using as, then test for null
IStorable isDoc = doc as IStorable;
if (isDoc != null)
{
isDoc.Read();
}
else
{
Console.WriteLine("Could not cast to IStorable");
}

ILoggedCompressible ilDoc = doc as ILoggedCompressible;
if (ilDoc != null)
{
Console.Write("\nCalling both ICompressible and ");
Console.WriteLine("ILoggedCompressible methods...");
ilDoc.rupress();
ilDoc.LogSavedBytes();
}
else
{
Console.WriteLine("Could not cast to ILoggedCompressible");
}
// cast using as, then test for null
ICompressible icDoc = doc as ICompressible;
if (icDoc != null)
{
Console.WriteLine(
"\nTreating the object as Compressible... ");
icDoc.rupress();
}
else
{
Console.WriteLine("Could not cast to ICompressible");
}
}
[STAThread]
static void Main()
{
TesterISAS t = new TesterISAS();
t.Run();
}
}
}

</source>

==Use is to avoid an invalid cast==

<source lang="csharp">
/*
C#: The Complete Reference
by Herbert Schildt
Publisher: Osborne/McGraw-Hill (March 8, 2002)
ISBN: 0072134852
*/

// Use is to avoid an invalid cast.

using System;

class A {}
class B : A {}

public class CheckCast {
public static void Main() {
A a = new A();
B b = new B();

// Check to see if a can be cast to B.
if(a is B) // if so, do the cast
b = (B) a;
else // if not, skip the cast
b = null;

if(b==null)
Console.WriteLine("Cast b = (B) a is NOT allowed.");
else
Console.WriteLine("Cast b = (B) a is allowed");
}
}

</source>

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