qr code generator c# example a b B a is is is is an A an A because it is derived from A a B an object in C#.NET

Drawing QR-Code in C#.NET a b B a is is is is an A an A because it is derived from A a B an object

Most of the is expressions are self-explanatory, but two may need a little discussion First, notice this statement:

if(b is A) ConsoleWriteLine("b is an A because it is derived from A");

The if succeeds because b is an object of type B, which is derived from type A Thus, b is an A However, the reverse is not true When this line is executed,

if(a is B) ConsoleWriteLine("This won t display -- a not derived from B");

Sometimes you will want to try a conversion at runtime, but not throw an exception if the conversion fails (which is the case when a cast is used) To do this, use the as operator, which has this general form: expr as type

17:

Here, expr is the expression being converted to type If the conversion succeeds, then a reference to type is returned Otherwise, a null reference is returned The as operator can be used to perform only reference, boxing, unboxing, or identity conversions The as operator offers a streamlined alternative to is in some cases For example, consider the following program that uses is to prevent an invalid cast from occurring:

// Use is to avoid an invalid cast using System; class A {} class B : A {} class CheckCast { 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) ConsoleWriteLine("The cast in b = (B) a is NOT allowed"); else ConsoleWriteLine("The cast in b = (B) a is allowed"); } }

This program displays the following output:

As the output shows, since a is not a B, the cast of a to B is invalid and is prevented by the if statement However, this approach requires two steps First, the validity of the cast must be confirmed Second, the cast must be made These two steps can be combined into one through the use of as, as the following program shows:

// Demonstrate as using System; class A {} class B : A {} class CheckCast { static void Main() { A a = new A(); B b = new B(); b = a as B; // cast, if possible

Part I:

if(b==null) ConsoleWriteLine("The cast in b = (B) a is NOT allowed"); else ConsoleWriteLine("The cast in b = (B) a is allowed"); } }

Here is the output, which is the same as before:

In this version, the as statement checks the validity of the cast and then, if valid, performs the cast, all in one statement

Although useful in their own ways, the as and is operators simply test the compatibility of two types Often, you will need to obtain information about a type To do this, C# supplies the typeof operator It retrieves a SystemType object for a given type Using this object, you can determine the type s characteristics The typeof operator has this general form: typeof(type) Here, type is the type being obtained The Type object returned encapsulates the information associated with type Once you have obtained a Type object for a given type, you can obtain information about it through the use of various properties, fields, and methods defined by Type Type is a large class with many members, and a discussion is deferred until the next section, where reflection is examined However, to briefly demonstrate Type, the following program uses three of its properties: FullName, IsClass, and IsAbstract To obtain the full name of the type, use FullName IsClass returns true if the type is a class IsAbstract returns true if a class is abstract

// Demonstrate typeof using System; using SystemIO; class UseTypeof { static void Main() { Type t = typeof(StreamReader); ConsoleWriteLine(tFullName); if(tIsClass) ConsoleWriteLine("Is a class"); if(tIsAbstract) ConsoleWriteLine("Is abstract"); else ConsoleWriteLine("Is concrete"); } }

This program outputs the following: