In the previous chapter, we looked at the user interface
guidelines for WP7 applications. We will now dive deeper into what
it takes to implement a WP7 application.
In this chapter, we will look at the various C# features that
map to the most common Objective-C features. We will provide code
snippets which will ease your way into C# code. We will point to
the key C# features that help you write safe code and enhance
productivity.
Introduction to Managed Programming
WP7 only supports managed
programming in C# or VB.NET. Before we jump into the details of
C#, let us briefly review managed programming.
The C# compiler (and similarly, the VB compiler) compiles the C#
(or VB.NET) code into an intermediate language (IL) bytecode and
metadata. The Common Language Runtime (CLR) executes the byte code.
It uses metadata to manage type safety, exception handling, array
bounds, etc. The CLR also manages memory and performs garbage
collection. In contrast, Objective-C code is compiled into ARM
binary code and executed directly.
Comparison between C# Features and Objective-C Classes
Class Declaration
Let us start with an example program. In contrast to
Objective-C, C# does not separate the class definition and the
implementation. The compiler derives the metadata about the classes
from the class implementation itself. You will also notice that you
do not need to define each class in a separate file as in
Objective-C.
In the example, the public signature of the class Person
consists of just the property, age, and the constructor. The rest
of the class implementation is opaque.
Instead of using the import statement, C# employs a using
statement to refer to the metadata of other classes. The namespace
declaration, shown at the top of the file, is used to both declare
scope and organize the code. You can access classes in other
namespaces by referring to a fully qualified name. See the
reference to System.Console.WriteLine in the example above, where
console is in the System namespace.
Objective-C uses a message passing syntax consisting of square
brackets, and a dot-notation for accessing properties. C# uniformly
uses the “.” notation for referring to all methods, fields and
properties.
Strong Typing
In contrast to Objective-C, C# is a very strongly typed
language. Types must be specified for variables as well as
input/output parameters. Types are enforced strictly by the
compiler. Objective-C uses weak typing for collection classes such
as NSArray and NSDictionary. In the section on generics below, we
will see how C# uses strong typing for collection classes.
The example above shows the strong typing for primitive types.
Strong typing works similarly for all classes.
Class Constructors
In contrast to the separate alloc and init statements of
Objective-C, in C#, instance constructors are used to create and
initialize instances. For example, p, an instance of the Person
class, can be both constructed and initialized with a given
birthdate, in a single statement.
Properties
Developers often need to decide about whether to implement a
member as a property or a method. In this case, the design pattern
is identical for Objective-C and C#. In general, the guidance is to
use properties when accessing data, and to use methods when there
is an action taken on the data.
As opposed to the Objective-C @property attribute, C# properties
are declared by the explicit definition of a getter, a setter, or
both. You can make the property read-only by providing just the
getter, write-only by providing just the setter or read-write, by
providing both.
Parameter Types
Similarly to Objective-C, C# uses value parameters by default.
While C# does not have pointers, it allows passing of parameters by
reference by using the ‘ref’ modifier. Instead of pointers,
parameters with ref can be used where you want to achieve side
effects in a method. In some cases, reference parameters are more
efficient, since they avoid data copying.
C# also provides parameters with an out modifier which denotes
parameters that must be initialized by the called method before
returning. This design pattern is often used to return the error in
addition to the value of the function.
Access Privileges
In Objective-C, access privilege can only be specified on
variables. Methods which are present only in the .m file are
private. On the other hand, C# allows access privileges on fields
(e.g., birthDate), properties (e.g., age) and methods (e.g.,
ageOn). It uses public, private and protected as modifiers to
denote three different levels of access privileges.
In the above example, the compiler will error out on p.birthDate
since that variable is private and therefore is not accessible from
the Program class. Similarly, the method ageOn is also private and
inaccessible from the Program class.
Methods with multiple parameters
Both Objective-C and C# support methods with multiple
parameters. In Objective-C method parameters are positional and
named, i.e., the names of formal parameters are used while passing
actual parameters. The name of the method is comprised of
everything to the left of the colon (“:”), for example, the name of
the Objective-C method below is addEmployee:name:age:. While C#
traditionally used positional and unnamed parameters, the latest
version of C# has also introduced named parameters. The following
example shows the comparative syntax for Objective-C and C#.
Objective-C does not support method overloading. While it does
not allow exactly the same method signature with different
parameter types, the following design pattern is commonly used in
Objetive-C programs:
– (void)insert:(myClass *)obj
atIndex:(NSInteger)index
– (void)insert:(myClass *)obj beforeObj:(myClass
*)obj
[mylist insert:obj1 atIndex:4];
[mylist insert:obj2 beforeObj:obj1];
As we saw earlier, the names of these two methods are different
and are “insert:atIndex” and “insert:beforeObj” respectively.
On the other hand, C# explicitly supports method overloading.
Using information about the parameter types, C# disambiguates
between methods with the same name.
void insert(myClass obj, int index);
void insert(myClass obj, myClass before);
The method insert may be called with both signatures:
list.insert(myObj1, 4);
list.insert(myObj1, myObj2);
Now that we have examined some of the basic class concepts in
C#, let us look at another example:
Inheritance
Like Objective-C, C# also uses a single inheritance mechanism.
Inheritance is specified by listing the parent class after the name
of the class as shown below. In the above example, the class
Rectangle inherits from the class Shape, whereas the class Square
inherits from the class Rectangle.
In C#, the constructor of the base class is automatically
invoked when constructing an instance of a derived class. However,
a derived class can invoke a specific constructor of the base class
if needed as shown in the constructor of the Square class.
In contrast to Objective-C, a C# derived class may not override
a method by just redefining it. The class must use the keyword
“override” in its method definition.
Protected Access
Objective-C provides protected variables, but methods cannot be
protected. In C#, access to fields, properties and methods can also
be controlled using the protected modifier. You can implement
protected variables in C# by using the protected access modifier,
as shown below:
Instance vs Class Level Access
While Objective-C and C# use different syntactic notation for
static methods or variables, they behave the same way. C# uses a
‘static’ modifier to denote class level methods, fields or
properties. Everything else is at an instance level. In the above
example, counter is a class level variable.
Abstract Classes
Abstract classes, are classes that cannot be instantiated. While
Objective-C does not provide a syntax for abstract classes, many
programmers use them by returning NULL from the abstract class init
method. The class Shape, defined above in C#, is an abstract class
and requires that both Area and the method contains must be
overridden in any derived classes.
Interfaces
Objective-C protocols and C# interfaces are similar. In the
example below, IThreeDShape defines an interface that is
implemented by the Cube class.
Polymorphism
Polymorphism works the same way in both Objective-C and C#. A C#
derived class can be passed as a parameter to a method that expects
a base class. Similarly, a class that implements a particular
interface can also be passed as a parameter to the method. This is
shown in the example below, where an object of the class Cube is
passed as a parameter, where the method expects an object of the
class IThreeDShape.
Structs
In contrast to the C-based structs used in Objective-C, C#
structs are closer to classes. C# structs can have constructors,
methods and properties as well as access modifiers. However, the
primary difference between a struct and a class is that a struct is
a value type, versus a class, which is a reference type.
Object Lifecycle – Creation and Deletion of Objects
Memory management is very different in Objective-C and C#. In
contrast to Objective-C, C# performs automatic memory management.
As we saw earlier, developers do not allocate memory, but use the
“new” operator to create objects on the heap and initialize them.
Equally important, in C#, the developer is not responsible for
tracking memory usage or knowing when to free memory. When the
object is no longer accessed by the code, the object is eligible
for garbage collection. Periodically, the .NET CLR garbage
collector frees up the memory for such objects.
In rare circumstances, developers may need to perform cleanup at
the time the object is destroyed. C# allows the use of destructors,
but in practice this is rare.
Other Topics
Type Checking v/s Reflection
In Objective-C, you can check the type of the class or determine
if an object supports a particular method and invoke the method on
that object. In C#, reflection is a versatile feature. You can use
reflection to get the type information from an existing object,
dynamically create an instance of a type, bind the type to an
existing object, invoke its methods or access its fields and
properties.
The following table explains the mapping between dynamic type
checking in Objective-C and the corresponding C# reflection
features.
Exception Handling
Exception handling is similar in C# and Objective-C. You use a
try-catch block to handle exceptions. Additionally, you can either
catch specific exceptions or use a catch-all statement. This is
similar to @try, @catch and @finally statements in Objective-C.
Key class libraries compared
Strings
C# provides a very comprehensive string class, which gives you
all the features that you are familiar with in the NSString
class.
Arrays
Dictionaries
C# provides a generic dictionary class that provides the
functionality of NSMutableDictionary. It allows addition, lookup
and removal of objects in the dictionary. Since it uses generics,
it also utilizes strong typing.
New features of C#
Generics
Generics introduce the notion of type parameters, that make it
possible to design classes that are type safe, even though the
actual type is deferred till the object instantiation. For example,
here is how you define a generic stack:
The Stack<T> uses T as a type parameter allowing you to
instantiate a stack of any type, e.g. Stack<int> or
Stack<string> and use them in a type safe manner.
Use of generics is closest to the use of id in Objective-C
collection classes such as NSDictionary.
Operator Overloading
Operator overloading permits a user defined implementation of
user-defined operators for user-defined classes. Consider the
following example of a Complex number struct. Operator overloading
allows you to define a ‘+’ operation using a natural syntax.
Delegates
Objective-C developers often use delegation for notification as
to when an asynchronous operation is completed. In C#, delegates
are similar to function pointers in C or Objective-C. In this
design pattern, a class delegates another class, not known at
compile time, to complete its action.
Let us look at the above example, where StringDelegate is
defined as a function that takes a string as a parameter and
returns void. Three different delegates, writer, logger and
multiLogger, are constructed by passing in methods that have the
same signature as the StringDelegate declaration. This can be
contrasted with Objective-C delegates, which are usually defined
using protocol declaration.
Calling Writer invokes the writeString method of ConsoleLogger
to print the message to the console. On the other hand, calling
Logger invokes the logString method of FileLogger to log the
message to the file. As you can see, delegates achieve indirection
while providing type safety. Delegates may be concatenated, as
shown by MultiLogger, which logs the message to both loggers.
Events
Events in C# are useful in the pub-sub (publisher and
subscriber) design pattern. Events are a very powerful design
pattern for asynchronous programming. An object can publish a set
of events that subscribers in turn subscribe to. When the publisher
raises the event, all subscribers are notified. The publisher
raises the event without knowing who is listening to the events.
Events are built using delegates, as shown below.
Summary
C# is a strongly typed, object oriented programming language
that uses static binding. Many Objective-C concepts map closely to
corresponding C# concepts. This chapter is a quick introduction to
how C# concepts map to those of Objective-C. It provides a starting
point for Objective-C users and points to additional resources.
Knowledge of object oriented programming, and Objective-C
experience, will provide a strong foundation to enable you to
master C# quickly.
Published: 2/2/2011