发信人: Erratic (漂砾), 信区: DotNET
标 题: C++ -> C#（转载）
发信站: BBS 水木清华站 (Thu Jun 28 21:03:30 2001)
From MSDN Magazine
C++ -> C#: What You Need to Know to Move from C++ to C#
Jesse Liberty
This article assumes you're familiar with C++
Level of Difficulty 1 2 3
Download the code for this article: CtoCsharp(41KB)
SUMMARY C# builds on the syntax and semantics of C++, allowing C programmers
to take advantage of .NET and the common language runtime. While the
transition from C++ to C# should be a smooth one, there are a few things to
watch out for including changes
to new, structs, constructors, and destructors. This article explores the
language features that are new to C# such as garbage collection, properties,
foreach loops, and interfaces. Following a discussion of interfaces, there's
a discussion of
properties, arrays, and the base class libraries. The article concludes with
an exploration of asynchronous I/O, attributes and reflection, type
discovery, and dynamic invocation.
------------------------------------------------------------------------------
--
very 10 years or so, developers must devote time and energy to learning a
new set of programming skills. In the early 1980s it was Unix and C; in the
early 1990s it was Windows? and C++; and today it is the Microsoft? .NET
Framework and C#. While this
process takes work, the benefits far outweigh the costs. The good news is
that with C# and .NET the analysis and design phases of most projects are
virtually unchanged from what they were with C++ and Windows. That said,
there are significant
differences in how you will approach programming in the new environment. In
this article I'll provide information about how to make the leap from
programming in C++ to programming in C#.
Many articles (for example, http://msdn.microsoft.com/msdnmag/issues/0900/csha
rp/csharp.asp) have explained the overall improvements that C# implements,
and I won't repeat that information here. Instead, I'll focus on what I see
as the most significant
change when moving from C++ to C#: going from an unmanaged to a managed
environment. I'll also warn you about a few significant traps awaiting the
unwary C++ programmer and I'll show some of the new features of the language
that will affect how you
implement your programs.
Moving to a Managed Environment
C++ was designed to be a low-level platform-neutral object-oriented
programming language. C# was designed to be a somewhat higher-level
component-oriented language. The move to a managed environment represents a
sea change in the way you think about
programming. C# is about letting go of precise control, and letting the
framework help you focus on the big picture.
For example, in C++ you have tremendous control over the creation and even
the layout of your objects. You can create an object on the stack, on the
heap, or even in a particular place in memory using the placement operator
new.
With the managed environment of .NET, you give up that level of control. When
you choose the type of your object, the choice of where the object will be
created is implicit. Simple types (ints, doubles, and longs) are always
created on the stack
(unless they are contained within other objects), and classes are always
created on the heap. You cannot control where on the heap an object is
created, you can't get its address, and you can't pin it down in a particular
memory location. (There are
ways around these restrictions, but they take you out of the mainstream.)
You no longer truly control the lifetime of your object. C# has no
destructor. The garbage collector will take your item's storage back sometime
after there are no longer any references to it, but finalization is
nondeterministic.
The very structure of C# reflects the underlying framework. There is no
multiple inheritance and there are no templates because multiple inheritance
is terribly difficult to implement efficiently in a managed, garbage-collected
environment, and because
generics have not been implemented in the framework.
The C# simple types are nothing more than a mapping to the underlying common
language runtime (CLR) types. For example, a C# int maps to a System.Int32.
The types in C# are determined not by the language, but by the common type
system. In fact, if you
want to preserve the ability to derive C# objects from Visual Basic? objects,
you must restrict yourself further, to the common language subset—those
features shared by all .NET languages.
On the other hand, the managed environment and CLR bring a number of tangible
benefits. In addition to garbage collection and a uniform type system across
all .NET languages, you get a greatly enhanced component-based language,
which fully supports
versioning and provides extensible metadata, available at runtime through
reflection. There is no need for special support for late binding; type
discovery and late binding are built into the language. In C#, enums and
properties are first-class
members of the language, fully supported by the underlying engine, as are
events and delegates (type-safe function pointers).
The key benefit of the managed environment, however, is the .NET Framework.
While the framework is available to any .NET language, C# is a language
that's well-designed for programming with the framework's rich set of
classes, interfaces, and objects.
Traps
C# looks a lot like C++, and while this makes the transition easy, there are
some traps along the way. If you write what looks like perfectly legitimate
code in C++, it won't compile, or worse, it won't behave as expected. Most of
the syntactic changes
from C++ to C# are trivial (no semicolon after a class declaration, Main is
now capitalized). I'm building a Web page which lists these for easy
reference (see http://www.LibertyAssociates.com/Books&Resources.htmand click
on the FAQ for Programming
C#), but most of these are easily caught by the compiler and I won't devote
space to them here. I do want to point out a few significant changes that
will cause problems, however.
Reference and Value Types
C# distinguishes between value types and reference types. Simple types (int,
long, double, and so on) and structs are value types, while all classes are
reference types, as are Objects. Value types hold their value on the stack,
like variables in C++,
unless they are embedded within a reference type. Reference type variables
sit on the stack, but they hold the address of an object on the heap, much
like pointers in C++. Value types are passed to methods by value (a copy is
made), while reference
types are effectively passed by reference.
Structs
Structs are significantly different in C#. In C++ a struct is exactly like a
class, except that the default inheritance and default access are public
rather than private. In C# structs are very different from classes. Structs
in C# are designed to
encapsulate lightweight objects. They are value types (not reference types),
so they're passed by value. In addition, they have limitations that do not
apply to classes. For example, they are sealed, which means they cannot be
derived from or have any
base class other than System.ValueType, which is derived from Object. Structs
cannot declare a default (parameterless) constructor.
On the other hand, structs are more efficient than classes so they're perfect
for the creation of lightweight objects. If you don't mind that the struct is
sealed and you don't mind value semantics, using a struct may be preferable
to using a class,
especially for very small objects.
Everything Derives from Object
In C# everything ultimately derives from Object. This includes classes you
create, as well as value types such as int or structs. The Object class
offers useful methods, such as ToString. An example of when you use ToString
is with the
System.Console.WriteLine method, which is the C# equivalent of cout. The
method is overloaded to take a string and an array of objects.
To use WriteLine you provide substitution parameters, not unlike the
old-fashioned printf. Assume for a moment that myEmployee is an instance of a
user-defined Employee class and myCounter is an instance of a user-defined
Counter class. If you write
the following code
Console.WriteLine("The employee: {0}, the counter value: {1}",
myEmployee, myCounter);
WriteLine will call the virtual method Object.ToString on each of the
objects, substituting the strings they return for the parameters. If the
Employee class does not override ToString, the default implementation
(derived from System.Object) will be
called, which will return the name of the class as a string. Counter might
override ToString to return an integer value. If so, the output might be:
The employee: Employee, the counter value: 12
What happens if you pass integer values to WriteLine? You can't call ToString
on an integer, but the compiler will implicitly box the int in an instance of
Object whose value will be set to the value of the integer. When WriteLine
calls ToString, the
object will return the string representation of the integer's value (see
Figure 1).
Reference Parameters and Out Parameters
In C#, as in C++, a method can only have one return value. You overcome this
in C++ by passing pointers or references as parameters. The called method
changes the parameters, and the new values are available to the calling
method.
When you pass a reference into a method, you do have access to the original
object in exactly the way that passing a reference or pointer provides you
access in C++. With value types, however, this does not work. If you want to
pass the value type by
reference, you mark the value type parameter with the ref keyword.
public void GetStats(ref int age, ref int ID, ref int yearsServed)
Note that you need to use the ref keyword in both the method declaration and
the actual call to the method.
Fred.GetStats(ref age, ref ID, ref yearsServed);
You can now declare age, ID, and yearsServed in the calling method and pass
them into GetStats and get back the changed values.
C# requires definite assignment, which means that the local variables, age,
ID, and yearsServed must be initialized before you call GetStats. This is
unnecessarily cumbersome; you're just using them to get values out of
GetStats. To address this
problem, C# also provides the out keyword, which indicates that you may pass
in uninitialized variables and they will be passed by reference. This is a
way of stating your intentions explicitly:
public void GetStats(out int age, out int ID, out int yearsServed)
Again, the calling method must match.
Fred.GetStats(out age,out ID, out yearsServed);
Calling New
In C++, the new keyword instantiates an object on the heap. Not so in C#.
With reference types, the new keyword does instantiate objects on the heap,
but with value types such as structs, the object is created on the stack and
a constructor is called.
You can, in fact, create a struct on the stack without using new, but be
careful! New initializes the object. If you don't use new, you must
initialize all the values in the struct by hand before you use it (before you
pass it to a method) or it won't
compile. Once again, definite assignment requires that every object be
initialized (see Figure 2).
Properties
Most C++ programmers try to keep member variables private. This data hiding
promotes encapsulation and allows you to change your implementation of the
class without breaking the interface your clients rely on. You typically want
to allow the client to
get and possibly set the value of these members, however, so C++ programmers
create accessor methods whose job is to modify the value of the private
member variables.
In C#, properties are first-class members of classes. To the client, a
property looks like a member variable, but to the implementor of the class it
looks like a method. This arrangement is perfect; it allows you total
encapsulation and data hiding
while giving your clients easy access to the members.
You can provide your Employee class with an Age property to allow clients to
get and set the employee's age member.
public int Age
{
get
{
return age;
}
set
{
age = value;
}
}
The keyword value is implicitly available to the property. If you write
Fred.Age = 17;
the compiler will pass in the value 17 as value.
You can create a read-only property for YearsServed by implementing the Get
and not the Set accessor.
public int YearsServed
{
get
{
return yearsServed;
}
}
If you change your driver program to use these accessors, you can see how
they work (see Figure 3).
You can get Fred's age through the property and then you can use that
property to set the age. You can access the YearsServed property to obtain
the value, but not to set it; if you uncomment the last line, the program
will not compile.
If you decide later to retrieve the Employee's age from a database, you need
change only the accessor implementation; the client will not be affected.
Arrays
C# provides an array class which is a smarter version of the traditional
C/C++ array. For example, it is not possible to write past the bounds of a C#
array. In addition, Array has an even smarter cousin, ArrayList, which can
grow dynamically to manage
the changing size requirements of your program.
Arrays in C# come in three flavors: single-dimensional, multidimensional
rectangular arrays (like the C++ multidimensional arrays), and jagged arrays
(arrays of arrays).
You can create a single-dimensional array like this:
int[] myIntArray = new int[5];
Otherwise, you can initialize it like this:
int[] myIntArray = { 2, 4, 6, 8, 10 };
You can create a 4×3 rectangular array like this:
int[,] myRectangularArray = new int[rows, columns];
Alternatively, you can simply initialize it, like this:
int[,] myRectangularArray =
{
{0,1,2}, {3,4,5}, {6,7,8}, {9,10,11}
};
Since jagged arrays are arrays of arrays, you supply only one dimension
int[][] myJaggedArray = new int[4][];
and then create each of the internal arrays, like so:
myJaggedArray[0] = new int[5];
myJaggedArray[1] = new int[2];
myJaggedArray[2] = new int[3];
myJaggedArray[3] = new int[5];
Because arrays derive from the System.Array object, they come with a number
of useful methods, including Sort and Reverse.
Indexers
It is possible to create your own array-like objects. For example, you might
create a listbox which has a set of strings that it will display. It would be
convenient to be able to access the contents of the box with an index, just
as if it were an
array.
string theFirstString = myListBox[0];
string theLastString = myListBox[Length-1];
This is accomplished with Indexers. An Indexer is much like a property, but
supports the syntax of the index operator. Figure 4 shows a property whose
name is followed by the index operator.
Figure 5 shows how to implement a very simple ListBox class and provide
indexing for it.
Interfaces
A software interface is a contract for how two types will interact. When a
type publishes an interface, it tells any potential client, "I guarantee I'll
support the following methods, properties, events, and indexers."
C# is an object-oriented language, so these contracts are encapsulated in
entities called interfaces. The interface keyword declares a reference type
which encapsulates a contract.
Conceptually, an interface is similar to an abstract class. The difference is
that an abstract class serves as the base class for a family of derived
classes, while interfaces are meant to be mixed in with other inheritance
trees.
The IEnumerable Interface
Returning to the previous example, it would be nice to be able to print the
strings from the ListBoxTest class using a foreach loop, as you can with a
normal array. You can accomplish this by implementing the IEnumerable
interface in your class, which
is used implicitly by the foreach construct. IEnumerable is implemented in
any class that wants to support enumeration and foreach loops.
IEnumerable has only one method, GetEnumerator, whose job is to return a
specialized implementation of IEnumerator. Thus the semantics of an
Enumerable class allow it to provide an Enumerator.
The Enumerator must implement the IEnumerator methods. This can be
implemented either directly by the container class or by a separate class.
The latter approach is generally preferred because it encapsulates this
responsibility in the Enumerator class
rather than cluttering up the container.
I'll add an Enumerator to the ListBoxTest that you have already seen in
Figure 5. Because the Enumerator class is specific to my container class
(that is, because ListBoxEnumerator must know a lot about ListBoxTest) I will
make it a private
implementation, contained within ListBoxTest.
In this version, ListBoxTest is defined to implement the IEnumerable
interface. The IEnumerable interface must return an Enumerator.
public IEnumerator GetEnumerator()
{
return (IEnumerator) new ListBoxEnumerator(this);
}
Notice that the method passes the current ListBoxTest object (this) to the
enumerator. That will allow the enumerator to enumerate this particular
ListBoxTest object.
The class to implement the Enumerator is implemented here as ListBoxEnumerator
, which is a private class defined within ListBoxTest. Its work is fairly
straightforward.
The ListBoxTest to be enumerated is passed in as an argument to the
constructor, where it is assigned to the member variable myLBT. The
constructor also sets the member variable index to -1, indicating that
enumerating the object has not yet begun.
public ListBoxEnumerator(ListBoxTest theLB)
{
myLBT = theLB;
index = -1;
}
The MoveNext method increments the index and then checks to ensure that you
have not run past the end of the object you're enumerating. If you have, you
return false; otherwise, true is returned.
public bool MoveNext()
{
index++;
if (index >= myLBT.myStrings.Length)
return false;
else
return true;
}
Reset does nothing but reset the index to -1.
The property Current is implemented to return the last string added. This is
an arbitrary decision; in other classes Current will have whatever meaning
the designer decides is appropriate. However it's defined, every enumerator
must be able to return
the current member, as accessing the current member is what enumerators are
for.
public object Current
{
get
{
return(myLBT[index]);
}
}
That's all there is to it. The call to foreach fetches the enumerator and
uses it to enumerate over the array. Since foreach will display every string
whether or not you've added a meaningful value, I've changed the
initialization of myStrings to eight
items to keep the display manageable.
myStrings = new String[8];
Using the Base Class Libraries
To get a better sense of how C# differs from C++ and how your approach to
solving problems might change, let's examine a slightly less trivial example.
I'll build a class to read a large text file and display its contents on the
screen. I'd like to
make this a multithreaded program so that while the data is being read from
the disk, I can do other work.
In C++ you would create a thread to read the file, and another thread to do
the other work. These threads would work independently, but they might need
synchronization. You can do all of that in C# as well, but most of the time
you won't need to write
your own threading because .NET provides very powerful mechanisms for
asynchronous I/O.
The asynchronous I/O support is built into the CLR and is nearly as easy to
use as the normal I/O stream classes. You start by informing the compiler
that you'll be using objects from a number of System namespaces:
using System;
using System.IO;
using System.Text;
When you include System, you do not automatically include all its subsidiary
namespaces, each must be explicitly included with the using keyword. Since
you'll be using the I/O stream classes, you'll need System.IO, and you want
System.Text to support
ASCII encoding of your byte stream, as you'll see shortly.
The steps involved in writing this program are surprisingly simple because
.NET will do most of the work for you. I'll use the BeginRead method of the
Stream class. This method provides asynchronous I/O, reading in a buffer full
of data, and then
calling your callback method when the buffer is ready for you to process.
You need to pass in a byte array as the buffer and a delegate for the
callback method. You'll declare both of these as private member variables of
your driver class.
public class AsynchIOTester
{
private Stream inputStream;
private byte[] buffer;
private AsyncCallback myCallBack;
The member variable inputStream is of type Stream, and it is on this object
that you will call the BeginRead method, passing in the buffer as well as the
delegate (myCallBack). A delegate is very much like a type-safe pointer to
member function. In C#,
delegates are first-class elements of the language.
.NET will call your delegated method when the byte has been filled from the
file on disk so that you can process the data. While you're waiting you can
do other work (in this case, incrementing an integer from 1 to 50,000, but in
a production program
you might be interacting with the user or doing other useful tasks).
The delegate in this case is declared to be of type AsyncCallback, which is
what the BeginRead method of Stream expects. An AsyncCallback delegate is
declared in the System namespace as follows:
public delegate void AsyncCallback (IAsyncResult ar);
Thus, this delegate may be associated with any method that returns void and
takes an IAsyncResult interface as a parameter. The CLR will pass in the
IAsyncResult interface object at runtime when the method is called; you only
have to declare the method
void OnCompletedRead(IAsyncResult asyncResult)
and then to hook up the delegate in the constructor:
AsynchIOTester()
{
???
myCallBack = new AsyncCallback(this.OnCompletedRead);
}
This assigns to the member variable myCallback (which was previously defined
to be of type AsyncCallback) the instance of the delegate created by calling
the AsyncCallback constructor and passing in the method you want to associate
with the delegate.
Here's how the entire program works, step by step. In Main you create an
instance of the class and tell it to run:
public static void Main()
{
AsynchIOTester theApp = new AsynchIOTester();
theApp.Run();
}
The call to new fires up the constructor. In the constructor you open a file
and get a Stream object back. You then allocate space in the buffer and hook
up the callback mechanism.
AsynchIOTester()
{
inputStream = File.OpenRead(@"C:\MSDN\fromCppToCS.txt");
buffer = new byte[BUFFER_SIZE];
myCallBack = new AsyncCallback(this.OnCompletedRead);
}
In the Run method, you call BeginRead, which will cause an asynchronous read
of the file.
inputStream.BeginRead(
buffer, // where to put the results
0, // offset
buffer.Length, // how many bytes (BUFFER_SIZE)
myCallBack, // call back delegate
null); // local state object
You then go on to do other work.
for (long i = 0; i < 50000; i++)
{
if (i%1000 == 0)
{
Console.WriteLine("i: {0}", i);
}
}
When the read completes, the CLR will call the callback method.
void OnCompletedRead(IAsyncResult asyncResult)
{
The first thing you do in OnCompletedRead is find out how many bytes were
read by calling the EndRead method of the Stream object, passing in the
IAsyncResult interface object passed in by the common language runtime.
int bytesRead = inputStream.EndRead(asyncResult);
The result of this call to EndRead is to get back the number of bytes read.
If the number is greater than zero, convert the buffer into a string and
write it to the console, then call BeginRead again for another asynchronous
read.
if (bytesRead > 0)
{
String s = Encoding.ASCII.GetString(buffer, 0, bytesRead);
Console.WriteLine(s);
inputStream.BeginRead(buffer, 0, buffer.Length,
myCallBack, null);
}
Now you can do other work (in this case, counting to 50,000) while the reads
are taking place, but you can handle the read data (in this case, by
outputting it to the console) each time a buffer is full. The complete source
code for this example,
AsynchIO.cs, is available for download from the link at the top of this
article.
Management of the asynchronous I/O is provided entirely by the CLR. It gets
even nicer when you read over the network.
Reading a File Across the Network
In C++, reading a file across the network is a nontrivial programming
exercise. .NET provides extensive support for this. In fact, reading files
across the network is just another use of the standard Base Class Library
Stream classes.
Start by creating an instance of the TCPListener class, to listen to a TCP/IP
port (port 65000 in this case).
TCPListener tcpListener = new TCPListener(65000);
Once constructed, ask the TCPListener object to start listening.
tcpListener.Start();
Now wait for a client to request a connection.
Socket socketForClient = tcpListener.Accept();
The Accept method of the TCPListener object returns a Socket object, which
represents a standard Berkeley socket interface and which is bound to a
specific end point (in this case, the client). Accept is a synchronous method
and will not return until
it receives a connection request. If the socket is connected, you're ready to
send the file to the client.
if (socketForClient.Connected)
{
???
Next you have to create a NetworkStream class, passing the socket in to the
constructor.
NetworkStream networkStream = new NetworkStream(socketForClient);
Then create a StreamWriter object much as you did before, except this time
not on a file, but on the NetworkStream you just created.
System.IO.StreamWriter streamWriter =
new System.IO.StreamWriter(networkStream);
When you write to this stream, the stream is sent over the network to the
client. The complete source code, TCPServer.cs, is also available for
download.
Creating the Client
The client instantiates a TCPClient class, which represents a TCP/IP client
connection to a host.
TCPClient socketForServer;
socketForServer = new TCPClient("localHost", 65000);
With this TCPClient, you can create a NetworkStream, and on that stream
create a StreamReader.
NetworkStream networkStream = socketForServer.GetStream();
System.IO.StreamReader streamReader =
new System.IO.StreamReader(networkStream);
Now, read the stream as long as there is data on it, and output the results
to the console.
do
{
outputString = streamReader.ReadLine();
if( outputString != null )
{
Console.WriteLine(outputString);
}
}
while( outputString != null );
To test this, you create a simple test file:
This is line one
This is line two
This is line three
This is line four
Here is the output from the server:
Output (Server)
Client connected
Sending This is line one
Sending This is line two
Sending This is line three
Sending This is line four
Disconnecting from client...
Exiting...
And here is the output from the client:
This is line one
This is line two
This is line three
This is line four
Attributes and Metadata
One significant difference between C# and C++ is that C# provides inherent
support for metadata: data about your classes, objects, methods, and so
forth. Attributes come in two flavors: those that are supplied as part of the
CLR and attributes you
create for your own purposes. CLR attributes are used to support
serialization, marshaling, and COM interoperability. A search of the CLR
reveals a great many attributes. As you've seen, some attributes are applied
to an assembly, others to a class or
interface. These are called the attribute targets.
Attributes are applied to their target by placing them in square brackets
immediately before the target item. Attributes may be combined, either by
stacking one on top of another
[assembly: AssemblyDelaySign(false)]
[assembly: AssemblyKeyFile(".\\keyFile.snk")]
or by separating the attributes with commas.
[assembly: AssemblyDelaySign(false),
assembly: AssemblyKeyFile(".\\keyFile.snk")]
Custom Attributes
You are free to create your own custom attributes and to use them at runtime
as you see fit. For example, you might create a documentation attribute to
tag sections of code with the URL of associated documentation. Or you might
tag your code with code
review comments or bug fix comments.
Suppose your development organization wants to keep track of bug fixes. It
turns out you keep a database of all your bugs, but you'd like to tie your
bug reports to specific fixes in the code. You might add comments to your
code similar to the
following:
// Bug 323 fixed by Jesse Liberty 1/1/2005.
This would make it easy to see in your source code, but it would be nice if
you could extract this information into a report or keep it in a database so
that you could search for it. It would also be nice if all the bug report
notations used the same
syntax. A custom attribute may be just what you need. You would then replace
your comment with something like this:
[BugFix(323,"Jesse Liberty","1/1/2005") Comment="Off by one error"]
Attributes, like most things in C#, are classes. To create a custom
attribute, you derive your new custom attribute class from System.Attribute.
public class BugFixAttribute : System.Attribute
You need to tell the compiler what kinds of elements this attribute can be
used with (the attribute target). You specify this with (what else?) an
attribute.
[AttributeUsage(AttributeTargets.ClassMembers, AllowMultiple = true)]
AttributeUsage is an attribute applied to attributes—a meta attribute. It
provides, if you will, meta-metadata; that is data about the metadata. In
this case, you pass two arguments: the first is the target (in this case
class members) and a flag
indicating whether a given element may receive more than one such attribute.
AllowMultiple has been set to true, which means a class member may have more
than one BugFixAttribute assigned.
If you wanted to combine Attribute targets, you can OR them together.
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Interface,
AllowMultiple = true)]
This would allow the attribute to be attached to either a Class or an
Interface.
The new custom attribute is named BugFixAttribute. The convention is to
append the word Attribute to your attribute name. The compiler supports this
by allowing you to call the attribute, when you assign it to an element, with
the shorter version of
the name. Thus, you can write this:
[BugFix(123, "Jesse Liberty", "01/01/05", Comment="Off by one")]
The compiler will first look for an attribute named BugFix and, not finding
that, will then look for BugFixAttribute.
Every attribute must have at least one constructor. Attributes take two types
of parameters, positional and named. In the previous example, the bug ID, the
programmer's name, and the date were positional parameters and comment was a
named parameter.
Positional parameters are passed in through the constructor and must be
passed in the order declared in the constructor.
public BugFixAttribute(int bugID, string programmer, string date)
{
this.bugID = bugID;
this.programmer = programmer;
this.date = date;
}
Named parameters are implemented as properties.
Using the Attribute
To test the attribute, create a simple class named MyMath and give it two
functions. Then assign the bug fix attributes to the class.
[BugFixAttribute(121,"Jesse Liberty","01/03/05")]
[BugFixAttribute(107,"Jesse Liberty","01/04/05",
Comment="Fixed off by one errors")]
public class MyMath
These attributes will be stored with the metadata. Figure 6 provides the
complete source code. The following shows the output:
Calling DoFunc(7). Result: 9.3333333333333339
As you can see, the attributes had absolutely no impact on the output, and
creating attributes has no impact on performance. In fact, for the moment,
you have only my word that the attributes exist at all. A quick look at the
metadata using ILDASM does
reveal that the attributes are in place, however, as shown in Figure 7.
Reflection
For this to be useful, you need a way to access the attributes from the
metadata—ideally during runtime. C# provides support for reflection for
examining the metadata. Start by initializing an object of type MemberInfo.
This object, in the
System.Reflection namespace, is provided to discover the attributes of a
member and to provide access to the metadata.
System.Reflection.MemberInfo inf = typeof(MyMath);
Call the typeof operator on the MyMath type, which returns an object of type
Type, which derives from MemberInfo.
The next step is to call GetCustomAttributes on this MemberInfo object,
passing in the type of the attribute you want to find. What you get back is
an array of objects, each of which is of type BugFixAttribute:
object[] attributes;
attributes = Attribute.GetCustomAttributes(inf,
typeof(BugFixAttribute));
You can now iterate through this array, printing out the properties of the
BugFixAttribute object, as shown in Figure 8. When this replacement code is
put into the listing in Figure 6, the metadata is displayed.
Type Discovery
You can use reflection to explore and examine the contents of an assembly.
You'll find this particularly useful if you're building a tool which needs to
display information about the assembly, or if you want to dynamically invoke
methods in the
assembly. You might want to do so if you're developing a scripting engine,
which would allow your users to generate scripts and run them through your
program.
With reflection, you can find the types associated with a module, the
methods, fields, properties, and events associated with a type, as well as
the signatures of each of the type's methods, the interfaces supported by the
type, and the type's
superclass.
To start, let's load an assembly dynamically with the Assembly.Load static
method. The signature for this method is as follows:
public static Assembly.Load(AssemblyName)
Then you should pass in the core library.
Assembly a = Assembly.Load("Mscorlib.dll");
Once the assembly is loaded, you can call GetTypes to return an array of Type
objects. The Type object is the heart of reflection. Type represents type
declarations: classes, interfaces, arrays, values, and enumerations.
Type[] types = a.GetTypes();
The assembly returns an array of types that can be displayed in a foreach
loop. The output from this will fill many pages. Here is a short excerpt from
the output:
Type is System.TypeCode
Type is System.Security.Util.StringExpressionSet
Type is System.Text.UTF7Encoding$Encoder
Type is System.ArgIterator
Type is System.Runtime.Remoting.JITLookupTable
1205 types found
You have obtained an array filled with the types from the core library, and
printed them one by one. As the output shows, the array contains 1,205
entries.
Reflecting on a Type
You can reflect on a single type in the assembly as well. To do so, you
extract a type from the assembly with the GetType method.
public class Tester
{
public static void Main()
{
// examine a single object
Type theType = Type.GetType("System.Reflection.Assembly");
Console.WriteLine("\nSingle Type is {0}\n", theType);
}
}
The output looks like this:
Single Type is System.Reflection.Assembly
Finding the Members
You can ask this type for all its members, listing all the methods,
properties, and fields, as you can see in Figure 9.
Once again the output is quite lengthy, but within the output you see fields,
methods, constructors, and properties, as shown in this excerpt:
System.String s_localFilePrefix is a Field
Boolean IsDefined(System.Type) is a Method
Void .ctor() is a Constructor
System.String CodeBase is a Property
System.String CopiedCodeBase is a Property
Finding Only Methods
You might want to focus on only the methods, excluding the fields,
properties, and so forth. To do so, you remove the call to GetMembers.
MemberInfo[] mbrInfoArray =
theType.GetMembers(BindingFlags.LookupAll);
Then you add a call to GetMethods.
mbrInfoArray = theType.GetMethods();
The output now is nothing but the methods.
Output (excerpt)
Boolean Equals(System.Object) is a Method
System.String ToString() is a Method
System.String CreateQualifiedName(System.String, System.String)
is a Method
System.Reflection.MethodInfo get_EntryPoint() is a Method
Finding Particular Members
Finally, to narrow down even further, you can use the FindMembers method to
find particular members of the type. For example, you can restrict your
search to methods whose names begin with the letters "Get", as you can see in
Figure 10.
An excerpt of the output looks like this:
System.Type[] GetTypes() is a Method
System.Type[] GetExportedTypes() is a Method
System.Type GetType(System.String, Boolean) is a Method
System.Type GetType(System.String) is a Method
System.Reflection.AssemblyName GetName(Boolean) is a Method
System.Reflection.AssemblyName GetName() is a Method
Int32 GetHashCode() is a Method
System.Reflection.Assembly GetAssembly(System.Type) is a Method
System.Type GetType(System.String, Boolean, Boolean) is a Method
Dynamic Invocation
Once you've discovered a method, it is possible to invoke it using
reflection. For example, you might like to invoke the Cos method of
System.Math, which returns the cosine of an angle.
To do so, you will get the Type information for the System.Math class, like
so:
Type theMathType = Type.GetType("System.Math");
With that type information, you can dynamically load an instance of that
class.
Object theObj = Activator.CreateInstance(theMathType);
CreateInstance is a static method of the Activator class, which can be used
to instantiate objects.
With an instance of System.Math in hand, you can call the Cos method. To do
so, you must prepare an array that will describe the types of the parameters.
Since Cos takes a single parameter (the angle whose cosine you want) you need
an array with a
single member. Into that array you'll put a Type object for the System.Double
type, which is the type of parameter expected by Cos.
Type[] paramTypes = new Type[1];
paramTypes[0]= Type.GetType("System.Double");
You can now pass the name of the method you want and this array describing
the types of the parameters to the GetMethod method of the type object
retrieved earlier.
MethodInfo CosineInfo =
theMathType.GetMethod("Cos",paramTypes);
You now have an object of type MethodInfo on which you can invoke the method.
To do so, you must pass in the actual value of the parameters, again in an
array.
Object[] parameters = new Object[1];
parameters[0] = 45;
Object returnVal = CosineInfo.Invoke(theObj,parameters);
Note that I've created two arrays. The first, paramTypes, held the type of
the parameters; the second, parameters, held the actual value. If the method
had taken two arguments, you would have declared these arrays to hold two
values. If the method took
no values, you still would create the array, but you would give it a size of
zero!
Type[] paramTypes = new Type[0];
Odd as this looks, it is correct. Figure 11 shows the complete code.
Conclusion
While there are a number of subtle traps waiting for the unwary C++
programmer, the syntax of C# is not very different from C++ and the
transition to the new language is fairly easy. The interesting part of
working with C# is working your way through
the new common language runtime library, which provides a host of
functionality that previously had to be written by hand. This article could
only touch on a few highlights. The CLR and the .NET Framework provide
extensive support for threading,
marshaling, Web application development, Windows-based application
development, and so forth.
The distinction between language features and CLR features is a bit blurry at
times, but the combination is a very powerful development tool.
------------------------------------------------------------------------------
--
For related articles see:
C# Offers the Power of C++ and Simplicity of Visual Basic
For background information see:
Getting Started with the .NET Framework
------------------------------------------------------------------------------
--
Jesse Liberty is the author of a dozen books on software development. He is
the president of Liberty Associates Inc. (http://www.LibertyAssociates.com)
where he provides training in .NET technology and contract programming. This
article has been
adapted from his upcoming book, Programming C#, to be published by O'Reilly &
Associates, Inc. in 2001.
------------------------------------------------------------------------------
--
From the July 2001 issue of MSDN Magazine.
--
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