A Closer Look at Intermediate Language C# Help

From what you learned in the previous section, Microsoft Intermediate Language obviously plays a fundamental role in the .NET Framework. As C# developers, we now understand that our C# code will be compiled into IL before it is executed (indeed, the C# compiler compiles only to managed code). It makes sense, then, to now take a closer look at the main characteristics of IL, because any language that targets .NET will logically need to support the main characteristics of IL, too .
Here are the important features of IL:

  • Object orientation and use of interfaces
  •  Strong distinction between value and reference types
  •  Strong data typing
  •  Error handling using exceptions
  •  Use of attributes

The following sections explore each of these characteristics.

Support for Object Orientation and Interfaces

The language independence of .NET does have some practical limitations. IL is inevitably going to implement some particular programming methodology, which means that languages targeting it need to be compatible with that methodology. The particular route that Microsoft has chosen to follow for IL is that of classic object-oriented programming, with single implementation ineritance of  classes.

          If you are unfamiliar with the concepts of object orientatioll, refer to Appendix B, “C#, Visual Basic, C++/CLI,” for more information.

In addition to classic object-oriented programming, IL also brings in the idea of interfaces, which saw their first implementation under Windows with COM. Interfaces built using .NET produce interfaces
that are not the same as COM interfaces. They do not need to support any of the COM infrastructure (for example, they are not derived from unknown, and they do not have associated globally unique identifiers, more commonly know as GUID’s). However, they do share with COM interfaces the idea that they provide a contract, and classes that implement a given interface must provide implementations of the methods and properties specified by that interface.

You have now seen that working with .NET means compiling to IL, and that in turn means that you will need to use traditional object-oriented methodologies. However, that alone is not sufficient to give you language interoperability. After all, C++ and Java both use the same object-oriented paradigms, but they are still not regarded as interoperable. We need to look a little more closely at the concept of language interoperability.

To start with, we need to consider exactly what we mean by language interoperability. After all, COM allowed components written in different languages to work together in the sense of calling each other’s methods. What was inadequate about that? COM, by virtue of being a binary standard, did allow components to instantiate other components and call methods or properties against them, without worrying about the language in which the respective components were written. To achieve this, however, each object had to be instantiated through the COM runtime, and accessed through an interface. Depending on the threading models of the relative components, there may have been large performance losses associated with marshaling data between apartments or running components or both on different threads. in the extreme case of components hosted as an executable rather than DLLfiles,
separate processes would need to be created to run them. The emphasis was very much that components could talk to each other but only via the COM runtime. In no way with COM did components written in different languages directly communicate with each other, or instantiate instances of each other – it was always done with COM as an intermediary. Not only that, but the COM architecture did not permit implementation inheritance, which meant that it lost many of the advantages of object-oriented programming.

An associated problem was that, when debugging, you would still need to debug components written in different languages independently. It was not possible to step between languages in the debugger.
Therefore, what we really mean by language interoperability is that classes written in one language should be able to talk directly to classes written in another language. In particular:

  •  A class written in one language can inherit from a class written in another language.
  • The class can contain an instance of another class, no matter what the languages of the two classes are.
  •  An object can directly call methods against another object written in another language.
  • Objects (or references to objects) can be passed around between methods.
  •  When calling methods between languages you can step between the method calls in the debugger, even when this means stepping between source code written in different languages.

This is all quite an ambitious aim, but amazingly, .NET and IL have achieved it. In the case of stepping between methods in the debugger, this facility is really offered by the Visual Studio integrated development environment (IDE) rather than by the CLR itself.

Distinct Value and Reference Types

As with any programming language, IL provides a number of predefined primitive data types. One characteristic of IL, however, is that it makes a strong distinction between value and reference types.
Value types are those for which a variable directly stores its data, whereas reference types are those for which a variable simply stores the address at which the corresponding data can be found.

In C++ terms, using reference types can be considered to be similar to accessing a variable through a pointer, whereas for Visual Basic, the best analogy for reference types are objects, which-in Visual Basic 6 are always accessed through references. a also lays down specifications about data storage: instances of reference types are always stored in an area of memory known as the managed heap, whereas value types are normally stored on the stack (although if value types are declared as fields within reference types,
they will be stored inline on the heap). Chapter 2, “C# Basics,” discusses the stack and the heap and
how they work.

Strong Data Typing

One very important aspect of IL is that it is based on exceptionally strong data typing. That means that all variables are clearly marked as being of a particular, specific data type (there is no room in IL, for
example, for the Variant data type recognized by Visual Basic and scripting languages). In particular, IL ,does not normally permit any operations that result in ambiguous data types.

For instance, Visual Basic6 developers are used to being able to pass variables around without worrying too much about their types, because Visual Basic 6 automatically performs type conversion. C++ developers are used to routinely casting pointers between different types. Being able to perform this kind of operation can be great for performance, but it breaks type safety. Hence, it is permitted only under certain circumstances in some of the languages that compile to managed code. Indeed, pointers (as opposed to references) are permitted only in marked blocks of code in C#, and not at all in Visual Basic (although they are allowed in managed C++). Using pointers in your code causes it to fail the
memory type-safety checks performed by the CLR.

You should note that some languages compatible with .NET, such as Visual Basic 2008, still allow some laxity in typing, but that is possible only because the compilers behind the scenes ensure that the type safety is enforced in the emitted IL.

Although, enforcing type safety might initially appear to hurt performance, in many cases the benefits gained from the services provided by .NET that rely on type safety far outweigh this performance loss.
Such services include:

  • Language interoperability
  • Garbage collection
  •  Security
  •  Application domains.

The following sections take a closer look at why strong data typing is particularly important for these features of .NET.

The Importance of Strong Data Typing for Language Interoperability

If a class is to derive from or contains instances of other classes, it needs to know about all the data types used by the other classes. This is why strong data typing is so important. Indeed, it is the absence of any agreed-on system for specifying this information in the past that has always been the real barrier to inheritance and interoperability across languages. This kind of information, is simply not present in a standard executable file or DLL.

Suppose-that one of the methods of a Visual Basic 2008 class is defined to return an Integer – one of the standard data types available in Visual Basic 2008. C# simply does not have any data type of that name. Clearly, you will be able to derive from the class, use this method, and use the return type from C# code, only if the compiler knows how to map Visual Basic 2008’s Integer type to some known type that is defined in C#. So, how is this problem circu mvented in .NET?

Common Type System

This data type problem is solved in .NET using the Common Type System (CTS). The CTS defines the predefined data types that are available in IL, so that all languages that target the .NET Framework will produce compiled code that is ultimately based on these types.

For the previous example, Visual Basic 2008’s Integer is actually a 32-bit signed integer, which maps exactly to the. IL type known as Int32. This will therefore be the data type specified in the IL code.
Because the C# compiler is aware of this type, there is no problem. At source code level, C# refers to Int32 with the keyword int, so the compiler will simply treat the Visual Basic 2008 method as if it returned an int.

The CTS does not specify merely primitive data types but a rich hierarchy of types, which includes well-defined points in the hierarchy at which code is permitted to define its own types. The hierarchical structure of the CTS reflects the single-inheritance object-oriented methodology of IL, and resembles.

Figure 1·1

Figure 1·1

The following table explains the types shown.

Type                                                         Meaning

Type                                      Base class that represents any type.
Value Type                     Base class that represents any value type.
Reference Types          Any data types that are accessed through a                                        reference and stored on the heap.
Built-in Value Types         Includes most of the standard primitive                                               types, which represent numbers. Boolean                                            values, or characters.
Enumerations                  Sets of enumerated values.
User-defined Value Types     Types that have been defined in                                                      source code and are stored as value                                                 types. In C# terms, this means any struct.
Interface Types                                      Interfaces.
Pointer Types                                          Pointers.
Self-describing Types        Data types that provide information about                                           themselves for the benefit of the garbage                                            collector (see the next section).
Arrays                               Any type that contains an array of objects.
Class Types                       Types that are self-describing but are not                                             arrays.
Delegates                           Types that are designed to hold                                                            references to methods.
User-defined Reference         Types that have been defined in             Types                                    source code and are stored                                                                as reference types. In C# terms, this                                                    means any class.
Boxed Value Types                A value type that is temporarily                                                            wrapped in a reference so that it can                                                   be stored on the heap.

We will not list all of the built-in value types here, because they are covered in detail in Chapter 3, “Objects and Types.” In C#, each predefined type recognized by the compiler maps onto one of the IL
built-in types. The same is true in Visual Basic 2008.

Common Language Specification

The Common Language Specification (CLS) works with the CTS to ensure language interoperability. The CLS is a set of minimum standards that all compilers targeting .NET must support. Because IL is a very rich language, writers of most compilers will prefer to restrict the capabilities of a given compiler to support only a subset of the facilities offered by IL and the CLS. That is fine, as long as the compiler supports everything that is defined in the CLS.

It is perfectly acceptable to write non-CLS-compliant code. However, if you do, the compiled IL code is not guaranteed to be fully language interoperable.

For example, take case sensitivity. IL is case sensitive. Developers who work with case-sensitive languages regularly take advantage of the flexibility that this case sensitivity gives them when selecting
variable names. Visual Basic2008,however, is not case sensitive. The CLS works around this by indicating that CLS-compliant code should not expose any two names that differ only in their case.
Therefore, Visual Basic2008 code can work with CLS-compliant code.

This example shows that the CLS works in two ways. First, it means that individual compilers do not have to be powerful enough to support the full features of .NET – this should encourage the
development of compilers for other programming languages that target :NET.Second, it provides a guarantee that, if you restrict your classes to exposing only CLS-compliant features, then code written in any other compliant language can use your classes.

The beauty of this idea is that the restriction to using CLS-compliant features applies only to public and protected members of classes and public classes. Within the private implementations of your classes, you can write whatever non-CLS code you want, because code in other assemblies (units of managed code, see later in this chapter) cannot access this part of your code anyway.

We will not go into the details of the CLS specifications here. In general, the CLS will not affect your C# code very much because there are very few non-CLS-compliant features of C# anyway.

Garbage Collection

The garbage collector is .NET’s answer to memory management and in particular to the question of what to do about reclaiming memory that running applications ask for. Up until now, two techniques have been used on the Windows platform for re-allocating memory that processes have dynamically requested from the system:

  • Make the application code do it all manually.
  • Make objects maintain reference counts.

Having the application code responsible for deallocating memory is the technique used by lower-level, high-performance languages such as C++. It is efficient, and it has the advantage that (in general) resources are never occupied for longer than necessary. The big disadvantage, however, is the frequency of bugs. Code that requests memory also should explicitly inform the system when it no longer requires that memory. However, it is easy to overlook this, resulting in memory leaks.

Although modem developer environments do provide tools to assist in detecting memory leaks, they remain difficult bugs to track down. That’s because they have no effect until so much memory has been
leaked that Windows refuses to grant any more to the process. By this point, the entire computer may have appreciably slowed down due to the memory demands being made on it.

Maintaining reference counts is favored in COM. The idea is that each COM component maintains a count of how many clients are currently maintaining references to it. When this count falls to zero; the component can destroy itself and free up associated memory and resources. The problem with this is that it still relies on the good behavior of clients to notify the component that they have finished with it.
It takes only one client what to do so, and the object sits in memory. In some ways, this is a potentially more serious problem an a simple C++-style memory leak because the COM object may exist in its
own process, which means that it will never be removed by the system. (At least with C++ memory leaks, the system can reclaim all memory when the process terminates.)

The .NETruntime relies on the garbage collector instead. The purpose of this program is to clean up memory. The idea is that all dynamically requested memory is allocated on the heap (that is true for all languages, although in the case of .NET,the CLRmaintains its own managed heap for .NET applications to use). Every so often, when .NET detects that the managed heap for a given process is becoming full and therefore needs tidying up, it calls the garbage collector. The garbage collector runs through variables currently in scope in your code, examining references to objects stored on the heap to identify which ones are accessible from your code – that is, which objects have references that refer to them. Any objects that are not referred to are deemed to be no longer accessible from your code and can therefore be removed. Java uses a system of garbage collection similar to this.

Garbage collection works in .NET because IL has been designed to facilitate the process. The principle requires that you cannot get references to existing objects other than by copying existing references and that IL be type safe. In this context, what we mean is that if any reference to an object exists, then there is sufficient information in the reference to exactly determine the type of the object.

It would not be possible to use the garbage collection mechanism with a language such as unmanaged C++, for example, because C++ allows pointers to be freely cast between types.

One important aspect of garbage collection is that it is not deterministic. In other words, you cannot guarantee when the garbage collector will be called; it will be called when the CLR decides that it is needed, though it is also possible to override this process and call up the garbage collector in your code.

Security

.NET can really excel in terms of complementing the security mechanisms provided by Windows because it can offer code-based security, whereas Windows really offers only role-based security.

Role-based securitv is based on the identity of the account under which the process is running (that is, who owns and is running the process). CQ re-based security, by contrast, is based on what the code actually does and on how much the code is trusted. Thanks to the strong type safety of IL, the CLR is able to inspect code before running it to determine required security permissions .. NET also offers a mechanism by which code can indicate in advance what security permissions it will require to run.

The importance at code-based security is that it reduces the risks associated with running code of dubious origin (such as code that you have downloaded from the Internet). For example, even if code is running under the administrator account, it is possible to use code-based security to indicate that that code should still not be permitted to perform certain types of operations that the administrator account would normally be allowed to do, such as read or write to environment variables, read or write to the registry, or access the .NET reflection features.

Security issues are covered in more depth in Chapter 20, “Security.”

Application Domains

Application domains are an important innovation in .NET and are designed to ease the overhead involved when running applications that need to be isolated from each other but that also need to be
able to communicate with each other. The classic example of this is a Web server application, which may be simultaneously responding to a number of browser request. It will, therefore, probably have a
number of instances of the component responsible for servicing these requests running simultaneously.

In pre- .NET days, the choice would be between allowing those instances to share a process (with the resultant risk of a problem in one running instance bringing the whole Web site down) or isolating those instances in separate processes (with the associated performance overhead).

Up until now, the only means of isolating code has been through processes. When you start a new application, it runs within the context of a process. Windows isolates processes from each other through address spaces. The idea is that each process has available 4GB of virtual memory in which to store its data and executable code (4GB is for 32-bit systems, 64-bit systems use more memory). Windows imposes an extra level of indirection by which this virtual memory maps into a particular area of actual
physical memory or disk space. Each process gets a different mapping, with no overlap between the actual physical memories that the blocks of virtual address space map.

Figure 1-2

Figure 1-2

In general, any process is able to access memory only by specifying an address in virtual memory processes do not have direct access to physical memory. Hence, it is simply impossible for one process to access the memory allocated tq another process. This provides an excellent guarantee that any badly behaved code will not be able to damage anything outside of its own address space. (Note that on
windows 95/98, these safeguards are not quite as thorough as they are on Windows NT/2000/2003/Vista, so the theoretical possibility exists of applications crashing Windows by writing to an appropriate memory.)

Processes do not just serve as a way to isolate instances of running code from each other. On Windows 2000/XP /2003/Vista systems, they also form the unit to which security privileges and permissions
re assigned. Each process has its own security token, which indicates to Windows precisely what functions that process is permitted to do.

Although processes are great for security reasons, their big disadvantage is in the area of performance a number of processes will actually be working together, and therefore need to communicate with other. The obvious example of this is where a process calls up a COM component, which is an table and therefore is required to run in its own process. The same thing happens in COM when dates are used. Because processes cannot share any memory, a complex marshaling process must be copy data between the processes. This results in a.very significant performance hit. If you need COM into its work together and do not want that performance hit, you must use DLL-based components re everything running in the same address space – with the associated risk that a badly behaved COM in with ill bring everything else down.

Application domains are designed as a way of separating components without resulting in the performance problems associated with passing data between processes. The idea is that anyone process is divided into a number of application domains. Each application domain roughly corresponds to a single application, and each thread of execution will be running in a particular application domain.

Figure 1-3

Figure 1-3

If different executables are running in the same process space, then they are clearly able to easily share data, because, theoretically, they can directly see each other’s data. However, although this is possible in principle, the CLR makes sure that this does not happen in practice by inspecting the code for each running application to ensure that the code cannot stray outside of its own data areas. This looks, at first, like an almost impossible task to pull off – after all, how can you tell what the program is going to do without actually running it?

In fact, it is usually possible to do this because of the strong type safety of the IL. In most cases, unless code is using unsafe features such as pointers, the data types it is using will ensure that memory is not accessed inappropriately. For example, .NET array types perform bounds checking to ensure that no out-of-bounds array operations are permitted. If a running application does need to communicate or share data with other applications Fuming in different application domains, it must do so by calling on .NET’s remoting services.

Code that has been verified to check that it cannot access data outside its application domain (other than through the explicit remoting mechanism) is said to be memory type safe. Such code can safely be run alongside other type-safe code in different application domains within the same process.

Error Handling with Exceptions

The .NET Framework is designed to facilitate handling of error conditions using the same mechanism, based on exceptions, that is employed by java and C++. C++ developers should note that because of IL’s stronger typing system, there is no performance penalty associated with the use of exceptions with IL in the way that there is in C++. Also, the finally block, which has long been on many C++ developers wish lists, is supported by .NET and by C#..

 “Errors and Exceptions.” Briefly, the idea is that certain areas of code are designated as exception handler routines, with each one able to deal with a particular error condition (for example, a file not being found, or being denied permission to perform some operation). These conditions can be defined as narrowly or as widely as you want. The exception architecture ensures that when an error condition occurs, execution can immediately jump to the exception handler routine that is not specifically geared to handle the exception condition in question.

The architecture of exception handling also provides a convenient means to pass an object containing, precise details of the exception condition to an exception handling routine, “This object might include an appropriate message for the user and details of exactly where in the code the exception was detected.

MQ exception-handling architecture, including the ‘control of program flow when an exception occurs, is “died by the high-level languages (C#, VisualBasic 2008,C++), and is not supported by any  speciaI commands. CI, for example, handles exceptions using try{}, catch{}, and finally{} blocks of code.

What .NET does do, however, is provide the infrastructure to allow compilers that target .NET to support exception handling. In particular, it provides a set of .NETclasses that can represent the
exceptions, and the language interoperability to allow the thrown exception objects to be interpreted by, the exception-handling code, regardless of what language the exception-handling code is written in. “This language independence is absent from both the C++ and Java Implementations of exception handling, although it is present to a limited extent in the COM mechanism for huge errors, which involves returning error codes from methods and passing error objects around. infact that exceptions are handled consistently in different languages is a crucial aspect of facilitating multi-language development.

Use of Attributes

Attributes are familiar to developers who use C++ to write COM components (through their use in Microsoft’s COM Interface Definition Language (IDL).The initial idea of an attribute was that it provided extra information concerning some item in the program that could be used by the compiler.

Attributes are supported in .NET- and hence now by C++, 01, and Visual Basic2008. What is however, particularly innovative about attributes in .NET is that you can define your own custom attributes in your source code. These user-defined attributes will be placed with the metadata for the corresponding data, types or methods. This can be useful for documentation purposes, in which they can be used in conjunction with reflection technology to perform programming tasks based on attributes, In addition, in common with the .NET philosophy of language independence, attributes can be defined in source code in one language and read by code that is written in another language.

Posted on October 27, 2015 in .NET Architecture

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