What is reflection?

All .NET assemblies have metadata information stored about the types defined in modules. This metadata information can be accessed by mechanism called as “Reflection”.System. Reflection can be used to browse through the metadata information.

Using reflection you can also dynamically invoke methods using System.Type.Invokemember. Below is sample source code if needed you can also get this code from CD provided, go to “Source code” folder in “Reflection Sample” folder.

Public Class Form1
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles MyBase.Load
Dim Pobjtype As Type
Dim PobjObject As Object
Dim PobjButtons As New Windows.Forms.Button()
Pobjtype = PobjButtons.GetType()
For Each PobjObject In Pobjtype.GetMembers
LstDisplay.Items.Add(PobjObject.ToString())
Next
End Sub
End Class

Sample source code uses reflection to browse through “Button” class of “Windows.Forms”. If you compile and run the program following is output as shown in “Sample Reflection Display”. Using reflection you can also dynamically invoke a method using “System.Type.InvokeMember”.

Note :- System.Type.InvokeMember is left as homework for readers. Believe me you will enjoy doing it yourself and the concept of reflection will be clearer.

Can we force garbage collector to run ?

System.GC.Collect() forces garbage collector to run. This is not recommended but can be used if situations arises.

What is garbage collection?

Garbage collection is a CLR feature which automatically manages memory. Programmers forget to release the objects while coding ..... Laziness (Remember in VB6 where one of the good practices is to set object to nothing). CLR automatically releases objects when they are no longer in use and refernced. CLR runs on non-deterministic to see the unused objects and cleans them. One side effect of this non-deterministic feature is that we cannot assume an object is destroyed when it goes out of the scope of a function. Therefore, we should not put code into a class destructor to release resources.

What is Delay signing?

During development process you will need strong name keys to be exposed to developer which

is not a good practice from security aspect point of view.In such situations you can assign the key

later on and during development you an use delay signing

Following is process to delay sign an assembly:

• First obtain your string name keys using SN.EXE.
• Annotate the source code for the assembly with two custom attributes from System.Reflection: AssemblyKeyFileAttribute, which passes the name of the file containing the public key as a parameter to its constructor. AssemblyDelaySignAttribute, which indicates that delay signing, is being used by passing true as a parameter to its constructor. For example as shown below: [Visual Basic]<Assembly:AssemblyKeyFileAttribute("myKey.snk")> <Assembly:AssemblyDelaySignAttribute(true)> [C#] [assembly:AssemblyKeyFileAttribute("myKey.snk")] [assembly:AssemblyDelaySignAttribute(true)] The compiler inserts the public key into the assembly manifest and reserves space in the PE file for the full strong name signature. The real public key must be stored while the assembly is built so that other assemblies that reference this assembly can obtain the key to store in their own assembly reference.
• Because the assembly does not have a valid strong name signature, the verification of that signature must be turned off. You can do this by using the –Vr option with theStrong Name tool.The following example turns off verification for an assembly called myAssembly.dll. Sn –Vr myAssembly.dllDLL name of the project.
• Just before shipping, you submit the assembly to your organization's signing authority for the actual strong name signing using the –R option with the Strong Name tool. The following example signs an assembly called myAssembly.dll with a strong name using the sgKey.snk key pair. Sn -R myAssembly.dll sgKey.snk

How to add and remove an assembly from GAC?

There are two ways to install .NET assembly in GAC:-

• Using Microsoft Installer Package. You can get download of installer from
• http://www.microsoft.com/.
• Using Gacutil. Goto “Visual Studio Command Prompt” and type “gacutil –i (assembly_name)”, where (assembly_name) is the DLL name of the project.

What is GAC ?

Twist :- What are situations when you register .NET assembly in GAC ?

GAC (Global Assembly Cache) is used where shared .NET assembly reside. GAC is used in the

following situations :-

• If the application has to be shared among several application.
• If the assembly has some special security requirements like only administratorscan remove the assembly. If the assembly is private then a simple delete of assembly the assembly file will remove the assembly.

Note :- Registering .NET assembly in GAC can lead to the old problem of DLL hell, where COM version was stored in central registry. So GAC should be used when absolutely necessary.

Is versioning applicable to private assemblies?

Versioning concept is only applicable to global assembly cache (GAC) as private assembly lie in their individual folders.

Where is version information stored of an assembly?

Version information is stored in assembly in manifest.

What is Manifest

Assembly metadata is stored in Manifest. Manifest contains all the metadata needed to do the following things (See Figure Manifest View for more details):

• Version of assembly
• Security identity
• Scope of the assembly
• Resolve references to resources and classes.
• The assembly manifest can be stored in either a PE file (an .exe or .dll) with Microsoft intermediate language (MSIL) code or in a stand-alone PE file that contains only assembly manifest information.

What is Difference between NameSpace and Assembly

Following are the differences between namespace and assembly :

• Assembly is physical grouping of logical units. Namespace logically groups classes.
• Namespace can span multiple assembly.

What is NameSpace

Namespace has two basic functionality :-

• NameSpace Logically group types, example System.Web.UI logically groups our UI related features.
• In Object Oriented world many times its possible that programmers will use the same class name.By qualifying NameSpace with classname this collision is able to be removed.

What are the different types of Assembly

There are two types of assembly Private and Public assembly. A private assembly is normally used by a single application, and is stored in the application's directory, or a sub-directory beneath. A shared assembly is normally stored in the global assembly cache, which is a repository of assemblies maintained by the .NET runtime. Shared assemblies are usually libraries of code which many applications will find useful, e.g. Crystal report classes which will be used by all application for Reports.

What is a Assembly

• Assembly is unit of deployment like EXE or a DLL.
• An assembly consists of one or more files (dlls, exe’s, html files etc.), and represents a group of resources, type definitions, and implementations of those types. An assembly may also contain references to other assemblies. These resources, types and references are described in a block of data called a manifest. The manifest is part of the assembly, thus making the assembly self-describing.
• An assembly is completely self-describing.An assembly contains metadata information, which is used by the CLR for everything from type checking and security to actually invoking the components methods. As all information is in the assembly itself, it is independent of registry. This is the basic advantage as compared to COM where the version was stored in registry.
• Multiple versions can be deployed side by side in different folders. These different versions can execute at the same time without interfering with each other. Assemblies can be private or shared. For private assembly deployment, the assembly is copied to the same directory as the client program that references it. No registration is needed, and no fancy installation program is required.When the component is removed, no registry cleanup is needed, and no uninstall program is required. Just delete it from the hard drive.
• In shared assembly deployment, an assembly is installed in the Global Assembly Cache (or GAC). The GAC contains shared assemblies that are globally accessible to all .NET applications on the machine.

What is a Managed Code

Managed code runs inside the environment of CLR i.e. .NET runtime. In short all IL are managed code. But if you are using some third party software example VB6 or VC++ component they are unmanaged code as .NET runtime (CLR) does not have control over the source code execution of the language.in a seamless manner.

What is a CLS(Common Language Specification)

This is a subset of the CTS which all .NET languages are expected to support. It was always a dream of Microsoft to unite all different languages in to one umbrella and CLS is one step towards that. Microsoft has defined CLS which are nothing but guidelines that language to follow so that it can communicate with other .NET languages in a seamless manner.

What is a CTS

In order that two language communicate smoothly CLR has CTS (Common Type System).Example in VB you have “Integer” and in C++ you have “long” these datatypes are not compatible so the interfacing between them is very complicated. In order to able that two different languages can communicate Microsoft introduced Common Type System. So “Integer” datatype in VB6 and “int” datatype in C++ will convert it to System.int32 which is datatype of CTS. CLS which is covered in the coming question is subset of CTS.

Note: If you have undergone COM programming period interfacing VB6 application with VC++ application was a real pain as the datatype of both languages did not have a common ground where they can come and interface, by having CTS interfacing is smooth.

What is a CLR

Full form of CLR is Common Language Runtime and it forms the heart of the .NET framework. All Languages have runtime and its the responsibility of the runtime to take care of the code execution of the program. For example VC++ has MSCRT40.DLL,VB6 has MSVBVM60.DLL, Java has Java Virtual Machine etc. Similarly .NET has CLR. Following are the responsibilities of CLR

•  Garbage Collection :- CLR automatically manages memory thus eliminating memory leaks. When objects are not referred GC automatically releases those memories thus providing efficient memory management.
• Code Access Security :- CAS grants rights to program depending on the security configuration of the machine. Example the program has rights to edit or create a new file but the security configuration of machine does not allow the program to delete a file. CAS will take care that the code runs under the environment of machines security configuration.
• Code Verification :- This ensures proper code execution and type safety while the code runs. It prevents the source code to perform illegal operation such as accessing invalid memory locations etc.
• IL( Intermediate language )-to-native translators and optimizer’s :- CLR uses JIT and compiles the IL code to machine code and then executes. CLR also determines depending on platform what is optimized way of running the IL code.

What is a IL

Twist :- What is MSIL or CIL , What is JIT?

(IL)Intermediate Language is also known as MSIL (Microsoft Intermediate Language) or CIL (Common Intermediate Language). All .NET source code is compiled to IL. This IL is then converted to machine code at the point where the software is installed, or at run-time by a Just-In-Time (JIT) compiler.

How does the Object Oriented approach improve software development

Ans). The key benefits are:

• Re-use of previous work: using implementation inheritance and object composition.
• Real mapping to the problem domain: Objects map to real world and represent vehicles, customers, products etc: with encapsulation.
• Modular Architecture: Objects, systems, frameworks etc are the building blocks of larger systems. The increased quality and reduced development time are the by-products of the key benefits discussed above. If 90% of the new application consists of proven existing components then only the remaining 10% of the code have to be tested from scratch.

What are the advantages of Object Oriented Programming Languages (OOPL)

Ans). The Object Oriented Programming Languages directly represent the real life objects like Car, Jeep, Account, Customer etc. The features of the OO programming languages like polymorphism, inheritance and
encapsulation make it powerful. [Tip: remember pie which, stands for Polymorphism, Inheritance and
Encapsulation are the 3 pillars of OOPL]

What is the difference between C++ and Java

Ans). Both C++ and Java use similar syntax and are Object Oriented, but:

• Java does not support pointers. Pointers are inherently tricky to use and troublesome.
• Java does not support multiple inheritances because it causes more problems than it solves. Instead Javasupports multiple interface inheritance, which allows an object to inherit many method signatures fromdifferent interfaces with the condition that the inheriting object must implement those inherited methods. Themultiple interface inheritance also allows an object to behave polymorphically on those methods. [Refer Q 8 and Q 10 in Java section.]
• Java does not support destructors but rather adds a finalize() method. Finalize methods are invoked by the garbage collector prior to reclaiming the memory occupied by the object, which has the finalize() method. This means you do not know when the objects are going to be finalized. Avoid using finalize() method to release non-memory resources like file handles, sockets, database connections etc because Java has only a finite number of these resources and you do not know when the garbage collection is going to kick in to release these resources through the finalize() method.
• Java does not include structures or unions because the traditional data structures are implemented as an object oriented framework (Java collection framework – Refer Q14, Q15 in Java section).
• All the code in Java program is encapsulated within classes therefore Java does not have global variables or functions.
• C++ requires explicit memory management, while Java includes automatic garbage collection. [Refer Q32 in Java section].

Give a few reasons for using Java

Ans). Java is a fun language. Let’s look at some of the reasons:

1. Built-in support for multi-threading, socket communication, and memory management (automatic garbage collection).
2. Object Oriented (OO).
3. Better portability than other languages across operating systems.
4. Supports Web based applications (Applet, Servlet, and JSP), distributed applications (sockets, RMI. EJB etc) and network protocols (HTTP, JRMP etc) with the help of extensive standardised APIs (Application Program Interfaces).