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It is a problem solving technique to develop software systems. It is a technique to think real world in terms of objects. Object maps the software model to real world concept. These objects have responsibilities and provide services to application or other objects.

 What’s a Class ?

 A class describes all the attributes of objects, as well as the methods that implement the behavior of member objects. It’s a comprehensive data type which represents a blue print of objects. It’s a template of object.

 What’s an Object ?

 It is a basic unit of a system. An object is an entity that has attributes, behavior, and identity. Objects are members of a class. Attributes and behavior of an object are defined by the class definition.

What is the relation between Classes and Objects ?

They look very much same but are not same. Class is a definition, while object is a instance of the class created. Class is a blue print while objects are actual objects existing in real world. Example we have class CAR which has attributes and methods like Speed, Brakes, Type of Car etc. Class CAR is just a prototype, now we can create real time objects which can be used to provide functionality. Example we can create a Maruti car object with 100 km speed and urgent brakes.

 What are different properties provided by Objectoriented systems ? or Can you explain different properties of Object Oriented Systems?

 .NET components communicate with COM using RCW (Runtime Callable Wrapper).

Following are the ways with which you can generate RCW :-

Adding reference in Visual Studio.net. Wrapper class is generated and placed in the “BIN” directory.

Using Type library import tool. Tlbimp.exe yourname.dll.

Using interopservices.System.runtime.Interopservices namespace contains class TypeLib Converter which provides methods to convert COM classes and interface in to assembly metadata.

Make your custom wrappe rs.If your COM component does not have type library then the only way to communicate is writing custom wrappers. That meanscommunicating directly with COM components.

  2) Once I have developed the COM wrapper do I have to still register the COM in registry?

      Yes.

 3) How can we use .NET components in COM? or What is CCW (COM callable wrapper) ? or How do we ensure that .NET components is compatible with COM ?

.NET components can not be used in straight forward way with COM. You will need to create CCW in order that COM components communicate with .NET assemblies.

Following are the different approaches to implement it :-

Explicitly declare interfaces.

The second way to create CCW is by using InteropServices attributes.

Once .NET assembly is created using either interface or using interopservices method we need to create a COM type library using Type library export tool. Tlbexp (AssemblyName)

 The final thing is registering the CCW in registry using regasm tool. regasm AssemblyName [Options] Finally refer the TLB in your COM IDE Below is figure showing VB6 IDE referencing the DLL

4) How can we make Windows API calls in .NET?

  Windows API call are not COM based and they are invoked through Platform Invoke Services.

  StringConversionType is for what type of conversion should take place. Either we can specify Unicode to  convert all strings to Unicode values, or Auto to convert strings according to the .NET runtime rules.

 MethodName is the name of the API to call.

 DllName is the name of the DLL.

 Args are any arguments to the API call.

5)When we use windows API in .NET is it managed or unmanaged code ?

Windows API in .NET is unmanaged code.

6)What is COM ?

Microsoft’s COM is a technology for component software development. It is a binary standard which is language independent. DCOM is a distributed extension of COM.

I found it’s most useful.

1) What is IL? Or 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.

2) 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.

3) 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.

4) 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.

5) 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.

6) 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.70 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.

7) 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.

8) 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 can be avoided.

9) 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.

10)If you want to view a Assembly how do you go about it ? Or What is ILDASM?

ILDASM basically converts the whole exe or dll in to IL code. To run ILDASM we have to go to “C:\Program Files\Microsoft71 Visual Studio .NET 2003\SDK\v1.1\Bin”. Note that i had v1.1 we have to probably change it depending on the type of framework version you have. If we run IDASM.EXE from the path we will be popped with the IDASM exe program . Click on file and browse to the respective directory for the DLL whose assembly we want to view. After we select the DLL you will be popped with a tree view details of the DLL as shown in figure ILDASM. On double clicking on manifest wewill be able to view details of assembly, internal IL code etc

Imp : The version number are in the manifest itself which is defined with the DLL or EXE thus making deployment much easier as compared to COM where the information was stored in registry.

11) What is Manifest?

Assembly metadata is stored in Manifest. Manifest contains all the metadata needed to do the following things

 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.

12) Where is version information stored of an assembly ?

Version information is stored in assembly in manifest.

13) Is versioning applicable to private assemblies?

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

14) What is GAC? Or 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 administrators can 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.

15) What is the concept of strong names? Or How do we generate strong names? Or What is use the of SN.EXE? Or How do we apply strong names to assembly? Or How do you sign an assembly?

Strong name is similar to GUID(It is supposed to be unique in space and time) in COM components. Strong Name is only needed when we need to deploy assembly in GAC. Strong Names helps GAC to differentiate between two versions. Strong names use public key cryptography (PKC) to ensure that no one can spoof it. PKC use public key and private key concept. Following are the step to generate a strong name and sign a assembly:-

 Go to “Visual studio Command Prompt”. Note the samples are compiled in 2005 but 2003 users do not have to worry about it. Same type of command prompt will be seen in 2003 also.
After you are in command prompt type sn.exe -k “c:\test.snk”.
After generation of the file you can view the SNK file in a simple notepad.
After the SNK file is generated its time to sign the project with this SNK file
Click on project — properties and the browse the SNK file to the respective folder and compile the project.

16)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.

17) 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 the Strong Name tool.

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

18)What is garbage collection?

Garbage collection is a CLR feature which automatically manages memory. Programmers forget to release the objects while coding . 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.we should avoid using destructors because before GC destroys the object it first executes destructor in that case it will have to wait for code to release the umanaged resource. resultin in additional delays in GC. So its recommended to implement IDisposable interface and write cleaup code in Dispose method and call GC.SuppressFinalize method so instructing GC not to call your constructor.

19) 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.

20)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.

21)What are different types of JIT ?

In Microsoft .NET there are three types of JIT compilers:

 Pre-JIT :- Pre-JIT compiles complete source code into native code in a single compilation cycle. This is done at the time of deployment of the application.

 Econo-JIT :- Econo-JIT compiles only those methods that are called at runtime. However, these compiled methods are removed when they are not required.

 Normal-JIT :- Normal-JIT compiles only those methods that are called at runtime. These methods are compiled the first time they are called, and then they are stored in cache. When the same methods are called again, the compiled code from cache is used for execution.

22) What are Value types and Reference types ?

Value types directly contain their data which are either allocated on the stack or allocated in-line in a structure.

Reference types store a reference to the value’s memory address, and are allocated on the heap. Reference types can be self-describing types, pointer types, or interface types.

Variables that are value types each have their own copy of the data, and therefore operations on one variable do not affect other variables.

Variables that are reference types can refer to the same object; therefore, operations on one variable can affect the same object referred to by another variable. All types derive from the System.Object base type.

23) What is concept of Boxing and Unboxing ?

Boxing permits any value type to be implicitly converted to type object or to any interface type implemented by value type. Boxing is a process in which object instances are created and copy values in to that instance.

Unboxing is vice versa of boxing operation where the value is copied from the instance in to appropriate storage location. Below is sample code of boxing and unboxing where integer data type is converted in to object and then vice versa.

Dim x As Integer

Dim y As Object x = 10

‘boxing process y = x  ‘unboxing process x = y

 24) What is the difference between VB.NET and C# ?

Both use the same framework and speed is also very much equivalents. But still let’s list down some major differences between them :

Advantages VB.NET :

Has support for optional parameters which makes COM interoperability much easy.

With Option Strict off late binding is supported.Legacy VB functionalities can be used by using Microsoft.VisualBasic namespace.

Has the WITH construct which is not in C#.

 The VB.NET part of Visual Studio .NET compiles your code in the background. While this is considered an advantage for small projects, people creating very large projects have found that the IDE slows down considerably as the project gets larger. Advantages of C#

 XML documentation is generated from source code but this is now been incorporated in Whidbey.

 Operator overloading which is not in current VB.NET but is been introduced in Whidbey.

 Use of this statement makes unmanaged resource disposal simple.

 Access to Unsafe code. This allows pointer arithmetic etc, and can improve performance in some situations. However, it is not to be used lightly, as a lot of the normal safety of C# is lost (as the name implies).This is the major difference that you can access unmanaged code in C# and not in VB.NET.

25)What is the difference between System exceptions and Application exceptions?

 All exception derives from Exception Base class. Exceptions can be generated programmatically or can be generated by system.

Application Exception serves as the base class for all application-specific exception classes. It derives from Exception but does not provide any extended functionality. You should derive your custom application exceptions from Application Exception. Application exception is used when we want to define user defined exception, while system exception is all which is defined by .NET.

26)What is CODE Access security?

CAS is part of .NET security model that determines whether or not a piece of code is allowed to run and what resources it can use while running. Example CAS will allow an application to read but not to write and delete a file or a resource from a folder.

27)How to prevent my .NET DLL to be decompiled?

By design .NET embeds rich Meta data inside the executable code using MSIL. Any one can easily decompile your DLL back using tools like ILDASM (owned by Microsoft) or Reflector for .NET which is a third party. Secondly there are many third party tools which make this decompiling process a click away. So any one can easily look in to your assemblies and reverse engineer them back in to actual source code and understand some real good logic which can make it easy to crack your application. The process by which you can stop this reverse engineering is using “obfuscation”. It’s a technique which will foil the decompilers. There are many third parties (XenoCode, Demeanor for .NET) which provide .NET obfuscation solution. Microsoft includes one that is Dotfuscator Community Edition with Visual Studio.NET.

28) What is the difference between Convert.toString and .toString() method ?

 int i =0;

 MessageBox.Show(i.ToString());

 MessageBox.Show(Convert.ToString(i));

The basic difference between them is “Convert” function handles NULLS while “i.ToString()” does not it will throw a NULL reference exception error. So as good coding practice using “convert” is always safe.

29) What is Native Image Generator (Ngen.exe)?

The Native Image Generator utility (Ngen.exe) allows you to run the JIT compiler on your assembly’s MSIL and generate native machine code which is cached to disk. After the image is created .NET runtime will use the image to run the code rather than from the hard disk. Running Ngen.exe on an assembly potentially allows the assembly to load and execute faster, because it restores code and data structures from the native image cache rather than generating them dynamically. Below are some points to be remembered for Native Image Generator:

Native images load faster than MSIL because JIT compilation and type-safety verification is eliminated.

If you are sharing code between process Ngen.exe improves the performance significantly. As Native image generated Windows PE file so a single DLL file can be shared across applications. By contrast JIT produced code are private to an assembly and can not be shared.

 Native images enable code sharing between processes.

 Native images require more storage space and more time to generate.

 Startup time performance improves lot. We can get considerable gains when applications share component assemblies because after the first application has been started the shared components are already loaded for subsequent applications. If assemblies in an application must be loaded from the hard disk, does not benefit as much from native images because the hard disk access time shadows everything.

 Assemblies in GAC do not benefit from Native image generator as the loader performs extra validation on the strong named assemblies thus shadowing the benefits of Native Image Generator.

 If any of the assemblies change then Native image should also be updated.

 You should have administrative privilege for running Ngen.exe.

While this can fasten your application startup times as the code is statically compiled but it can be somewhat slower than the code generated dynamically by the JIT compiler. So you need to compare how the whole application performance with Ngen.exe and with out it.

 To run Ngen.exe, use the following command line. ngen.exe install <assemblyname> This will synchronously precompile the specified assembly and all of its dependencies. The generated native images are stored in the native image cache. In .NET Framework 2.0 there is a service (.NET Runtime Optimization Service) which can precompile managed assemblies in the background. You can schedule your assemblies to be precompiled asynchronously by queueing them up with the NGEN Service. Use the following command line. ngen.exe install <assemblyname> /queue:<priority> Assemblies which are critical to your application’s start up time should either be precompiled synchronously or asynchronously with priority 1. Priority 1 and 2 assemblies are precompiled aggressively while Priority 3 assemblies are only precompiled during machine idle-time. Synchronously precompiling your critical assemblies guarantees that the native images will be available prior to the first time your end user launches the application but increases the time taken to run your application’s set up program. You can uninstall an assembly and its dependencies (if no other assemblies are dependent on them) from the native image cache by running the following command. ngen.exe uninstall <assemblyname> Native images created using Ngen.exe cannot be deployed; instead they need to be created on the end user’s machine. These commands therefore need to be issued as part of the application’s setup program. Visual Studio .NET can be used to implement this behavior by defining custom actions in a Microsoft Installer (MSI) package.

Note: – I really want to explain this in depth for two reasons. First I have seen this question been frequently asked and second it’s of real practical importance. I have faced this in every of my .NET projects…So let’s try to get this fundamental not in our brain but in our heart.

OK first let’s try to understand what the interviewer is talking about. Let’s say you have made an application and its using a DLL which is present in GAC. Now for some reason you make second version of the same DLL and put it in GAC. Now which DLL does the application refer? Ok by default it always uses the version by which you have compiled you application in IDE. But you want that it should actually use the older version. So first we answer in short. You need to specify “bindingRedirect” in your config file. For instance in the below case “ClassLibraryVersion” has two versions “1.1.1830.10493″ and “1.0.1830.10461″ from which “1.1.1830.10493″ is the recent version. But using the bindingRedirect we can specify saying “1.0.1830.10461″ is the new version. So the client will not use “1.1.1830.10493″.

<configuration> <runtime>

<assemblyBinding xmlns=”urn:schemas-microsoft-com:asm.v1″> <dependentAssembly> <assemblyIdentity name=”ClassLibraryVersion” publicKeyToken=”b035c4774706cc72″ culture=”neutral”/>

 <bindingRedirect oldVersion= “1.1.1830.10493″ newVersion= “1.0.1830.10461″/> </dependentAssembly> </assemblyBinding> </runtime>

 </configuration>