Basic .NET Framework Interview Questions

1. 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-InTime (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 :

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.

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.


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.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 is able to be removed.


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 ?

Twist : What is ILDASM ?

When it comes to understanding of  internals nothing can beat ILDASM. ILDASM basically converts the whole exe or dll in to IL code. To run ILDASM you have to go to "C:\Program Files\ Microsoft  Visual Studio .NET 2003\SDK\v1.1\Bin". Note that i had v1.1 you have to probably change it depending on the type of framework version you have. If  you run IDASM.EXE from the path you will be popped with the IDASM exe program as shown in figure ILDASM. Click on file and browse to the respective directory for the DLL whose assembly you want to view. After you 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 you will be able to view details of  assembly,  internal  IL code etc as shown in Figure Manifest View.
Note : 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. Note the version information in Figure Manifest view. You can expand the tree for detail information regarding the DLL like methods etc.


11. 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) withMicrosoft 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 ?

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 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 ?

Twist :- How do we generate strong names  or what is the process of generating strong names, What is use the of  SN.EXE , How do we apply strong names to assembly, 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.


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.The following example turns off verification for an assembly called myAssembly.dll.
√ 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.


18. 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.


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.
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


21. What are different types  of JIT ?

JIT compiler is a part of the runtime execution environment. 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# ?

Well this is the most debatable issue in .NET community and people treat there languages like eligion. Its a subjective matter which language is best. Some like VB.NET’s natural style and some like professional and terse C# syntaxes. 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 applicationspecific 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 ?

Just to give an understanding of what the above question means seethe below code.

int i =0;
MessageBox.Show(i.ToString());
MessageBox.Show(Convert.ToString(i));

We can convert the integer “i” using “i.ToString()” or “Convert.ToString” so what’s the difference.
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.

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