C# Interview Questions

What is the difference between IEnumerable and IQueryable?

IEnumerable and IQuerable are used to query data from database and collections. The IEnumerable is basically inherited from IQueryable, so it’s having all the features of the IQueryable in addition to its own features. Both have its own importance to query data and data manipulation

S.No IEnumerable IQueryable
1. Exists in System.Collections Namespace Exists in System.Linq Namespace
2. Can move to forward only in the collection. Can move to forward, backward or in between the items.
3. Best for collection like List, Array or in-memory collection Best for the collections which is out memory like services, remote databases etc 
4. Suitable for LINQ to Object and Linq to XML queries. Suitable for Linq to SQL queries
5. It doesn’t supports custom query, Lazy loading so it’s not good for paging scenarios. It supports custom query using CreateQuery and Execute methods. Also support lazy loading so good for the paging scenarios.
6. The extension methods which supports be IEnumrable takes the functional objects as the parameters. The extension methods which supports be IEnumrable takes the expression tree expressions as the parameters.

What is cross site scripting? (XSS)

Cross-site scripting is the way to attacks and inserts the vulnerabilities in Web page. This attack is injected by client-side code. The script which is injected by the client can embed itself in response data. The response data which send back to the malicious user. The browser can’t recognize the scripts as it is responded from the trusted source.
The cross-site scripting attacks also work on the HTTP and HTTPS.
There are 2 ways to prevent the cross site scripting attack: 
a. Constrain input- Validate the input length, type, formatting, range etc.
b. Encode output- Send the input data with encode e.g. Encode to HTML
To prevent the cross-site attack, we can set the below attributes in web.config file: 

If I want to see my website similar in all the browsers then what I need to do for that?

If you want to see the website with the same look and feel then you need to write the common css style which should be same for all browsers. Actually every browser will not support every css elements so it is not possible to use the same css to support all the browsers. You need to write the common css elements in the file and then apply it across the website. Else you need to write the separate css file as per the browser.

If say 1 Lac users are using any particular website then what you will do to prevent crashing of server? Which care you will take at the time of coding?

There are the ways like we can have multiple servers to handle the requests from the users. In this, we can have the Web Gardening concept where we can have the multiple web servers and then one main server to handle the number of requests and switch the requests to other servers.

Why to use design patterns?

Design Pattern is the way to solve the recurring problems which occur during the designing of the applications. As the requirements increases, the projects becomes complex and due to the complexity, it’s difficult to maintain it.
With the help of design patterns, we can reduce the complexity and with the help of OOPs paradigm, we can make our applications more efficient in all the ways.
According to the GoF (Gang of Four) company, the Design Patterns can be classified to 3 ways:-
• Creational Design Pattern
• Behavioral Design Pattern
• Structural Design pattern

If I have a class C and two interfaces I1 and I2 and I have add method inside I1 and I2 then how to specify which one has to be called?

By using the explicit implement interface, we can implement the same method which is defined in both the interfaces.

Garbage collection uses which type of algorithm? How it will find which object is unused?

Mark-And-Release is the algorithm which the garbage collection uses to reclaim the memory of the unreferenced objects which are no longer used.
The algorithm Mark-and-Release work in 2 steps:-
a. In the first steps, it marks all the accessible objects of the heap memory. This is called as mark phase.
b. In the second step, scan the heap and reclaim all the unmarked objects by the Garbage Collection algorithm. This step is called as sweep phase. 

What is the difference between out and ref?

Ref and Out are the parameters which are used in the methods. [ ref] and [out] both allow the called method to modify a parameter. The difference between them is what happens before you make the call. 
• [ref] means that the parameter has a value on it before going into the function. The called function can read and or change the value any time. The parameter goes in, then comes out
• [out] means that the parameter has no official value before going into the function. The called function must initialize it. The parameter only goes out.
So the main difference between the ref and out parameters is that the out parameter doesn’t need to be initialized while the ref parameter must be initialized before passing to the function. 

Is it possible to use more than one out parameter?

As we know that the method always returns a single value but by using the out parameter, we can return multiple values from the method or function.
In C# we can write the small code snippet which will describe to return the multiple values using the out parameters:

We can also use the struct which will contain the multiple values as the return:

There is another way to return multiple values like using Tuple class (newly introduces in .Net 4.0). The tuple class can return the object which can contain multiple values in it. 

Is it possible to use .js files used under script will be in body and not in header? Why?

The .js file is used in header because first the .js file should be loaded to the application and then rest of the content should be load.

What are delegates?
Another useful innovation of C# language is called delegates which basically serve the same purpose as function pointers in C++. However, delegates are type-safe, secure managed objects. That means run-time guarantee that a delegate point into a valid method.

What are multicast delegates?
Multicast Delegates one of the special feature of delegates that a single delegates may encapsulate into more than one method. Multicast delegates are the sub set of System.Multicast Delegates.
There are following example of Multicast Delegates 
delegate Void Your MultiDelegates (int a,int b)
Youy can add Multicast Delegates by using "+ ="sign assignment operators like YourMethod += new yourMulticastDelegates(Addition);