Interview Questions

Difference between onBlur and onChange events.
onBlur is when you have moved away from an object without necessarily changing it.
onChange is only called afer you have moved away from an object that you have changed the value of.
Difference between a postback and a callback function.
A Postback occurs when the data (the whole page) on the page is posted from the client to the server..ie the data is posted-back to the server, and thus the page is refreshed (redrawn)...think of it as 'sending the server the whole page (asp.net) full of data'.
On the other hand, a callback is also a special kind of postback, but it is just a quick round-trip to the server to get a small set of data (normally), and thus the page is not refreshed, unlike with the postback...think of it as 'calling the server, and receiving some data back'.For example Update panel is used in asp.net for call backs.

With Asp.Net, the ViewState is not refreshed when a callback is invoked, unlike with a postback.




Suppose I want a function that will work only during initial page load and not during postbacks where to write it?
Inside if(!Page.IsPostBack) condition.
Difference between runtime and compile time?
Compile time

• Syntax errors
• Typechecking errors

Run time

• Division by zero
• Deferencing a null pointer
• Running out of memory
• Trying to open a file that isn't there

Difference between primary key and unique key
A table can have more than one unique key constraint columns but primary key columns should be only one.
Primary key does not allow null values.Unique key allows a null value.
Primary key Supports Auto Increment value but Unique doesn't support auto Increment value.
Difference between stored procedure and User Defined function
Stored procedures are compiled for the first time and executes compiled code whenever it is called. But function is compiled and executed every time when it is called.
Function must return a value but in stored procedure it is optional.
Functions take one input parameter,it is mandatory whereas stored procedure takes 0 to n input parameters.
Functions can be called from select statement whereas stored procedure cannot.
We can use try catch statements in stored procedures but in functions cannot.
We cannot use insert,update,delete and create statements in functions whereas in stored procedure we can.
Functions can be called from procedure.Procedures cannot be called from function.
 Difference between truncate and delete

• You can use WHERE clause(conditions) with DELETE but you can't use WHERE clause with TRUNCATE .
  
• You cann't rollback data in TRUNCATE but in DELETE you can rollback data.TRUNCATE removes(delete) the record permanently.
  
• A trigger doesn’t get fired in case of TRUNCATE whereas Triggers get fired in DELETE command.
  
• If tables which are referenced by one or more FOREIGN KEY constraints then TRUNCATE will not work.
  
• TRUNCATE resets the Identity counter if there is any identity column present in the table where delete not resets the identity counter. 
• TRUNCATE is faster than DELETE.      
 Difference between collections and generics 

- Non-Generic collections - These are the collections that can hold elements of different data types. It holds all elements as object type.
So it includes overhead of type conversions.
- Generic collections - These are the collections that can hold data of same type and we can decide what type of data that collections can hold.
Some advantages of generic collections - Type Safe, Secure, reduced overhead of type conversions.

Collections are enumerable data structures that can be accessed using indexes or keys.Using generic collections,classes provide increased type safety and in some cases,better performance especially when storing value types.

Difference between compile time polymorphism and runtime polymorphism

 Compile time polymorphism also known as function overloading.In this we can have two or more methods with the same name but different signatures.

Runtime polymorphism also known as function overriding. In this, we can have two or more methods with the same name,same signature but with different implementation.

Difference between GET and POST method.

GET method is not secure since your informations will be appeared in the url.Can transfer only limited data.

POST method is secure and can transfer unlimited data. 

Difference between UserControl and ServerControl

A User Control is a partial web page, created in the same way as any other web page in ASP.NET, except that it has an .ASCX extension, and it can be embedded in your other ASPX pages.

User controls are registered with the web page in which they are used, like this:

<%@ Register TagPrefix="UC" TagName="TestControl" Src="test.ascx" %>

They are then declared in the web page they are to be used in, like this:

<UC:TestControl id="Test1" runat="server"/>

Server controls are controls that execute on the server and render markup to the browser. User controls and custom controls are both examples of server controls.

Difference between char and varchar in sql

The char is a fixed-length character data type, the varchar is a variable-length character data type.

Because char is a fixed-length data type, the storage size of the char value is equal to the maximum size for this column. Because varchar is a variable-length data type, the storage size of the varchar value is the actual length of the data entered, not the maximum size for this column.

You can use char when the data entries in a column are expected to be the same size.(For eg: Phone number).

You can use varchar when the data entries in a column are expected to vary considerably in size.(For eg: Address).

Difference between output parameters and return statement in sql stored procedure

 In short, return is used to give the control back to the calling program and also return information like success or failure. Note that this shouldn't be used as a means to return actual user data.

Output parameter is used to share the user data from the SP to the calling program. One needs to use caution while using this as you don't want to have many output parameters. Also output parameters give only singleton values. And if there are too many, its best to use select statements to return data.

In addition, the RETURN statement can return only integer value, while output parameters support other data types.

Useful link:

http://www.sqlteam.com/article/stored-procedures-returning-data

Difference between is and as keyword in c#

The is operator checks if an object can be cast to a specific type. Example: if (someObject is StringBuilder) ... The as operator attempts to cast an object to a specific type, and returns null if it fails. Example: StringBuilder b = someObject as StringBuilder; if (b != null) ... Also related: The cast operator attempts to cast an object to a specific type, and throws an exeption if it fails. Example: StringBuilder b = (StringBuilder)someObject.

The as keyword attempts to convert one object to another, similar to a cast. If the conversion fails, a null is returned instead of an exception being thrown. For example:

object n=123;
object obj="abc";

string s1=n as string;
string s2=obj as string;

The is operator tests whether an object is of a particular type. For example:

object n1=123;

bool b1=n1 is int;
bool b2=n1 is string;

Console.WriteLine("n1 is int ? "+b1.ToString());
Console.WriteLine("n1 is string ? "+b2.ToString()); 

What is Doctype in html

The <!DOCTYPE> declaration must be the very first thing in your HTML document, before the <html> tag.
The <!DOCTYPE> declaration is not an HTML tag; it is an instruction to the web browser about what version of HTML the page is written in.
The <!DOCTYPE> declaration refers to a Document Type Definition (DTD). The DTD specifies the rules for the markup language, so that the browsers render the content correctly.

If cookies are disabled in client browser, will session work?
No. Identities of client get destroy.

What is the @Register directive used for? What is the purpose of @Register directive in ASP.NET? How to create Web User Controls in ASP.NET?
Directives in ASP.NET are used to set attributes for a page. The @Register directive is a directive used to register user defined controls on a web page. A user created server control has an ascx extension. These controls inherit from the namespace System.Web.UI.UserControl. This namespace inherits from the System.Web.UI.Control.

A user control may be embedded in an aspx web page using the @Register directive. A user control cannot be executed on its own independently, but may be registered on a web page and then used out there. Below is the syntax for registering an @register directive in an aspx page in ASP.NET

<%@ Register TagPrefix="UC1" TagName="UserControl1" Src="UserControl1.ascx" %>

The TagPrefix attributes is used to specify a unique namespace for the user control. The TagName is a name used to refer a user control uniquely by its name. Say we want to use this control in a webpage, we may use the code below...

<UC1:UserControl1 runat="server"/>

What is Global.asax file?
The Global.asax file, also known as the ASP.NET application file, is an optional file that contains code for responding to application-level and session-level events raised by ASP.NET or by HTTP modules.
What is caching?
It is a way to store the frequently used data into the server memory which can be retrieved very quickly. And so provides both scalability and performance. For example if user is required to fetch the same data from database frequently then the resultant data can be stored into the server memory and later retrieved in very less time (better performance).
What are the different sql tuning/optimisation methods?
1) Use column names in select statement instead of '*'.
2) HAVING clause is used to filter the rows after all the rows are selected. It is just like a filter. Do not use HAVING clause for any other purposes.
3)Sometimes you may have more than one subqueries in your main query. Try to minimize the number of subquery block in your query.
4) Use operator EXISTS, IN and table joins appropriately in your query.
a) Usually IN has the slowest performance.
b) IN is efficient when most of the filter criteria is in the sub-query.
c) EXISTS is efficient when most of the filter criteria is in the main query.
5)Use EXISTS instead of DISTINCT when using joins which involves tables having one-to-many relationship.
6)Try to use UNION ALL in place of UNION.

What are temporary tables?
 Temporary tables are created in tempdb database.There are two types of temp tables, local temporary tables and global temporary tables.
 Local temp table starts with a symbol "#" and they are visible only in the current session. They gets deleted when the user disconnects from instance of microsoft sql server.Global temp tables are written with "##" and they are visible to all sessions or any user.They gets deleted when all users disconnect from sql.
For Example:
If you create a table named employees, the table can be used by any person who has the security permissions in the database to use it, until the table is deleted. If you create a local temporary table named #employees, you are the only person who can work with the table, and it is deleted when you disconnect. If you create a global temporary table named ##employees, any user in the database can work with this table. If no other user works with this table after you create it, the table is deleted when you disconnect. If another user works with the table after you create it, SQL Server deletes it when both of you disconnect. SQL statements for creating the temporary table using the CREATE TABLE statement:

What is the default session timeout in asp.net?
20 min
Difference between varchar and nvarchar in sql?
The key difference between the two data types is how they're stored. VARCHAR is stored as regular 8-bit data. But NVARCHAR strings are stored in the database as UTF-16 — 16 bits