Execute multiple SELECT statements using a SqlCommand object and read the results using a SqlDataReader object

using System;
using System.Data;
using System.Data.SqlClient;

class ExecuteSelect
{
public static void Main()
{
SqlConnection mySqlConnection =new SqlConnection("server=(local)\\SQLEXPRESS;database=MyDatabase; Integrated Security=SSPI;");

SqlCommand mySqlCommand = mySqlConnection.CreateCommand();

mySqlCommand.CommandText =
"SELECT TOP 5 ProductID, ProductName " +
"FROM Products " +
"ORDER BY ProductID;" +
"SELECT TOP 3 CustomerID, CompanyName " +
"FROM Customers " +
"ORDER BY CustomerID;" +
"SELECT TOP 6 OrderID, CustomerID " +
"FROM Orders " +
"ORDER BY OrderID;";

mySqlConnection.Open();

SqlDataReader mySqlDataReader = mySqlCommand.ExecuteReader();

do
{
while (mySqlDataReader.Read())
{
Console.WriteLine("mySqlDataReader[0] = " + mySqlDataReader[0]);
Console.WriteLine("mySqlDataReader[1] = " + mySqlDataReader[1]);
}
Console.WriteLine("");
} while (mySqlDataReader.NextResult());

mySqlDataReader.Close();
mySqlConnection.Close();
}
}

What is the difference between const and static read-only member?

A const field must be initialized at the place where it is declared as shown in the example below.
class Program
{
      public const int Number = 100;
}
It is a compile time error to declare a const without a value. The code below will generate a compiler error stating "A const field requires a value to be provided"

class Program
{
      public const int Number;
}
It is a compile time error to change the value of a constant. The following code will generate a compiler error stating "The left-hand side of an assignment must be a variable, property or indexer"

class Program
{
     public const int Number = 100;
     static void Main()
     {
           Number = 200;
     }
}


It is not mandatory to initialize a static readonly field where it is declared. You can declare a static readonly field without an initial value and can later initialize the static field in a static constructor as shown below.
class Program
{
     public static readonly int Number;
     static Program()
     {
           Number = 100;
     }
}


Once a static readonly field is initialized, the value cannot be changed. The code below will generate a compiler error stating "A static readonly field cannot be assigned to (except in a static constructor or a variable initializer)"
class Program
{
       public static readonly int Number;
       static Program()
       {
             Number = 100;
       }
       static void Main()
       {
              Number = 200;
        }
}


In short, the difference is that static readonly field can be modified by the containing class, but const field can never be modified and must be initialized where it is declared. A static readonly field can be changed by the containing class using static constructor as shown below.
class Program
{
       // Initialize the static readonly field
       // to an initial value of 100
          public static readonly int Number=100;
          static Program()
          {
               //Value changed to 200 in the static constructor
                 Number = 200;
          }
}

LINQ Interview Questions

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What are the three main components of LINQ or Language INtegrated Query?

1. Standard Query Operators

2. Language Extensions

3. LINQ Providers

How are Standard Query Operators implemented in LINQ?

Standard Query Operators are implemented as extension methods in .NET Framework. These Standard Query Operators can be used to work with any collection of objects that implements the IEnumerable interface. A class that inherits from the IEnumerable interface must provide an enumerator for iterating over a collection of a specific type. All arrays implement IEnumerable. Also, most of the generic collection classes implement IEnumerable interface.

How are Standard Query Operators useful in LINQ?

Standard Query Operators in LINQ can be used for working with collections for any of the following and more.

1. Get total count of elements in a collection.
2. Order the results of a collection.
3. Grouping.
4. Computing average.
5. Joining two collections based on matching keys.
6. Filter the results


List the important language extensions made in C# to make LINQ a reality?
1. Implicitly Typed Variables
2. Anonymous Types
3. Object Initializers
4. Lambda Expressions


What is the purpose of LINQ Providers in LINQ?
LINQ Providers are a set of classes that takes a LINQ query and dynamically generates a method that executes an equivalent query against a specific data source.


What are the four LINQ Providers that .NET Framework ships?
1. LINQ to Objects - Executes a LINQ query against a collection of objects
2. LINQ to XML - Executes an XPATH query against XML documents
3. LINQ to SQL - Executes LINQ queries against Microsoft SQL Server.
4. LINQ to DataSets - Executes LINQ queries against ADO.NET DataSets.

Write a program using LINQ to find the sum of first 5 prime numbers?

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LINQ Interview Questions

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What is LINQ?
LINQ, or Language INtegrated Query, is a set of classes added to the .NET Framework 3.5. LINQ adds a rich, standardized query syntax to .NET programming languages that allows developers to interact with any type of data.



In any data driven application, you get data either from a Database, or an XML file or from collection classes. Prior to LINQ, working with each data source requires writing a different style of code. Moreover, working with external resources like data bases, XML files involves communicating with that external resource in some syntax specific to that resource. To retrieve data from a database you need to send it a string that contains the SQL query to execute, similarly, to work with an XML document involves specifying an XPath expression in the form of a string. The idea is that using LINQ you can work with disparate data sources using a similar style without having to know a separate syntax for communicating with the data source (e.g., SQL or XPath) and without having to resort to passing opaque strings to external resources.

In any data driven web application or windows application, we use database as a datasource for the application. In order to get data from the database and display it in a web or windows application, we typically do the following.
1. Prepare your SQL Statements.
2. Execute SQL Statements against the database.
3. Retrieve the results.
4. Populate the Business Objects.
5. Display the Data in the Web Form or Windows From.
What are the advantages of using LINQ or Language INtegrated Query?

In order to send a query to the database we must first establish a connection to the database. We then must encode the logic - the SQL query, its parameters, and the parameters' values - into strings that are supplied to the SqlCommand object. And because these inputs are encoded into opaque strings, there is no compile-time error checking and very limited debugging support. For example, if there is a spelling mistake in the SELECT query causing the Customets table name to be misspelled, this typographical error won't show up until runtime when this page is viewed in a web browser. These typographical errors are easy to make as there is no IntelliSense support. When we use LINQ, Visual Studio would display an error message alerting us about the incorrect table name.

Another mismatch between the programming language and the database is that the data returned by the database is transformed for us into objects accessible through the SqlDataReader, but these objects are not strongly-typed objects like we'd like. To get this data into strongly-typed objects we must write code ourselves that enumerates the database results and populates each record into a corresponding object.


LINQ was designed to address all these issues. LINQ also offers a unified syntax for working with data, be it data from a database, an XML file, or a collection of objects. With LINQ you don't need to know the intricacies of SQL, the ins and outs of XPath, or various ways to work with a collection of objects. All you need be familiar with is LINQ's classes and the associated language enhancements centered around LINQ.

In other words, LINQ provides type safety, IntelliSense support, compile-time error checking, and enhanced debugging scenarios when working with different datasources.

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