How do I create a connection to a database?

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To do an interaction with a database system you have to create a connection to the server. This connection basically contains the location of the database, database name, a username and password of a database and some other parameters.

To create a connection object you create an instance of SqlConnection class. You need to pass a connection string to the constructor of this object. After creating a connection you have to call the Open() method to open the database connection.

After having the connection object ready you can start using it to manipulate the database with the help of the SqlCommand object. Let’s see an example below for creating and opening a connection.

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

namespace Kodecsharp
{
    class SqlConnectionExample
    {
        static void Main(string[] args)
        {
            //
            // Define the connection string to the database.
            //
            string connectionString = 
                @"Data Source=.\\SQLEXPRESS; Initial Catalog=Northwind; Integrated Security=SSPI;";

            using (SqlConnection connection = new SqlConnection(connectionString))
            {
                connection.Open();
                if (connection.State == ConnectionState.Open)
                {
                    Console.WriteLine("Connection is ready to be used.");
                }

                // Do something with the connection object.
            }
        }
    }
}

using System;

using System.Data;

using System.Data.SqlClient;

namespace Kodecsharp

{

class SqlConnectionExample

{

static void Main(string[] args)

{

// Define the connection string to the database.

string connectionString =

@"Data Source=.\\SQLEXPRESS; Initial Catalog=Northwind; Integrated Security=SSPI;";

using (SqlConnection connection = new SqlConnection(connectionString))

{

connection.Open();

if (connection.State == ConnectionState.Open)

{

Console.WriteLine("Connection is ready to be used.");

}

// Do something with the connection object.

}

How to display a numeric value in hexadecimal format?

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To display a numeric value in its hexadecimal format we can use the x or X formatting specifier when using the Console.WriteLine() method. The letter case of the formatting specifier determines whether the hexadecimal letters appear in lowercase or uppercase.

The code snippet below show you how to do this:

using System;

namespace Kodecsharp.System.Example
{
    class HexadecimalFormat
    {
        [STAThread]
        static void Main(string[] args)
        {
            Console.WriteLine("255 = 0x{0:X}", 255);
            Console.WriteLine("255 = 0x{0:x}", 255);
            Console.WriteLine("64  = 0x{0:X}", 64);
        }
    }
}

using System;

namespace Kodecsharp.System.Example

{

class HexadecimalFormat

{

[STAThread]

static void Main(string[] args)

{

Console.WriteLine("255 = 0x{0:X}", 255);

Console.WriteLine("255 = 0x{0:x}", 255);

Console.WriteLine("64 = 0x{0:X}", 64);

}

The output below show the result of our code snippet:

255 = 0xFF
255 = 0xff
64  = 0x40

255 = 0xFF

255 = 0xff

64 = 0x40

How to select top node in XML using XPath?

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In this example you will learn how to select top node from an Xml document. We can use an XPath expression to select the nodes. For example using the following expression /Users/User[position() <= 3] we can select the first three of the User node.

using System;
using System.Xml;

namespace Kodecsharp.System.Xml.Example
{
    class SelectTopNode
    {
        static void Main(string[] args)
        {
            string xml =
                "<Users>" +
                "    <User Name='Alice'/>" +
                "    <User Name='Bob'/>" +
                "    <User Name='Carol'/>" +
                "    <User Name='Jack'/>" +
                "    <User Name='Malory'/>" +
                "</Users>";

            //
            // Creates an XmlDocument to load the xml data.
            //
            XmlDocument document = new XmlDocument();
            document.LoadXml(xml);

            //
            // Select top 3 user nodes from the xml document.
            //
            string expression = "/Users/User[position() <= 3]";
            XmlNodeList nodes = document.SelectNodes(expression);
            foreach (XmlNode node in nodes)
            {
                Console.WriteLine("User: {0}", 
                    node.Attributes["Name"].Value);
            }
        }
    }
}

using System;

using System.Xml;

namespace Kodecsharp.System.Xml.Example

{

class SelectTopNode

{

static void Main(string[] args)

{

string xml =

"<Users>" +

" <User Name='Alice'/>" +

" <User Name='Bob'/>" +

" <User Name='Carol'/>" +

" <User Name='Jack'/>" +

" <User Name='Malory'/>" +

"</Users>";

// Creates an XmlDocument to load the xml data.

XmlDocument document = new XmlDocument();

document.LoadXml(xml);

// Select top 3 user nodes from the xml document.

string expression = "/Users/User[position() <= 3]";

XmlNodeList nodes = document.SelectNodes(expression);

foreach (XmlNode node in nodes)

{

Console.WriteLine("User: {0}",

node.Attributes["Name"].Value);

}

The code snippet above select the first top three node from the xml document. It will print the following output:

User: Alice
User: Bob
User: Carol

User: Alice

User: Bob

User: Carol

How to find XML node using multiple attribute values?

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In this example you will learn how to select xml nodes based on multiple attribute values. The code snippet below selects User nodes that have an Active attribute equals to True and Expired attribute equals to False.

We define this selection criteria using an XPath expression that can be seen in the criteria variable in the snippet. We use the and operator in the XPath expression so that it matches both Active and Expired attributes.

Base on the defined xml document in the code snippet it is going to find two xml nodes. They are the nodes for user with username admin and jdoe. Let’s see the code below:

using System;
using System.Xml;

namespace Kodecsharp.System.Xml.Example
{
    class FindNodeByMultipleAttribute
    {
        static void Main(string[] args)
        {
            string xml =
                "<Users>" +
                "   <User UserName='admin' Active='True' Expired='False'/>" +
                "   <User UserName='jdoe' Active='True' Expired='False'/>" +
                "   <User UserName='foobar' Active='False' Expired='True'/>" +
                "   <User UserName='system' Active='True' Expired='True'/>" +
                "</Users>";

            XmlDocument xmlDoc = new XmlDocument();
            xmlDoc.LoadXml(xml);

            string criteria = 
                "/Users/User[@Active = 'True' and @Expired = 'False']";

            XmlNodeList activeUsers = xmlDoc.SelectNodes(criteria);
            foreach (XmlNode node in activeUsers)
            {
                Console.WriteLine("Username: {0}", 
                    node.Attributes["UserName"].Value);
            }            
        }
    }
}

using System;

using System.Xml;

namespace Kodecsharp.System.Xml.Example

{

class FindNodeByMultipleAttribute

{

static void Main(string[] args)

{

string xml =

"<Users>" +

" <User UserName='admin' Active='True' Expired='False'/>" +

" <User UserName='jdoe' Active='True' Expired='False'/>" +

" <User UserName='foobar' Active='False' Expired='True'/>" +

" <User UserName='system' Active='True' Expired='True'/>" +

"</Users>";

XmlDocument xmlDoc = new XmlDocument();

xmlDoc.LoadXml(xml);

string criteria =

"/Users/User[@Active = 'True' and @Expired = 'False']";

XmlNodeList activeUsers = xmlDoc.SelectNodes(criteria);

foreach (XmlNode node in activeUsers)

{

Console.WriteLine("Username: {0}",

node.Attributes["UserName"].Value);

}

If you run the snippet you’ll get the following output:

Username: admin
Username: jdoe

1 2	Username: admin Username: jdoe

How do I read xml node attribute values?

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using System;
using System.Xml;

namespace Kodecsharp.System.Xml.Example
{
    class ReadNodeAttributeValue
    {
        static void Main(string[] args)
        {
            string xmlString =
                "<Users>" +
                "    <User Id='1' Status='1'>" +
                "        <UserName>admin</UserName>" +
                "    </User>" +
                "    <User Id='2' Status='0'>" +
                "        <UserName>jdoe</UserName>" +
                "    </User>" +
                "</Users>";

            XmlDocument document = new XmlDocument();
            document.LoadXml(xmlString);

            //
            // Select a user node that has an id value equals to 1.
            //
            XmlNode node = document.SelectSingleNode("/Users/User[@Id='1']");
            if (node != null)
            {
                string username = node["UserName"].InnerText;

                //
                // Read the node attribute values.
                //
                string id = node.Attributes["Id"].Value;
                string status = node.Attributes["Status"].Value;

                Console.WriteLine("Username: {0}; Id: {1}; Status: {2}", 
                    username, id, status);
            }
        }
    }
}

using System;

using System.Xml;

namespace Kodecsharp.System.Xml.Example

{

class ReadNodeAttributeValue

{

static void Main(string[] args)

{

string xmlString =

"<Users>" +

" <User Id='1' Status='1'>" +

" <UserName>admin</UserName>" +

" </User>" +

" <User Id='2' Status='0'>" +

" <UserName>jdoe</UserName>" +

" </User>" +

"</Users>";

XmlDocument document = new XmlDocument();

document.LoadXml(xmlString);

// Select a user node that has an id value equals to 1.

XmlNode node = document.SelectSingleNode("/Users/User[@Id='1']");

if (node != null)

{

string username = node["UserName"].InnerText;

// Read the node attribute values.

string id = node.Attributes["Id"].Value;

string status = node.Attributes["Status"].Value;

Console.WriteLine("Username: {0}; Id: {1}; Status: {2}",

username, id, status);

}

How do I select xml node by attribute value?

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This example show you how to select nodes from an xml document based on the node’s attribute values. We can use the XmlDocument.SelectNodes or XmlDocument.SelectSingleNode method to get a list of nodes or a single node from an xml document. For selecting node based on its attribute values we use an XPath expression in the following format: /Users/User[@UserName='foobar'].

using System;
using System.Xml;

namespace Kodecsharp.System.Xml.Example
{
    class SelectNodeByAttributeValue
    {
        static void Main(string[] args)
        {
            string xmlString =
                "<Users>" +
                "    <User UserName='jdoe' Status='1'/>" +
                "    <User UserName='admin' Status='1'/>" +
                "    <User UserName='foobar' Status='0'/>" +
                "</Users>";

            //
            // Load the xml string document.
            //
            XmlDocument xmlDoc = new XmlDocument();
            xmlDoc.LoadXml(xmlString);

            //
            // Find an XmlNode that has a UserName attribute equals 
            // to 'foobar'.
            //
            string xpath = "/Users/User[@UserName='foobar']";
            XmlNode node = xmlDoc.SelectSingleNode(xpath);
            Console.WriteLine("Username: {0}; Status: {1}",
                node.Attributes["UserName"].Value, 
                node.Attributes["Status"].Value);

            //
            // Find a list of XmlNode that has a Status attribute
            // equals to '1'.
            //
            xpath = "/Users/User[@Status='1']";
            XmlNodeList nodes = xmlDoc.SelectNodes(xpath);
            foreach (XmlNode n in nodes)
            {
                Console.WriteLine("Username: {0}; Status: {1}",
                    n.Attributes["UserName"].Value,
                    n.Attributes["Status"].Value);
            }
        }
    }
}

using System;

using System.Xml;

namespace Kodecsharp.System.Xml.Example

{

class SelectNodeByAttributeValue

{

static void Main(string[] args)

{

string xmlString =

"<Users>" +

" <User UserName='jdoe' Status='1'/>" +

" <User UserName='admin' Status='1'/>" +

" <User UserName='foobar' Status='0'/>" +

"</Users>";

// Load the xml string document.

XmlDocument xmlDoc = new XmlDocument();

xmlDoc.LoadXml(xmlString);

// Find an XmlNode that has a UserName attribute equals

// to 'foobar'.

string xpath = "/Users/User[@UserName='foobar']";

XmlNode node = xmlDoc.SelectSingleNode(xpath);

Console.WriteLine("Username: {0}; Status: {1}",

node.Attributes["UserName"].Value,

node.Attributes["Status"].Value);

// Find a list of XmlNode that has a Status attribute

// equals to '1'.

xpath = "/Users/User[@Status='1']";

XmlNodeList nodes = xmlDoc.SelectNodes(xpath);

foreach (XmlNode n in nodes)

{

Console.WriteLine("Username: {0}; Status: {1}",

n.Attributes["UserName"].Value,

n.Attributes["Status"].Value);

}

This code snippet will print the following output:

Username: foobar; Status: 0
Username: jdoe; Status: 1
Username: admin; Status: 1

Username: foobar; Status: 0

Username: jdoe; Status: 1

Username: admin; Status: 1

How to select the first matched XmlNode using XPath?

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This example show you how to read the first XmlNode that matches the XPath expression. To find the first node we can use the XmlDocument.SelectSingleNode() method. This method takes a string of XPath expression and return the first XmlNode found in the xml document.

using System;
using System.Xml;

namespace Kodecsharp.System.Xml.Example
{
    class SelectSingleNodeDemo
    {
        static void Main(string[] args)
        {
            //
            // Users Xml Data
            //
            string usersXml =
                "<Users>" +
                "    <User>" +
                "        <UserName>admin</UserName>" +
                "        <FirstName>System</FirstName>" +
                "        <LastName>Administrator</LastName>" +
                "    </User>" +
                "    <User>" +
                "        <UserName>jdoe</UserName>" +
                "        <FirstName>John</FirstName>" +
                "        <LastName>Doe</LastName>" +
                "    </User>" +
                "</Users>";

            //
            // Creates XmlDocument and load xml document from the userXml 
            // string.
            //
            XmlDocument xmlDoc = new XmlDocument();
            xmlDoc.LoadXml(usersXml);

            //
            // Select the first XmlNode that matches the XPath
            // expression.
            //
            XmlNode node = xmlDoc.SelectSingleNode("/Users/User");
            if (node != null) 
            {
                string username = node["UserName"].InnerText;
                string firstName = node["FirstName"].InnerText;
                string lastName = node["LastName"].InnerText;

                Console.WriteLine(
                    String.Format("User: {0}; FName: {1}; LName: {2}",
                    username, firstName, lastName));
            }
        }
    }
}

using System;

using System.Xml;

namespace Kodecsharp.System.Xml.Example

{

class SelectSingleNodeDemo

{

static void Main(string[] args)

{

// Users Xml Data

string usersXml =

"<Users>" +

" <User>" +

" <UserName>admin</UserName>" +

" <FirstName>System</FirstName>" +

" <LastName>Administrator</LastName>" +

" </User>" +

" <User>" +

" <UserName>jdoe</UserName>" +

" <FirstName>John</FirstName>" +

" <LastName>Doe</LastName>" +

" </User>" +

"</Users>";

// Creates XmlDocument and load xml document from the userXml

// string.

XmlDocument xmlDoc = new XmlDocument();

xmlDoc.LoadXml(usersXml);

// Select the first XmlNode that matches the XPath

// expression.

XmlNode node = xmlDoc.SelectSingleNode("/Users/User");

if (node != null)

{

string username = node["UserName"].InnerText;

string firstName = node["FirstName"].InnerText;

string lastName = node["LastName"].InnerText;

Console.WriteLine(

String.Format("User: {0}; FName: {1}; LName: {2}",

username, firstName, lastName));

}

This code will give you the first node which is the node that contains the admin user information.

User: admin; FName: System; LName: Administrator

1	User: admin; FName: System; LName: Administrator

C# Examples

Code snippets for C# developers

How do I create a connection to a database?

How to display a numeric value in hexadecimal format?

How to select top node in XML using XPath?

How to find XML node using multiple attribute values?

How do I read xml node attribute values?

How do I select xml node by attribute value?

How to select the first matched XmlNode using XPath?