We can connect our C# application to data in a SQL Server database by using following ways:

1. First Method

Firstly, we need a connection string to connect to database.This connection string contain the information about the server to which we are going to connect.

SQL Connection string

string connetionString = "Data Source=ServerName; Initial Catalog=DatabaseName;User ID=UserName;Password=Password";

Add the following namespace in your code:
using System;
using System.Windows.Forms;
using System.Data.SqlClient;

Now, we have to create an instance of the SqlConnection and pass the Connection String as an argument in it.

using ( SqlConnection connection = new SqlConnection( connetionString ))
 {
    connection.Open();    
    
}

A Sample C# program that connect SQL Server using connection string.

using System; 
using System.Windows.Forms; 
using System.Data.SqlClient; 
namespace ConnectToDatabase
{ 
    public partial class FormSample : Form 
  {
      InitializeComponent(); 
  }

private void button_Click1(object sender, EventArgs e) 
{
  string conString = "Data Source=(local); Initial Catalog=EmployeeData;Integrated Security=True";
try 
{
   using ( SqlConnection connection = new SqlConnection( conString ))
   {
       connection.Open();        
 }
}
catch (Exception ex) 
 { 
    thrown ex
 }
}

2. Second method

We can also connect to SQL Server database by providing a connection string in Web.config file.

To configure a connection string for SQL Server in the Web.config file use the following steps:

1. Create or open the Web.config file of an application.

2. Define a ConnectionStrings element in Configuration tag. In this tag, create an add tag which define the following properties:

• name  

      It define the value to the name which is use to give a reference to the connection string.

  Example:-

  name="EmpDataConnectionString"

• connectionString  

       It define a connection string.

  Example:

  connectionString="Data Source=localhost;Integrated Security=SSPI;Initial Catalog=(local)"

•  providerName:-

              It define the value "System.Data.SqlClient" which specifies that use the ADO.NET provider.

<connectionStrings>

    <add name="EmpDataConnectionString" connectionString="Data Source=localhost;Integrated Security=SSPI;Initial Catalog=(local)" providerName="System.Data.SqlClient" />

</connectionStrings>

4. At the end save the Web.config file and close it.  

Add the following namespace in your code:

using System.Configuration;

using System.Data

To read the connection string use the ConfigurationManager class.

Example:

string connStr = ConfigurationManager.ConnectionStrings["myConnectionString"].ConnectionString;

A Sample C# program that connect SQL Server using connection string.

using System.Configuration;
using System.Data;

using System.Configuration;
using System.Data;

namespace ConnectToDatabase
{ 
    public partial class FormSample : Form 
{
InitializeComponent(); 
}

private void button2_Click(object sender, EventArgs e) 
{
   try 
   {
         using ( SqlConnection connection = new SqlConnection( ConfigurationManager.ConnectionStrings["EmpDataConnectionString"].ToString() ))
       {
           connection.Open();     
    }
}
 catch (Exception ex) 
 { 
    thrown ex
 }
}

3. Third method

To connect to database we use an App.Config file in which we define a connection string.

For example:

<configuration>

 <connectionStrings>

   <add name="EmpConString" connectionString="Data Source=(local);Initial Catalog=EmpRecord;Integrated Security=True"/>

 </connectionStrings>

</configuration>

In C# Code

Add the following references

using System;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;
using System.Windows.Forms;

Then, to read the Connection String value from the App.Config file use the below code:

string connectionString = ConfigurationManager.ConnectionStrings["EmpConString"].ConnectionString;

A Sample C# program that connect SQL Server using connection string.

<configuration>

<appSettings>

<add key="ConnectionString" value="Data Source=MY-PC;Initial Catalog=DBA;User ID=sa;Password=MYSQL123@" />

</appSettings>

</configuration>

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

namespace ConnectToDatabaseSample
{
public partial class EmpRecord : Form
{
   SqlConnection MyConnection = new SqlConnection(System.Configuration.ConfigurationSettings.AppSettings["ConnectionString"]);

    public CommodityEdit()
    {
        InitializeComponent();
    }

    private void button1_Click(object sender, EventArgs e)
    {
        try
        {
           MyConnection.Open();
        }
        catch (Exception)
        {
           throw;
        }
     }
  }
}