Conexion a la base de datos con SQL SERVER en C#

Descargar e Instalar:

Microsoft.Data.SqlClient

System.Configuration.ConfigurationManager

En el App.config del proyecto principal:
Configuramos la cadena de conexión para SQL SERVER

<connectionStrings>
    <add name="SqlCn" connectionString="Data Source=TI\MSSQLSERVER2022;Initial Catalog=salesdb;Integrated Security=True" />

</connectionStrings>

ó

<connectionStrings> 

     <add name="SqlServerCn" connectionString="Server=TI;Database=testdb;uid=sa;pwd=123456";Encrypt=False />

</connectionStrings>

Otra Forma:

string cadena = @"server = TI\MSSQLSERVER;database = Negocios2022;Trusted_Connection = True;MultipleActiveResultSets = True;TrustServerCertificate = False;Encrypt = False";

En el código C#

Creamos la clase Connection o Database

------------------------------------------------------------------------------

public class Connection

{

    private static Connection instance;

    private Connection() { }

    public static Connection GetInstance()

    {

        if (instance == null)

        {

            instance = new Connection();

        }

        return instance;

    }

    public SqlConnection GetConnection()

    {

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

        return new SqlConnection(connectionString);

    }

}

---------------------------------------------------------------------------------------------------------------

public class Connection

{

    private static Connection instance;

    private static readonly object _lock = new object();

    private Connection() { }

    public static Connection GetInstance()

    {

        if (instance == null)

        {

            lock (_lock)

            {

                if(instance == null)

                {

                    instance = new Connection();

                }

             }

        }

        return instance;

    }

    public SqlConnection GetConnection()

    {

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

        return new SqlConnection(connectionString);

    }

}

-----------------------------------------------------------------------------------------------------------------

Validar la conexion a la base de datos en C# Windows Form

private void btnConnection_Click(object sender, EventArgs e)

{

    try

    {

        Database database = new Database();

        using (SqlConnection conn = database.GetConnection())

        {

            conn.Open();

            MessageBox.Show("Conexión exitosa");

        }

    }

    catch (Exception ex)

    {

        MessageBox.Show("Conexión fallida: " + ex.Message);

    }

}