System.Data.SqlClient.SqlException: An error has occurred while establishing a connection to the server.  When connecting to SQL Server 2005, this failure may be caused by the fact that under the default settings SQL Server does not allow remote connections. (provider: Named Pipes Provider, error: 40 - Could not open a connection to SQL Server)
   at System.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection)
   at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj)
   at System.Data.SqlClient.TdsParser.Connect(ServerInfo serverInfo, SqlInternalConnectionTds connHandler, Boolean ignoreSniOpenTimeout, Int64 timerExpire, Boolean encrypt, Boolean trustServerCert, Boolean integratedSecurity, SqlConnection owningObject)
   at System.Data.SqlClient.SqlInternalConnectionTds.AttemptOneLogin(ServerInfo serverInfo, String newPassword, Boolean ignoreSniOpenTimeout, Int64 timerExpire, SqlConnection owningObject)
   at System.Data.SqlClient.SqlInternalConnectionTds.LoginNoFailover(String host, String newPassword, Boolean redirectedUserInstance, SqlConnection owningObject, SqlConnectionString connectionOptions, Int64 timerStart)
   at System.Data.SqlClient.SqlInternalConnectionTds.OpenLoginEnlist(SqlConnection owningObject, SqlConnectionString connectionOptions, String newPassword, Boolean redirectedUserInstance)
   at System.Data.SqlClient.SqlInternalConnectionTds..ctor(DbConnectionPoolIdentity identity, SqlConnectionString connectionOptions, Object providerInfo, String newPassword, SqlConnection owningObject, Boolean redirectedUserInstance)
   at System.Data.SqlClient.SqlConnectionFactory.CreateConnection(DbConnectionOptions options, Object poolGroupProviderInfo, DbConnectionPool pool, DbConnection owningConnection)
   at System.Data.ProviderBase.DbConnectionFactory.CreatePooledConnection(DbConnection owningConnection, DbConnectionPool pool, DbConnectionOptions options)
   at System.Data.ProviderBase.DbConnectionPool.CreateObject(DbConnection owningObject)
   at System.Data.ProviderBase.DbConnectionPool.UserCreateRequest(DbConnection owningObject)
   at System.Data.ProviderBase.DbConnectionPool.GetConnection(DbConnection owningObject)
   at System.Data.ProviderBase.DbConnectionFactory.GetConnection(DbConnection owningConnection)
   at System.Data.ProviderBase.DbConnectionClosed.OpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory)
   at System.Data.SqlClient.SqlConnection.Open()
   at WindowsFormsApplication1.Form1.conexion() in C:\akuma\Proyectos c#\practicas\cajero automatico\cajero automatico\Form1.cs:line 

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Data.OleDb;
using System.Data.Sql;
using System.Data.SqlClient;
namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {            
        }

        private void button2_Click(object sender, EventArgs e)
        {
            conexion();
        }
                
        private void conexion()
        {
            SqlConnection conn = new SqlConnection("Data Source=/.SQLEXPRESS;AttachDbFilename=cajero.mdf;Integrated Security=True;Connect Timeout=30;User Instance=True");
            try
            {

                conn.Open();
                MessageBox.Show("se conecto exitosamente");

            }
            catch (Exception e)
            {
                MessageBox.Show("no se pudo conectar error");
                txtus.AppendText(Convert.ToString(e));
            }             

        }
    }
}

SqlConnection conn = new SqlConnection("Data Source=./SQLEXPRESS;AttachDbFilename=cajero.mdf;Integrated Security=True;Connect Timeout=30;User Instance=True");

Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\cajero.mdf;Integrated Security=True;Connect Timeout=30;User Instance=True

using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Windows.Forms;
using System.Data.OleDb;
using System.Data.Sql;
using System.Data.SqlClient;
namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            
        }

        private void button2_Click(object sender, EventArgs e)
        {
            SqlConnection conn = new SqlConnection("Data Source=.\\SQLEXPRESS;AttachDbFilename=|DataDirectory|\\cajero.mdf;Integrated Security=True;Connect Timeout=10;User Instance=True");
            try
            {
                conn.Open();
                MessageBox.Show("se conecto exitosamente");
            }
            
            catch (Exception)
            {
                MessageBox.Show("no se pudo conectar error");
            }
            
        }

        private void dataGridView1_CellContentClick(object sender, DataGridViewCellEventArgs e)
        {
           
        } 
    }
}