How to connect to database

 SqlConnection MyConnection = new SqlConnection("Your Connection String");
 SqlCommand TheCommandL = new SqlCommand("select Name from tblCustomer", MyConnection);
 MyConnection.Open();
 SqlDataReader rdr = null;
 rdr = TheCommandL.ExecuteReader();
 while (rdr.Read())
 {
   Console.Write(rdr["Cunm"].ToString());

 }

.NET Data Provider -- Default Relative Path -- Trusted Connection
using System. Data. SqlClient;
var conn = new SqlConnection();
conn. ConnectionString =
"Data Source=.\SQLExpress;" +
"User Instance=true;" +
"Integrated Security=true;" +
"AttachDbFilename=|DataDirectory|DataBaseName.mdf;"
conn. Open();

<connectionStrings>
    <add name="CharityManagement" 
    connectionString="Data Source=.;Initial Catalog=CharityManagement;Integrated Security=True"/>
  </connectionStrings>

var connectionString=ConfigurationManager.ConnectionStrings["CharityManagement"].ConnectionString;

  SqlConnection con = new SqlConnection(SqlDataSource2.ConnectionString);
  con.Open();
  SqlDataAdapter a = new SqlDataAdapter("SELECT * FROM TableName", con);
  DataTable dt = new DataTable();
  a.Fill(dt);
  con.Close();
  //... traverse dt for retrieved data.

 private void button1_Click(object sender, EventArgs e)
        {
            string connetionString = null;
            SqlConnection cnn ;
			connetionString = "Data Source=ServerName;Initial Catalog=DatabaseName;User ID=UserName;Password=Password"
            cnn = new SqlConnection(connetionString);
            try
            {
                cnn.Open();
                MessageBox.Show ("Connection Open ! ");
                cnn.Close();
            }
            catch (Exception ex)
            {
                MessageBox.Show("Can not open connection ! ");
            }
        }

public class  employee
{
OleDbConnection cn=new OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;Data Source=E://srid documents/mydocsssssss/products.accdb;User ID=admin");
OleDbCommand cmd=new OleDbCommand();

    public DataSet getemploye(int id)
    {
        OleDbDataAdapter da = new OleDbDataAdapter("select * from emp where empid=" + id,cn);
        DataSet ds = new DataSet();
        da.Fill(ds,"p");
        return ds;
    }

    public bool checkemploye(int eno)
    {
        try
        {
            cmd.CommandText = "select count(*) from emp where empid=" + eno;
            cmd.Connection = cn;
            cn.Open();
            int i = (int)cmd.ExecuteScalar();
            if (i==1)
            {
                return true;
            }
            else
            return false;
        }
        catch (Exception ex)
        {
            throw new Exception(ex.Message);
        }
        finally
        {
           cn.Close();
        }
    }
}

protected void Button1_Click(object sender, EventArgs e)
    {
       employee obj=new employee();
        bool b=obj.checkemploye(int.Parse(TextBox1.Text));
        if (b == true)
        {
            System.Data.DataSet ds = obj.getemploye(int.Parse(TextBox1.Text));
            GridView1.DataSource= ds.Tables[0];
            GridView1.DataBind();
            GridView1.Visible = true;
        }
        else
        {
            Response.Write("Emp id "+TextBox1.Text+" Doesnot exist");
            GridView1.Visible = false;
        }
    }

    protected void BindData()
    {        
        cmd = new SqlCommand("Get_Employees", con);
        cmd.CommandType = CommandType.StoredProcedure;
        cmd.Parameters.AddWithValue("@DeptNo", DeptNo);
        dt = new DataTable();

        try
        {
            con.Open();
            da = new SqlDataAdapter(cmd);
            da.Fill(dt);
            if (dt.Rows.Count > 0)
            {
                GV.DataSource = dt;
                GV.DataBind();
            }
            
        }
        catch (Exception ex)
        {

        }
        finally
        {
            con.Close();
            con.Dispose();
        }
    }