[SQL] how to build a SQL connection using C# in Visual Studio 2017?

I've always used Oledb Connection. but now I need to connect with my Database via Sql connection yet I don't know how to do so, can some one provide me an example of a database connected with sql connection?

this code needs a sql connection to be done successfully.

protected void Button1_Click(object sender, EventArgs e)
{
string st = this.TextBox1.Text;
string sqlstr2 = "select * from hsinfo WHERE rname='"+st+ "'";

SqlCommand cmd = new SqlCommand(sqlstr2,);
using (SqlDataReader rd = cmd.ExecuteReader())
{
this.Label1.Text = rd["rmail"].ToString();
}
}

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