how to get connection string from app config in c#

Question

My app.config file:

Answer 1

try

SqlConnection con = new SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["ConnectionStringNameFromWebConfig"].ConnectionString);

It should work

Answer 2

Basing my answer on this solution, try:

conn.ConnectionString = ConfigurationManager.ConnectionStrings["MegaPixelBizConn"].ConnectionString;

Make sure you have a reference to System.Configuration as well.

Also remember to close your connection.

EDIT

Based on your new edit, try this for your code (Note I unfortunately can't test if this works due to my PC being broken).

private void btnLogin_Click(object sender, EventArgs e)
    {
        try
        {
            if (txtUserName.Text == "" || txtPassword.Text == "")
            {
                lblError.Visible = true;
                lblError.Text = "*Enter UserName and Password";
                //MessageBox.Show(" Enter UserName and Password .");
                return;
            }
            else
            {

                using (SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["MegaPixelBizConn"].ConnectionString))
                {

                    con.Open();

                    using (SqlCommand cmd = new SqlCommand("SELECT * FROM login_info WHERE userName = @Username AND password = @Password", con))
                    {
                        // If these two lines don't work, replace "Username" and "Password" with "@Username"/"@Password"
                        cmd.Parameters.Add(new SqlParameter("Username", txtUserName.Text);
                        cmd.Parameters.Add(new SqlParameter("Password", txtPassword.Text);
                        SqlDataReader r = cmd.ExecuteReader();
                        if(r.HasRows())
                        {
                            // this assumes that there is only one user/password and no two users with same UID and PWORD
                            this.Hide();
                            Home hm = new Home();
                            hm.Show();
                        }

                        else
                        {
                            lblError.Text = "*Not Registered User or Invalid Name/Password";
                            //MessageBox.Show("Not Registered User or Invalid Name/Password");
                            txtPassword.Text = "";
                        }
                    }
                }
            }
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }


    }

This will help prevent against SqlInjections as well.