Select values from different db with same app C#

I think you should create a configuration form that allows you to change your connection string at runtime as the following

• Server: Contains your sql server name, you can enter your server name or select from the available server list
• Database: Enter your database name that you want to connect
• User name: Use the sql username to connect to your sql server
• Password: Use the sql password to connect to your sql server

Put your connection string in the app.config file

<configuration>
  <connectionStrings>
    <add name="cn" connectionString="Data Source=YourServerName;Initial Catalog=YourDatabase;User ID=YourUserSql;Password=YourPasswordSql;" providerName="System.Data.SqlClient"/>
  </connectionStrings>
</configuration>

Create an AppSetting class to read and update your connection string

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace WindowsForms1
{
    public class AppSetting
    {
        Configuration config;

        public AppSetting()
        {
            config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
        }

        //Get connection string from App.Config file
        public string GetConnectionString(string key)
        {
            return config.ConnectionStrings.ConnectionStrings[key].ConnectionString;
        }

        //Save connection string to App.config file
        public void SaveConnectionString(string key,string value)
        {
            config.ConnectionStrings.ConnectionStrings[key].ConnectionString = value;
            config.ConnectionStrings.ConnectionStrings[key].ProviderName = "System.Data.SqlClient";
            config.Save(ConfigurationSaveMode.Modified);
        }
    }
}

Open your configuration form, then add code to handle connect button and save button

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsForm1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void btnConnect_Click(object sender, EventArgs e)
        {
            string connectionString = string.Format("Data Source={0};Initial Catalog={1};User ID={2};Password={3};", cboServer.Text, txtDatabase.Text, txtUsername.Text, txtPassword.Text);
            try
            {
                SqlHelper helper = new SqlHelper(connectionString);
                if (helper.IsConnection)
                    MessageBox.Show("Test connection succeeded.", "Message", MessageBoxButtons.OK, MessageBoxIcon.Information);
            }
            catch(Exception ex)
            {
                MessageBox.Show(ex.Message, "Message", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            //Add server name to combobox
            cboServer.Items.Add(".");
            cboServer.Items.Add("(local)");
            cboServer.Items.Add(@".\SQLEXPRESS");
            cboServer.Items.Add(string.Format(@"{0}\SQLEXPRESS", Environment.MachineName));
            cboServer.SelectedIndex = 3;
        }

        private void btnSave_Click(object sender, EventArgs e)
        {
            //Set connection string
            string connectionString = string.Format("Data Source={0};Initial Catalog={1};User ID={2};Password={3};", cboServer.Text, txtDatabase.Text, txtUsername.Text, txtPassword.Text);
            try
            {
                SqlHelper helper = new SqlHelper(connectionString);
                if (helper.IsConnection)
                {
                    AppSetting setting = new AppSetting();
                    setting.SaveConnectionString("cn", connectionString);
                    MessageBox.Show("Your connection string has been successfully saved.", "Message", MessageBoxButtons.OK, MessageBoxIcon.Information);                      
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, "Message", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }
    }
}

You should change your connection string

SqlConnection con = new SqlConnection(cs)

to

SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["cn"].ConnectionString)

or you can create a global class that's help you easily to change the connection string

public static class Global
{
    public static string ConnectionString => ConfigurationManager.ConnectionStrings["cn"].ConnectionString;
}

then change your connection string as the following

SqlConnection con = new SqlConnection(Global.ConnectionString)