error 26- error locating server/instance specified

Question

I designed a C# desktop app in visual studio 2019 and for database used sql server express 2019 edition. i am trying to run this app on another pc. i have installed sql server express 2019 in the other pc also MS server management studio 2019 installed and restored the database. everything works fine like login,saving updating,deleting but when i try to fetch data to datagridview it shows "system.data.sqlclient.sqlexception - a network related or instance specific error occurred while establishing a connection to sql server. the server was not found or was not accessible. verify instance name is correct and sql server is configured to allow remote connections.(provider: sql network interfaces, error: 26 - error locating server/instance specified)."

all the ports are enabled and firewall rule is also enabled in the client pc.

i am using the below connection string for the connection.

class Connection
  {
       SqlConnection con = new SqlConnection(@"Data Source=.\SQLEXPRESS;Initial Catalog=icon;Integrated Security=True");

    public SqlConnection active()
    {
        if (con.State == ConnectionState.Closed)
        {
            con.Open();
        }
        return con;
    }
}

Please help anyone as i am not able to get what is the problem going on.

Belowcode is working

 private void loginBtn_Click(object sender, EventArgs e)
     {
        Connection con = new Connection();
        SqlCommand cmd = new SqlCommand("select * from [user] where 
        Username='" + usernameTxt.Text + "'and password='" + passwordTxt.Text 
        + "'", con.active());
        SqlDataAdapter sda = new SqlDataAdapter(cmd);
        DataTable dt = new DataTable();
        sda.Fill(dt);
        if (dt.Rows.Count > 0)
        {
            MessageBox.Show("Login Successful", "Sucsess", 
              MessageBoxButtons.OK, MessageBoxIcon.Information);
            new dashboard().Show();
            this.Hide();
        }

but this is not working.it shows the error when i try to fetch the data.

public partial class AllSudent : Form
{
    public AllSudent()
    {
        InitializeComponent();
    }

    Connection con = new Connection();
    public int studentID;
    private void AllSudent_Load(object sender, EventArgs e)
    {
       
        GetStudentsRecord();
    }

    public void GetStudentsRecord()
    {
        SqlCommand cmd = new SqlCommand("Select * From [student]", 
        con.active());
        DataTable dt = new DataTable();
        SqlDataReader sdr = cmd.ExecuteReader();
        dt.Load(sdr);

        sdataGridView.DataSource = dt;
    }

Answer 1

Throw your Connection class away, and pass the connection string to the DataAdapter. Don't bother opening or closing the connection; DataAdapter knows how to open a connection if it's closed

Put the connectionstring into the Settings

Use parameters

 private void loginBtn_Click(object sender, EventArgs e)
 {
    using(var sda = new SqlDataAdapter("select * from [user] where Username=@user and password=@pass", Properties.Settings.Default.ConStr)
    {
      //USE PARAMETERS
      sda.SelectCommand.Parameters.Add("@user", SqlDbType.VarChar, usernameTxt.Text.Length).Value = usernameTxt.Text;
      sda.SelectCommand.Parameters.Add("@pass", SqlDbType.VarChar, passwordTxt.Text.Length).Value = passwordTxt.Text.GetHashcode(); //DO NOT store your passwords in plain text!!

      var dt = new DataTable();
      sda.Fill(dt);
      if (dt.Rows.Count > 0)
      {
        MessageBox.Show("Login Successful", "Sucsess", 
          MessageBoxButtons.OK, MessageBoxIcon.Information);
        new dashboard().Show();
        this.Hide();
      }

   }
}

Use parameters

Just in case you missed it: USE PARAMETERS. Never again, in your life ever, should you concatenate a value into an SQL string. Ever. There is no reason to do it, and doing it will result in the software you create being hacked / you getting fired / both

Also, don't store passwords in plain text, ever. Salt and hash them. I've used string.GetHashcode() for demo purposes, which is not good but better than plaintext

---

Do the same thing to the not working code:

public void GetStudentsRecord()
{
    using(var sda = new SqlDataAdapter("Select * From [student]", Properties.Settings.Default.ConStr)){
      var dt = new DataTable();
      sda.Fill(dt);

      sdataGridView.DataSource = dt; 
    }
}

Answer 2

this issue also confusing me a few days after the IT guy do some security settings to the SQL Server. i have an EntityFramework for the Web application and a desktop application. after i did some setting on the SQL Server, the Web application comeback to work, but the desktop still with issue. but i used the some connection string for the both application, it make no sense one is work but the other doesn't. then i searched a lot until i found some one said need add a port number 1433 after the $ServerName$DatabaseInstanceName,1433 at here http://www.windows-tech.info/15/9f6dedc097727100.php . after i added it. the exception became: System.Data.SqlClient.SqlException: Login failed for user 'domain\name-PC$'. then i found this link System.Data.SqlClient.SqlException: Login failed for user: System.Data.SqlClient.SqlException: Login failed for user it said need add Trusted_Connection=False;. the whole connection string should be like: data source=XXXXX\SQLSERVER,1433;initial catalog=XXXDB;user id=UserID;password=PWD;Trusted_Connection=False;MultipleActiveResultSets=True;

hope this answer will help the ones out off Generic exception: "Error: 26-Error Locating Server/Instance Specified)