An unhandled exception of type 'System.InvalidOperationException' occurred in System.Data.dll in C#

Question

Code :

private void button2_Click(object sender, EventArgs e)
{
    DataTable dt = new DataTable();
    SqlConnection CON = new SqlConnection(@"Data Source(LocalDB)\MSSQLLocalDB;AttachDbFilename=C:\Users\stud\Documents\ronak.mdf;Integrated Security=True;Connect Timeout=30");

      
    SqlCommand cmd = new SqlCommand("Select * from Table where username= ' " + textBox1.Text + "' and password= ' " + textBox2.Text + "' ", CON);
         
    SqlDataReader sda = cmd.ExecuteReader();

    dt.Load(sda);
    if (dt.Rows[0][0].ToString() == "1")
     {

        this.Hide();
        login2 rk = new login2();
        rk.Show();
     }
     else
     {
        MessageBox.Show("please chack you username and password");
     }
 }

This code is totally true but I have face some problem.

Answer 1

Your code has quite a number of different issues:

• Your connection string is missing = after Data Source
• You need to actually open the connection.
• Do not use AttachDbFilename instead create and connect to a normal database.
• Do not store plain-text passwords. Salt and hash them instead. Then compare the hash server-side, do not return it to the client.
• You don't need a DataTable or DataAdapter , you can just use ExecuteScalar to retrieve a single value.
• Dispose the connection and command with using .
• Do not inject data into your queries. Use parameters instead.

const string query = @"
Select 1
from Table
where username= @username
  and password= @password
";

using (var CON = new SqlConnection(@"Data Source=(LocalDB)\MSSQLLocalDB;Initial Catalog=ronak;Integrated Security=True;Connect Timeout=30"))
using (var cmd = new SqlCommand(query, CON))
{
    cmd.Parameters.Add("@username", SqlDbType.NVarChar, 100).Value = textBox1.Text;
    cmd.Parameters.Add("@password", SqlDbType.VarBinary, 256).Value = SaltAndHashPassword(textBox2.Text, textBox1.Text);
    CON.Open();
    var exists = cmd.ExecuteScalar() == 1;
    CON.Close();

    if (exists)
    {
        this.Hide();
        login2 rk = new login2();
        rk.Show();
    }
    else
    {
        MessageBox.Show("please chack you username and password");
    }
}

Answer 2

First of all you forget = after Data source in sqlConnection Second you are using bad names for variables Third you dont need to use SqlCommand, you can replace it with SqlDataAdapter it s more simple Fourth you must use ( Using ) to dispose connection

Here`s the full code

  private void button2_Click(object sender, EventArgs e)
        {
            DataTable table = new DataTable();
            using (SqlConnection connection = new SqlConnection(@"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=C:\Users\stud\Documents\ronak.mdf;Integrated Security=True;Connection  Timeout=30"))
            {
                connection.Open();
                using (SqlDataAdapter adapter = new SqlDataAdapter($"Select * from Table where username= '{textBox1.Text}' and password= '{textBox2.Text}' ", connection))
                {
                    adapter.Fill(table);

                    if (table.Rows.Count == 0)
                    {
                        MessageBox.Show("please chack you username and password");
                        return;
                    }
                    if (table.Rows[0][0].ToString() == "1")
                    {
                        this.Hide();
                        login2 rk = new login2();
                        rk.Show();
                    }
                }
            }

        }