Invalid attempt to read when no data is present in ASP.NET

Question

I am using C# to learn ASP.NET, I tried to write authentication code as

but i am always got these error plz give some solution.. 

 

protected void Button3_Click(object sender, EventArgs e)

{

Button3.Visible = true;

txtdms.Visible = true;

lbldms.Visible = true;

str3 = "Select [Coloum Name1],[Coloum Name2] from LPsupplier where [DMS Vendor Code] = '" + txtbox.Text + "'";

sqlCmd.Connection = conn;

sqlCmd.Connection.Open();

sqlCmd.CommandText = str3;

sqlDr = sqlCmd.ExecuteReader();

sqlDr.Read();

txtaxcode.Text = sqlDr["Coloum Name1"].ToString();

txtvendorname.Text = sqlDr["Coloum Name2"].ToString();

}