C#

private bool DoesUserExists(string sUserID) {
  ... perform a select statement here with a prepared statement
  ... then check if you have exactly one record
  bool bResult = odr.Read(); --> true if you have one record
  ... close everything
  return b;
}

public void CheckUser() {
    ... get the value from your textbox
    // call method to check if user exists
    if(this.DoesUserExists(sUserID)) {
       // show success dialog
    } else {
       // show failure dialog
    }
}

private bool DoesUserExists(string sUserID) {
   OdbcConnection cn;
   OdbcCommand cmd;
   OdbcParameter dbParameter;
   OdbcDataReader odr;

   string MyString = "Select * from users WHERE userid = @userid";

    cn = new OdbcConnection("Driver={Microsoft ODBC for Oracle};Server=orcl8i;UID=itehad;PWD=creative;");
    cn.Open();
    cmd = new OdbcCommand(MyString, cn);
    dbParameter = new OdbcParameter("userid", sUserID");
    cmd.Parameters.Add(dbParameter);
    odr = cmd.ExecuteReader();

    bool bResult = (odr.HasRows && odr.Read());
    odr.Close();
    cn.Close();
    return bResult;
}

private void login_Click(object sender, EventArgs e)
{
   if(this.DoesUserExists(usr.Text)) {
     MessageBox.Show("User Login", "Information", MessageBoxButtons.OK, MessageBoxIcon.Information);
   } else {
       DialogResult drsave = MessageBox.Show("Login Fail", "Erorr", MessageBoxButtons.OKCancel, MessageBoxIcon.Error);
       if (drsave == DialogResult.OK)
       {
          usr.Text = "";
          pass.Text = "";
          usr.Focus();
        } else {
            this.Close();
        }
   }
}

SeMartens wrote:

odr.Read(); --> true if you have one record

a == b

a.Equals(b)