private void button1_Click(object sender, EventArgs e)
{
    isUserExist(textBox1.Text,textBox2.Text);
}

public bool isUserExist(string username,string password)
{
    try
    {
        string connstring = "data source=test_db;user id=system;password=password;";
        string statementcmd = "SELECT * FROM register_user Where UserName=@username";

        OracleConnection conn = new OracleConnection(connstring);
        OracleCommand cmd = new OracleCommand();
        cmd.Connection = conn;
        cmd.CommandText = statementcmd;
        cmd.Parameters.Add("@username", username);
        if (conn.State != ConnectionState.Open)
        {
            conn.Open();
            OracleDataReader reader = cmd.ExecuteReader();

            if (!reader.HasRows)
            { MessageBox.Show("User Name Not Found"); }

            if (!password.Equals(reader["password"].ToString()))
                 MessageBox.Show("Incorrect Password");
            reader.Close();
            }
            return true;

        }
    catch (Exception ex)
    {
        MessageBox.Show(ex.ToString());
        return false;
    }

}

if (reader.Read())
{
    // Do stuff
}

public bool isUserExist(string username,string password)
{
    bool status=false;
    try
    {
        string connstring = "data source=test_db;user id=system;password=password;";
        string statementcmd = "SELECT * FROM register_user Where [UserName]=@username";

        using(OracleConnection conn = new OracleConnection(connstring))
       {

        using(OracleCommand cmd = new OracleCommand())
       {
        cmd.Connection = conn;
        cmd.Parameters.Add("@username", username);//add parameters before assigning it to CommandText
        cmd.CommandText = statementcmd;

        if (conn.State != ConnectionState.Open)
        {
            conn.Open();
           OracleDataReader reader = cmd.ExecuteReader();


            if (!reader.Read())
            { MessageBox.Show("User Name Not Found"); }

            if (!password.Equals(reader["password"].ToString()))
            {
                 status=true;
                 MessageBox.Show("Incorrect Password");

            }


        }
      }

     }

   }
    catch (Exception ex)
    {
        MessageBox.Show(ex.ToString());
        status=false;
    }

return status;

}