Login code in c#

using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Data.SqlClient;
using System.Net.Mail;

public int select(string str, string username, string password)
{
int f = 0;
cmd.CommandText = str;
cmd.Connection = con;
cmd.CommandType = CommandType.StoredProcedure;
dr = cmd.ExecuteReader();
while (dr.Read())
{

if (username.Trim().ToString() == dr.GetString(0).Trim().ToString() && password.Trim().ToString() == dr.GetString(1).Trim().ToString())
{
f = 1;
return f;
}

}
return f;
}

Code are good but i have one Query...

When username and password is already a string type variable than why we need to convert them into string.
in if statement.


Thanks
AARIF SHAIKH
aarifshaikh4u@gmail.com