Validation on FromDate and ToDate

public boolean validateWrite()

{

    boolean ret;

    SS_CategoryTradeAgreement      categoryTradeAgreement;

    ;

    ret = super();

    if(this.ToDate < this.FromDate)

        ret = checkFailed("Todate should be greater than or equal to Fromdate");

    while select categoryTradeAgreement where  categoryTradeAgreement.Category == this.Category

                                            && categoryTradeAgreement.Vendor == this.Vendor

                                            && categoryTradeAgreement.RecId != this.RecId

                                            && (( categoryTradeAgreement.FromDate <= this.FromDate && categoryTradeAgreement.ToDate >= this.FromDate)

                                                       ||(categoryTradeAgreement.FromDate <= this.ToDate   && categoryTradeAgreement.ToDate >= this.ToDate))

    {

    ret = checkfailed(strfmt("Duplicate Entries are not allowed - Same record is there for Vendor - %1 with Category %2 of period %3 - %4" ,

                         categoryTradeAgreement.Vendor,categoryTradeAgreement.Category,categoryTradeAgreement.FromDate,categoryTradeAgreement.ToDate));

    break;

    }

    return ret;

}