ajax呼叫完成后，readonly prop丢失了

Question

我使用下面的代码但是在我的表单提交中出现验证错误后，readonly属性丢失了 我正在为ajax中的文本框设置readonly prop，但在完成之后就丢失了。

if (txnDate != undefined && txnDate != 'undefined' && schCode != '0' && schCode != undefined)
{
    $("#updateProgress").dialog("open");
    $.ajax({
        url: '@Url.Content("~/AddExtPortfolio/MFSelectOnChange")' + '?txnType=' +  
        selectedOption + &brkCode='brkCode + '&amcCode=' + amcCode + '&schCode=' + schCode +
        '&txnDate=' +   txnDate + '&ID=' + ID,
        Type: 'POST',
        datatype: 'json',
        contentType: 'application/json; charset=utf-8',
        success: function (data) {
            if (
                selectedOption == "Buy" ||
                selectedOption == "SIP" ||
                selectedOption == "Redeem" ||
                selectedOption == "Switch" ||
                selectedOption == "STP" ||
                selectedOption == "SWP" ||
                selectedOption == "DR")
            {
                if (selectedOption == "Buy") {
                    NAV = $("#MutualFundList_" + ID + "__TxnNAV");
                    NAV_Value = data.MutualFundList[ID].TxnNAV;
                }
                else if (selectedOption == "SIP") {
                    NAV = $("#MutualFundList_" + ID + "__TxnNAVSIP");
                    NAV_Value = data.MutualFundList[ID].TxnNAVSIP;
                }
                else if (selectedOption == "Redeem" || selectedOption == "Switch" || selectedOption == "STP" || selectedOption == "SWP") {
                    ExtUnits = $("#add_external_form_" + selectedOption + "_" + ID + " #MutualFundList_" + ID + "__ExtUnits" + selectedOption);
                    ExtAmt = $("#add_external_form_" + selectedOption + "_" + ID + " #MutualFundList_" + ID + "__ExtAmount" + selectedOption);
                }
                if (selectedOption == "Redeem") {
                    ExtUnits.val(data.MutualFundList[ID].ExtUnitsRedeem);
                    //ExtAmt.val(data.MutualFundList[ID].ExtAmountRedeem);
                    NAV = $("#add_external_form_" + selectedOption + "_" + ID + " #MutualFundList_" + ID + "__TxnNAVRedeem");
                    NAV_Value = data.MutualFundList[ID].TxnNAVRedeem;
                    var new_redeem_amount = data.MutualFundList[ID].ExtUnitsRedeem * NAV_Value;
                    if (NAV_Value != 0) {
                        ExtAmt.val(new_redeem_amount);
                    }
                    else {
                        ExtAmt.val(new_redeem_amount);
                        $(NAV).change(function () {
                            result = $(ExtUnits).val() * $(NAV).val();
                            result = Math.round(result * 100) / 100;
                            $(ExtAmt).val(result);
                        });
                    }
                }
                NAV.val(NAV_Value);
                if (NAV_Value == null || NAV_Value == 0) {
                    $(NAV).prop("readonly", false);
                }
                else {
                    $(NAV).prop("readonly", true);
                    if (selectedOption != 'Buy' && selectedOption != 'SIP' && selectedOption != 'DP') {
                        GetAmtUnits(ID, "");
                    }
                }
            }
        }
    });/*end of ajax call*/
}