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我有一个客户对象。Customer 对象可能包含多个 Address 对象。

例如，我想在 Customer 对象上设置一个方法GetCity。

现在，每个 Address 对象都有一个 addressId，所以我试图通过 linq 查询该属性。

下面是我的代码（我已经为这个问题剪掉了它）

客户对象...

public class Customer
{
    private string _customerNo;
    private List<CustomerAddress> _addresses = new List<CustomerAddress>();
    //other members immitted from this sample


    //constructor code, immitted from this sample

    public List<CustomerAddress> Addresses
    {
        get { return _addresses; }
    }
    //other properties immitted from this sample    

    public string GetCity(string id)
    {
        var city = (from a in _addresses
                        where a.AddressID == id
                        select a.City).Single();

        return city;
    }   
}

the Address Object...
public class CustomerAddress
{
    private string _addressid;
    private string _city;
    //other members immitted from this sample

    public CustomerAddress(string strAddressID, string strAddressLister, string strAddress1, string strAddress2, string strAddress3, string strCity, string strState, string strPostCode, string strCountry,
        bool booDefaultAddress, string strDeliveryAddress, string strInvoiceAddress, string strPayAddress, string strVisitAddress, string strDeliveryTerms, string strShipVia, string strRouteId)
    {
        _addressid = strAddressID;
        _addresslister = strAddressLister;
        _address1 = strAddress1;
        _address2 = strAddress2;
        _address3 = strAddress3;
        _city = strCity;
        _state = strState;
        _postcode = strPostCode;
        _country = strCountry;
        _defaultaddress = booDefaultAddress;
        _deliveryaddress = strDeliveryAddress;
        _invoiceaddress = strInvoiceAddress;
        _payaddress = strPayAddress;
        _visitaddress = strVisitAddress;
        _deliveryterms = strDeliveryTerms;
        _shipvia = strShipVia;
        _routeid = strRouteId;
    }

    ////////////////////////////
    //CustomerAddress Properties
    ////////////////////////////
    public string AddressID
    {
        get { return _addressid; }
    }    
    public string City
    {
        get { return _city; }
    }
    //other properties immitted from this sample
}

当我GetCity(id)在 Customer 对象上运行该方法时，我得到了错误。

System.InvalidOperationException：序列不包含任何元素

我还在 linq 查询中尝试了以下代码，但得到了相同的结果。

public string GetCity(string id)
{
    var city = (from a in this.Addresses
                    where a.AddressID == id
                    select a.City).Single();

    return city;
}

和...

public string GetCity(string id)
    {
        var city = (from a in this._addresses
                        where a.AddressID == id
                        select a.City).Single();

        return city;
    }

我知道 Customer 对象中确实包含 Address 对象，但我不确定如何让GetCityMethod 工作。

任何帮助，将不胜感激。如果我错过了任何重要信息，请告诉我。

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3 回答 3

4

这里的问题要么是 _addresses 集合根本不包含任何元素，要么是 _addresses 集合不包含任何AddressID等于 id 的元素（可能更有可能）。

如果Single()少于 1 (0) 项或多于一项符合您的给定条件，它会引发异常。如果您只是想null在没有匹配项时返回，请使用SingleOrDefault().

于 2013-03-06T15:05:36.617 回答
3 
where a.AddressID == id

正在过滤掉您的所有地址，因此当您调用Single空枚举时会收到该错误消息。

您可以切换Single到SingleOrDefault，但我认为主要问题是地址比较

于 2013-03-06T15:06:09.683 回答
1

似乎 linq 查询没有返回您在列表中填充的任何项目？你确定请求的 id 存在吗？

Single 需要一个 IEnumerable，其中只有 1 个项目

于 2013-03-06T15:05:16.170 回答