LINQ to XML-Let语句中的where子句

Question

More troubles with LINQ. Just hoping that someone could please help.

Here's the structure I'm working with;

<root>
   <Level1>
     <Level2>
       <Level3>
         <Car>
           <Price PaymentType="Cash">200</Price>
           <Price PaymentType="Credit">500</Price>
         </Car>
       </Level3>
     </Level2>
   </Level1>
</root>

I'm trying to create two Let variables, that do this;

Let priceCash = ??? // should return 200
Let priceCredit = ??? // should return 500

To complicate things more, not all Cars have a Credit Price. For these I'd like to return -1.

Here's the butchered code I came up with;

// get all Price elements
let PriceList = CarList.Elements("Price") 

// try to get the first Price element where PaymentType==Cash
let priceCash = PriceList.Where(c => PriceList.Attributes("PaymentType").First().Value == "Cash")

// try to get the first Price element where PaymentType==Credit
let priceCredit = PriceList.Where(c => PriceList.Attributes("PaymentType").First().Value == "Credit")

Is there a better way to do that? It seems to work, but then I run into trouble here;

select new MyObj
{
    Price1 = priceCash == null ? -1 : priceCash.ElementAt(0).Value,
    Price2 = priceCredit == null ? -1 : priceCredit.ElementAt(0).Value,
}).ToList<MyObj>();

ElementAt(0) is causing an exception when the element isn't found.

cheers

Answer 1

The following should be more robust:

// try to get the first Price element where PaymentType==Cash
let priceCash = PriceList.FirstOrDefault(c => ((string)c.Attribute("PaymentType")) == "Cash")

// try to get the first Price element where PaymentType==Credit
let priceCredit = PriceList.FirstOrDefault(c => ((string)c.Attribute("PaymentType")) == "Credit")

select new MyObj
{
    Price1 = priceCash == null ? -1 : (int)priceCash,
    Price2 = priceCredit == null ? -1 : (int)priceCredit
}).ToList<MyObj>();

Answer 2

First of all, use Attribute method, instead of Attributes().First chain and (string)XAttribute conversion instead of XAttribute.Value property:

// get all Price elements
let PriceList = CarList.Elements("Price") 

// try to get the first Price element where PaymentType==Cash
let priceCash = PriceList.Where(c => (string)c.Attribute("PaymentType") == "Cash")

// try to get the first Price element where PaymentType==Credit
let priceCredit = PriceList.Where(c => (string)c.Attribute("PaymentType") == "Credit")

And what even more important, use DefaultIfEmpty to get -1 when elements are not present:

select new MyObj
{
    Price1 = priceCash.Select(x => (int)x).DefaultIfEmpty(-1).First(),
    Price2 = priceCredit.Select(x => (int)x).DefaultIfEmpty(-1).First()
}).ToList<MyObj>();