在使用LINQ：XML创建类的实例方面需要帮助

Question

I have a class which defines meteorological measurements with an API. I have created classes to store these measurements within an XML file. I have been able to write to the XML file, but am unable to read from it. I need to be able to read the data, then instantiate the Measurement Class via the constructor, and then add these instances to a List Here is the code for the Measurement constructor

  public Measurement(string longit, string latitud, string cty, string pcode, DateTime dte, decimal huy, decimal pre, string windDir, int windSp) : base(longit, latitud, cty,pcode)
    {

        dte = date;
        huy = humidity;
        pre = precipitation;           
        windDir = windDirection;
        windSp = windSpeed;
    }

and here is a sample of the XML file

<Measurement>
<longitude>-76.2858726</longitude>
<latitude>36.8507689</latitude>
<city>Thetford</city>
<postcode>IP24 1ED</postcode>
<date>01/04/2011</date>
<windDirection>SE</windDirection>
<windSpeed>8</windSpeed>
<humidity>2.6</humidity>
<precipitation>0.0</precipitation>
</Measurement>

And here is the method I have written to read the file, and make an instance of the class Measurement. I receive no errors, however have not managed to create the List<>

public List<Measurement> GetList()
    {
        List<Measurement> measurements = new List<Measurement>();
        XDocument doc = XDocument.Load(@"C:\Users\Sonya\Documents\Visual Studio 2010\Projects\WeatherAPI\WeatherAPI\measurement_store.xml");
        XElement root = doc.Root;

     var d = (from s in root.Descendants("Measurement")
                 select new Measurement
                 {
                     Longitude = s.Element("longitude").Value,
                     Latitude = s.Element("latitude").Value,
                     City = s.Element("city").Value,
                     Postcode = s.Element("postcode").Value,
                     Date = DateTime.Parse(s.Element("date").Value),
                     Humidity = decimal.Parse(s.Element("humidity").Value),
                     Precipitation = decimal.Parse(s.Element("precipitation").Value),
                     WindSpeed = Convert.ToInt32(s.Element("windSpeed").Value),
                     WindDirection = s.Element("windDirection").Value,

                 }).ToList();


     d.ForEach(measurements.Add);

        return measurements;
    }//end GetList()

I've also written another method, of a slightly different format..

public List<Measurement> createXMLListMeasurements()
    {
        //load the xml document
        XDocument doc = XDocument.Load(@"C:\Users\Sonya\Documents\Visual Studio 2010\Projects\WeatherAPI\WeatherAPI\measurement_store.xml");
        //XElement root = doc.Root;

        //instantiate a new list of measurements
        List<Measurement> measurements = new List<Measurement>();

            //get a list of measurement elements
           var d = from s in doc.Descendants("Measurement")
                    //where
                    select new 
                    { //assign Measurement variables to data from xml doc
                        Longitude = (string)s.Element("longitude").Value,
                        Latitude = (string)s.Element("latitude").Value,
                        City = (string)s.Element("city").Value,
                        Postcode = (string)s.Element("postcode").Value,
                        Date = DateTime.Parse(s.Element("date").Value),
                        Humidity = decimal.Parse(s.Element("humidity").Value),
                        Precipitation = decimal.Parse(s.Element("precipitation").Value),
                        WindSpeed = Convert.ToInt32(s.Element("windSpeed").Value),
                        WindDirection = (string)s.Element("windDirection").Value,
                    };

           foreach (var s in d)
           {   //for each element found, instantiate Measurements class, and add to the measurements list.
               Measurement m = new Measurement(s.Longitude, s.Latitude, s.City, s.Postcode, s.Date, s.Humidity, s.Precipitation, s.WindDirection, s.WindSpeed);
               measurements.Add(m);

           }


        return measurements;
    }

Apologies if these questions seem silly, am VERY new to LINQ and XML, so finding my way very slowly..any help much appreciated! A console application calls this method for testing and produces nothing but WeatherAPI.Measurement WeatherAPI.Measurement

Help? thanks!

Answer 1

Overall, your code looks fine. As Jon Skeet pointed out in his comment, you don't need to bother adding each element to a list -- you can simply return the result of the query after calling .ToList() .

Most likely, there's either something wrong with your xml file, or you're reading it incorrectly.

If your xml file is truly just:

<Measurement>
    <longitude>-76.2858726</longitude>
    <latitude>36.8507689</latitude>
    <city>Thetford</city>
</Measurement>

Then your code won't work, because the root of the xml file is Measurement . Therefore, calling doc.Root.Descendants("Measurement") will give you 0 results. Instead, your xml file should have a unique root element, for example:

<root>
    <Measurement>
        <longitude>-76.2858726</longitude>
        <latitude>36.8507689</latitude>
        <city>Thetford</city>
    </Measurement>
    <Measurement>
        <longitude>-71.2858726</longitude>
        <latitude>32.1507689</latitude>
        <city>Other City</city>
    </Measurement>
</root>

Furthermore, you don't need to bother obtaining the Root of the xml document. If all you want to do is find elements named Measurement , just say doc.Descendants("Measurement") .

Try this code with the above xml file:

void Main()
{
    var measurements = GetMeasurements(@"C:\path\to\file\measurements.xml");
}

public List<Measurement> GetMeasurements(string path)
{
    XDocument doc = XDocument.Load(path);

    return (from s in doc.Descendants("Measurement")
            select new Measurement
            {
                 Longitude = Convert.ToDecimal(s.Element("longitude").Value),
                 Latitude = Convert.ToDecimal(s.Element("latitude").Value),
                 City = s.Element("city").Value,
            }).ToList();
}

public class Measurement
{
    public decimal Longitude { get; set; }
    public decimal Latitude { get; set; }
    public string City { get; set; }
}

When I run it in LINQPad, I get the following result: