Question

In this question Jon Skeet offered a very interesting solution to making a LINQ-to-XML statement dynamic, but my knowledge of lambdas and delegates is not yet advanced enough to implement it:

I’ve got it this far, but of course I get the error “smartForm does not exist in the current context”:

private void LoadWithId(int id)
{
    XDocument xmlDoc = null;
    try
    {
        xmlDoc = XDocument.Load(FullXmlDataStorePathAndFileName);
    }
    catch (Exception ex)
    {
        throw new Exception(String.Format("Cannot load XML file: {0}", ex.Message));
    }

    Func<XElement, bool> whereClause = (int)smartForm.Element("id") == id";

    var smartForms = xmlDoc.Descendants("smartForm")
        .Where(whereClause)
        .Select(smartForm => new SmartForm
                     {
                         Id = (int)smartForm.Element("id"),
                         WhenCreated = (DateTime)smartForm.Element("whenCreated"),
                         ItemOwner = smartForm.Element("itemOwner").Value,
                         PublishStatus = smartForm.Element("publishStatus").Value,
                         CorrectionOfId = (int)smartForm.Element("correctionOfId"),
                         IdCode = smartForm.Element("idCode").Value,
                         Title = smartForm.Element("title").Value,
                         Description = smartForm.Element("description").Value,
                         LabelWidth = (int)smartForm.Element("labelWidth")
                     });

    foreach (SmartForm smartForm in smartForms)
    {
        _collection.Add(smartForm);
    }
}

Ideally I want to be able to just say:

var smartForms = GetSmartForms(smartForm=> (int) smartForm.Element("DisplayOrder").Value > 50);

I’ve got it this far, but I’m just not grokking the lambda magic, how do I do this?

public List<SmartForm> GetSmartForms(XDocument xmlDoc, XElement whereClause)
{
    var smartForms = xmlDoc.Descendants("smartForm")
        .Where(whereClause)
        .Select(smartForm => new SmartForm
                     {
                         Id = (int)smartForm.Element("id"),
                         WhenCreated = (DateTime)smartForm.Element("whenCreated"),
                         ItemOwner = smartForm.Element("itemOwner").Value,
                         PublishStatus = smartForm.Element("publishStatus").Value,
                         CorrectionOfId = (int)smartForm.Element("correctionOfId"),
                         IdCode = smartForm.Element("idCode").Value,
                         Title = smartForm.Element("title").Value,
                         Description = smartForm.Element("description").Value,
                         LabelWidth = (int)smartForm.Element("labelWidth")
                     });
}

Answer 1

I expect you mean:

public List<SmartForm> GetSmartForms(
       XDocument xmlDoc, Func<XElement,bool> whereClause)

and:

Func<XElement, bool> whereClause = smartForm => (int)smartForm.Element("id") == id;

To use as a method, I would make it IEnumerable<T>:

public static IEnumerable<SmartForm> GetSmartForms(
       XDocument xmlDoc, Func<XElement,bool> predicate)
    {
        return xmlDoc.Descendants("smartForm")
            .Where(predicate)
            .Select(smartForm => new SmartForm
            {
                ... snip
            });
    }

and call as:

            foreach (SmartForm smartForm in GetSmartForms(xmlDoc,
                sf => (int)sf.Element("id") == id))
            {
                _collection.Add(smartForm);
            }