Sample code - creating lookup columns as a feature

I promised in an earlier post to post the code I used to create lookup columns as a feature. The basic idea here to use CAML as far as possible to define the site column, but use code to inject the list ID which the column refers to (unknown at feature design-time since SharePoint decides the GUID for lists). Extracted below is the code for the feature receiver.

A couple of key points:-

• this assumes a file exists at the location D:\COB.Demo.ListBasedSiteColumns.Fields.xml with the following contents (note the 'List' attribute with an empty value):-

  <?xml version="1.0" encoding="utf-8"?>
  <elements xmlns="http://schemas.microsoft.com/sharepoint/">
  <!-- _filecategory="ContentType" _filetype="File" _filename="fields.xml" _uniqueid="c0188da6-e320-44c0-b50c-cb6eaecec512" --> 
  <Field
  Type="Lookup"
  Displayname="Locations"
  Required="FALSE"
  List=""
  ShowField="LinkTitleNoMenu"
  UnlimitedLengthInDocumentlibrary="FALSE"
  Group="COBDemo"
  ID="{ae8e6ab8-b151-4fa4-8694-3cd24ad3b1bc}"
  SourceId="{8c066b26-5a3e-4e1b-85ec-7e584cf178d7}"
  StaticName="Locations"
  Name="Locations">
  </elements>

  
  
• Since most of the column definition is in CAML, this allows you to specify the ID of the column. This is useful later on when we deploy content types or lists which need to reference this site column by ID.
  
• Site columns cannot be deleted when they are in use i.e. used in content types or lists. I prefer to try to delete the column on feature deactivation or reactivation, but omit error-handling so SharePoint throws an exception. This makes it clear to me why the feature cannot be deactivated. See comments in code for more information.
  
• Note that the assembly which contains the code below must be available (in the GAC, or site bin directory with appropriate CAS policy) when the feature is activated. You must also specify the FeatureAssembly and FeatureReceiver attributes in your feature.xml file to register the feature receiver.
  
• In terms of deploying the assembly, note that whilst you certainly can deploy assemblies to the GAC or site bin using the feature framework, you can't do it in this feature. Despite the 'FeatureActivated' name of the event handler, when your code executes the CAML won't yet have been processed. Hence your assembly won't yet have been copied and you'll get a FileNotFoundException when it tries to load the assembly. A good solution is to use a feature dependency. Here you can create a 2nd feature which deploys the assembly, and set the main feature to be dependent on this one. Additionally you can mark the assembly deployment feature to be hidden thus making the implementation a bit tidier. If it's useful to see the entire set of files I used leave me a comment and I'll put them up somewhere.
  

Apologies for the formatting, I will try to do something about this when I get time:-

public class FeatureReceiver : SPFeatureReceiver
{
    private readonly string f_csSITE_COLS_DEFINITION_PATH = @"D:\COB.Demo.ListBasedSiteColumns.Fields.xml";
 
    public override void FeatureActivated(SPFeatureReceiverProperties properties)
    {
       // feature is scoped at Site, so the parent is type SPSite rather than SPWeb..
       using (SPSite site = properties.Feature.Parent as SPSite)
       {
          SPWeb currentWeb = null;
          Guid gRootWebId = Guid.Empty;
          if (site != null)
          {
             currentWeb = site.RootWeb;
             gRootWebId = currentWeb.ID;
          }
          else
          { 
             currentWeb = properties.Feature.Parent as SPWeb;
             gRootWebId = currentWeb.Site.RootWeb.ID;
          }





          using (currentWeb)
          {
             // get reference to the list..
             SPList referencedList = currentWeb.Site.RootWeb.Lists["LocationsList"];
             string sFieldElement = null;
             XmlTextReader xReader = new XmlTextReader(f_csSITE_COLS_DEFINITION_PATH);
             while (xReader.Read())
             {
                if (xReader.LocalName == "Field")
                {
                   sFieldElement = xReader.ReadOuterXml();
                   break;
                }
             }


             string sFinalCaml = replaceListGuidString(sFieldElement, referencedList);
             createLookupColumn(currentWeb, sFinalCaml, "Locations");
             currentWeb.Update();
         }
     }
   }

   private string replaceListGuidString(string sFieldElement, SPList referencedList)
   {
      string sPopulatedGuid = string.Format("List=\"{0}\"", referencedList.ID);
      return sFieldElement.Replace("List=\"\"", sPopulatedGuid);
   }

   /// <summary>
   /// Attempt to delete the column. Note that this will fail if the column is inuse,
   /// i.e. it is used in a content type or list. I prefer to not catch the exception
   /// (though it may be useful to add extra logging), hence feature deactivation/re- /// activation will fail. This effectively means this feature cannot be deactivated whilst the column is in use.
   /// </summary>
   /// <param name="column">Column to delete.</param>

   private void attemptColumnDelete(SPFieldLookup column)
   {
      try
      {
         column.Delete();
      }
      catch (SPException e)
      {
         // consider logging full explanation..
         throw;
      }
   }

   private void createLookupColumn(SPWeb web, string sColumnDefinitionXml, string sColumnName)
   {
      // delete the column if it exists already and is not yet in use..
      SPFieldLookup lookupColumn = null;
      lookupColumn = web.Fields[sColumnName] as SPFieldLookup;
      if (lookupColumn != null)
      {
         attemptColumnDelete(lookupColumn);
      }

      // now create the column from the CAML definition..
      string sCreatedColName = web.Fields.AddFieldAsXml(sColumnDefinitionXml);
  
      // also set LookupWebId so column can be used in webs other than web which hosts list..
      lookupColumn = web.Fields[sCreatedColName] as SPFieldLookup;
      lookupColumn.LookupWebId = web.Site.RootWeb.ID;
      lookupColumn.Update();
   }

   public override void FeatureDeactivating(SPFeatureReceiverProperties properties)
   {
      // delete the column if it exists already and is not yet in use..

// feature is scoped at Site, so the parent is type SPSite rather than SPWeb..
using (SPSite site = properties.Feature.Parent as SPSite)
{
SPWeb currentWeb = null;
Guid gRootWebId = Guid.Empty;
if (site != null)
{
currentWeb = site.RootWeb;
gRootWebId = currentWeb.ID;
}
else
{
currentWeb = properties.Feature.Parent as SPWeb;
gRootWebId = currentWeb.Site.RootWeb.ID;
}

SPFieldLookup lookupColumn = null;
lookupColumn = currentWeb.Fields["LocationsList"] as SPFieldLookup;

if (lookupColumn != null)
{
attemptColumnDelete(lookupColumn);
}
}
}

   public override void FeatureInstalled(SPFeatureReceiverProperties properties)
   {
   }


   public override void FeatureUninstalling(SPFeatureReceiverProperties properties)
   {
   }
}

If it's useful to have the full set of files illustrating the 2 features(particularly since the above isn't very readable!), leave me a comment and I'll make them available somewhere..

[Update - these have now been uploaded to Codeplex, see http://sharepointnutsandbolts.blogspot.com/2007/04/feature-to-create-lookup-fields-on.html]