LINQ Tip: Enumerable.OfType

In the past I’ve mentioned LINQ’s Cast<T>() as an efficient way to convert a SharePoint collection into an IEnumerable<T> that has access to LINQ’s various extension methods. Fundamentally, Cast<T>() is implemented like this:

public IEnumerable<T> Cast<T>(this IEnumerable source)
{
  foreach(object o in source)
    yield return (T) o;
}

Using an explicit cast performs well, but will result in an InvalidCastException if the cast fails. A less efficient yet useful variation on this idea is OfType<T>():

public IEnumerable<T> OfType<T>(this IEnumerable source)
{
  foreach(object o in source)
    if(o is T)
      yield return (T) o;
}

The returned enumeration will only include elements that can safely be cast to the specified type. Why would this be useful?

Example 1: SPWindowsServiceInstance

SharePoint, especially with MOSS, has several different services that can run on the various servers in a farm. We know where our web services are running, but where are the various windows services running?

var winsvc = from svr in SPFarm.Local.Servers
             from inst in svr.ServiceInstances.OfType<SPWindowsServiceInstance>()
             select new
             {
                 Server = svr.Name,
                 ID = inst.Id,
                 ServiceType = inst.Service.GetType().Name
             };

Example 2: SPDocumentLibrary

SharePoint provides a few special subclasses of SPList for specific kinds of lists. These include SPDocumentLibrary, SPPictureLibrary and the essentially obsolete SPIssueList. We can use OfType() to retrieve only lists of a certain type, like this LINQified MSDN sample that enumerates all files in a site collection’s libraries, excluding catalogs and form libraries:

SPSite site = SPContext.Current.Site;
var docs = from web in site.AllWebs.AsSafeEnumerable()
           from lib in web.Lists.OfType<SPDocumentLibrary>()
           from SPListItem doc in lib.Items
           where !lib.IsCatalog && lib.BaseTemplate != SPListTemplateType.XMLForm
           select new { WebTitle = web.Title, ListTitle = lib.Title,
                        ItemTitle = doc.Fields.ContainsField("Title") ? doc.Title : "" };

foreach (var doc in docs)
  Label1.Text += SPEncode.HtmlEncode(doc.WebTitle) + " -- " +
                 SPEncode.HtmlEncode(doc.ListTitle) + " -- " +
                 SPEncode.HtmlEncode(doc.ItemTitle) + "<BR>";

Example 3: SPFieldUser

Finally, let’s pull a list of all user fields attached to lists in the root web. This could also be used to easily find instances of a custom field type.

var userFields = from SPList list in site.RootWeb.Lists
                 from fld in list.Fields.OfType<SPFieldUser>()
                 select new
                 {
                     ListTitle = list.Title,
                     FieldTitle = fld.Title,
                     InternalName = fld.InternalName,
                     PresenceEnabled = fld.Presence
                 };

Contrived examples, perhaps, but potentially useful nonetheless.

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