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Building a SPQuery ViewFields string

If you’re querying SharePoint content using a CAML query from code it’s a good habit to always populate the SPQuery instance’s ViewFields property. Otherwise the returned SPListItem instances might not contain any data for certain fields and throw an exception when you try to access those fields.

Specifying ViewFields involves creating a string of CAML FieldRef elements. For instance if we want our query to return items that contain data for the Title, Created and ID field we use this:

   1: <FieldRef Name='Title'/><FieldRef Name='Created'/><FieldRef Name='ID'/>

As you can see there’s some overhead of boilerplate markup involved. I’ve written a small piece of code that I always use to make my life a little easier. Today I happened to post this code in a reply I wrote on the MSDN forums and also decided to submit it as Community Content to the official SPQuery docs on MSDN. Then I thought I might as well share it with you here. So here it is:

   1: public static string BuildViewFieldsXml(params string[] fieldNames)
   2: {
   3:     const string TEMPLATE = @"<FieldRef Name='{0:S}'/>";
   4:     StringBuilder sb = new StringBuilder();
   5:     foreach (string fieldName in fieldNames)
   6:     {
   7:         sb.AppendFormat(TEMPLATE, fieldName);
   8:     }
   9:     return sb.ToString();
  10: }
  12: // Use it like this:
  13: SPQuery query = new SPQuery();
  14: query.ViewFields = BuildViewFieldsXml("Title", "Created", "ID");
  16: // Note that you can specify a variable amount of string parameters, i.e.
  17: query.ViewFields = BuildViewFieldsXml("Title", "Created", "ID", "Author", "Gender");

Yeah, you’re right. This piece of code isn’t exactly rocket science. But you might appreciate it anyway 🙂

Do not reuse SPQuery!

Last week I was refactoring some of my SharePoint code. I stumbled on a loop that created a new SPQuery instance for each iteration. The code was something like this:

   1: SPList list = GetCommentsList();
   2: const string VIEWFIELDS = "";
   4: foreach (string param in params)
   5: {
   6:     // Create fresh new SPQuery instance...
   7:     SPQuery query = new SPQuery();
   8:     query.ViewFields = VIEWFIELDS;
   9:     query.Query = BuildQuery(param);
  11:     // And use it...
  12:     SPListItemCollection items = list.GetItems(query);
  13:     ProcessItems(items);
  14: }

As the value of the ViewFields property remained the same for each iteration I decided to create just one SPQuery instance and reuse it, like this:

   1: SPList list = GetCommentsList();
   2: const string VIEWFIELDS = "";
   4: // Create just one SPQuery instance...
   5: SPQuery query = new SPQuery();
   6: query.ViewFields = VIEWFIELDS;
   8: foreach (string param in params)
   9: {
  10:     // And reuse it...
  11:     query.Query = BuildQuery(param);
  12:     SPListItemCollection items = list.GetItems(query);
  13:     ProcessItems(items);
  14: }

To my surprise this code didn’t work! The first time the SPQuery instance was used it worked just fine. However, during the next iterations of the foreach loop it didn’t seem to get updated.

So, what have we learned today? Never reuse SPQuery instances!

Well… That’s not entirely true. You can reuse SPQuery instances for a very valid reason. If you use the RowLimit property you can limit the number of items returned in the query, which is useful for paging as seen in this sample code (taken from MSDN):

   1: using (SPWeb oWebsiteRoot = SPContext.Current.Site.RootWeb)
   2: {
   3:     SPList oList = oWebsiteRoot.Lists["Announcements"];
   4:     SPQuery oQuery = new SPQuery();
   5:     oQuery.RowLimit = 10;
   6:     int intIndex = 1;
   8:     do
   9:     {
  10:         Console.WriteLine("Page: " + intIndex);
  11:         SPListItemCollection collListItems = oList.GetItems(oQuery);
  13:         foreach(SPListItem oListItem in collListItems)
  14:         {
  15:             Console.WriteLine(oListItem["Title"]);
  16:         }
  17:         oQuery.ListItemCollectionPosition = 
  18:           collListItems.ListItemCollectionPosition;
  19:         intIndex++;
  20:     } while(oQuery.ListItemCollectionPosition != null);
  21: }

So, only reuse SPQuery instances if you use paging. If you change the actual CAML query you should create a new SPQuery instance for it.