CamlQuery that traverses multiple folders

Question

I've got a CamlQuery that traverses the /public/ folder and returns all files that match a certain criteria and it works gorgeously. Now I need it to do the same for a separate library but have the results returned in the same instance.

Code

 private static SPListItemCollection retrieveDocs(SPWeb web, String meta)
        {
            SPQuery query = new SPQuery();
            query.ViewXml = "<View Scope='Recursive'><Query><Where><Contains><FieldRef Name='Key_x0020_Topics'/><Value Type='SPFieldLookupMulti'>" + meta + "</Value></Contains></Where></Query></View>";
            SPList docLib = web.Lists["public"];
            SPListItemCollection files = docLib.GetItems(query);

            return files;
        }

Can I make my query traverse both the public and secure folder and then return the files as one Collection?

Answer 1

Based on your description, it's possible only If there is a relation between the first and second library. so you will be able to use List Join CAML Query to return the related data from two libraries in one SPListItemCollection

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But I don't think you have this case, so Alternatively, you can merge multiple SPListItemCollection to a Data Table as the following:

First, Define only one function that accepts three parameters with the SPList Object.

private static SPListItemCollection retrieveDocs(SPWeb web, String meta,SPList list)
        {
            SPQuery query = new SPQuery();
            query.ViewXml = "<View Scope='Recursive'><Query><Where><Contains><FieldRef Name='Key_x0020_Topics'/><Value Type='SPFieldLookupMulti'>" + meta + "</Value></Contains></Where></Query></View>";

            SPListItemCollection files = list.GetItems(query);

            return files;
        }

Second, call the function for each list and set each result in an independent site collection.

First Collection

SPList docLibpublic = web.Lists["public"];
SPListItemCollection  SPLCol1 = retrieveDocs(web, meta,docLibpublic);

Second Collection

SPList docLibsecure = web.Lists["secure"];
SPListItemCollection  SPLCol2 = retrieveDocs(web, meta,docLibsecure);

Now, Convert each SPListItemCollection to data table then merge the data tables

DataTable dtCol1 = SPLCol1.GetDataTable();
DataTable dtCol2 = SPLCol2.GetDataTable();
dtCol1.Merge(dtCol2);

Finally, you have data table (dtCol1) that contains the items from multiple SPListItemCollection

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Note: You can use SPSiteDataQuery to combine two lists. but unfortunately, the returned collection cannot be parsed to SPListItemCollection