I am trying to add indexes to the Title, FileRef and UniqueId fields in my library to avoid a list query threshold error. My code is as follows:

private void CreateListIndexes()
        var list = ClientContext.Web.Lists.GetByTitle(AppConstants.Office365SourceDocuments);
        CreateListIndex(list, "Title");
        CreateListIndex(list, "FileRef");
        CreateListIndex(list, "UniqueId");

    private void CreateListIndex(List list, string fieldName)
        var field = list.Fields.GetByInternalNameOrTitle(fieldName);
        field.Indexed = true;

The index on Title is added OK, but the other two fields fail with the following error:

"Microsoft.SharePoint.Client.ServerException: Cannot complete this action."

How can I add an index to these 2 fields?


I found that I did not need to add an index to these fields... rather I updated to release 16 of the Microsoft.SharePoint.Client and Microsoft.SharePoint.ClientRuntime dlls (this is the version that supports SharePoint online) and then used the following methods instead of CAML queries:

  • For the FileRef queries: clientContext.Web.GetFolderByServerRelativeUrl or clientContext.Web.GetFileByServerRelativeUrl
  • For the UniqueId queries: clientContext.Web.GetFolderById or clientContext.Web.GetFileById

NB the update from version 15 to version 16 of the dlls was only necessary to use the GetFolderById and GetFileById methods.

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