SOAP API - Get root test case (folder)

Hi,

I would like to get a list of the 'root folders' trough the SOAP API. I can either use:

• TestCase_RetrieveByFolder(int testCaseFolderId). But how do I get the root folder? From there, I can browse the rest of the tree.
• TestCase_Retrieve(ArrayOfRemoteFilter? remoteFilters, int startingRow, int numberOfRows) with using a filter on the IndentLevel parameter. However, this seems not to be working as expected.

In the last code, I used (for testing):

var Filter = new RemoteFilter();
Filter.PropertyName = "IndentLevel";
Filter.StringValue = "AAA";

var RootTestCases = Client.TestCase_Retrieve(remoteFilters: new RemoteFilter[] { Filter }, startingRow: 1, numberOfRows: 50);

However, it still gives me all results ignoring the filter. What am I doing wrong?