You have two options.
In case you need the string in a specific format, you can easily build it yourself by traversing the FilterDescriptors collection of the control. The logical operator property of this collection is in fact the root node of a tree of filter criteria. The collection consists of filter descriptors -- they can be either simple ones (member-operator-value) or composite ones. The composite ones are filter descriptors that contain other filter descriptors and have a logical operator, i.e. they provide the hierarchy. So you can traverse this tree in the order that you want and build the string.
Out-of-the-box, if you call the ToString method of the FilterDescriptors collection it will return a similar string, but chances are it will not be 100% the format that you need to build. Anyway, try this and see what will come out.
I hope this helps. Let me know if you have problems.
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