Gh_structure of type string

Hi, How can I pass datatree of type string in the component visual studio?

I use this one but compiler gives error that the out variable txt has to be GH_Structure. When I use GH_Structure I get error that string is not part of IGH_Goo. How to pass string tree correctly?

    protected override void SolveInstance(IGH_DataAccess DA) {

        var txt = new DataTree<string>();
        DA.GetDataTree(0,out txt);


If it must be a datatree, then you must use GH_Structure<GH_String>.

Thank you