/// <summary>
/// z.B.: //com.android.externalstorage.documents/tree/19F4-0903:abc/def/ghi.txt aus /abc/def/ghi.txt (API level 21 / Lollipop / 5.0)
/// </summary>
/// <param name="storagename">z.B. "primary" oder "19F4-0903"</param>
/// <param name="volpath">abs. Pfad im Volume</param>
/// <returns></returns>
global::Android.Net.Uri GetTreeDocumentUri(string storagename, string volpath = "")
{
if (volpath.Length > 0 &&
volpath[0] == '/')
{
volpath = volpath.Substring(1);
}
// Build URI representing access to descendant documents of the given Document#COLUMN_DOCUMENT_ID. (API level 21 / Lollipop / 5.0)
return(DocumentsContract.BuildTreeDocumentUri(AUTHORITY_EXTERNALSTORAGE_DOCUMENTS, storagename + ":" + volpath));
}