|EwsDownloadFolderByFullName Method (FolderId, String)|
public EwsFolder DownloadFolderByFullName( FolderId containingFolderId, string folderFullName )
Public Function DownloadFolderByFullName ( containingFolderId As FolderId, folderFullName As String ) As EwsFolder
To delimit levels of hierarchy in the folder name, use the same character which is set by FolderLevelDelimiter property. By default, it's '/' (forward slash).
containingFolderId is just for optimization so you can leave it a null reference. However, if you already know that the folder you're looking for is contained in some upper level folder, you can specify it there. This will optimize performance as only a subset of all folders in the account will be searched. If you don't know the containing folder, you can use another optimization provided by DownloadFolderByFullName(string, int) overload.
containingFolderId does not need to be the immediate parent of the folder you're looking for, it can be any level up (e.g. you can specify WellKnownFolderName.Inbox while searching for "Inbox/Nested folder/Deeply nested folder"). Still, more close containingFolderId's level is to folderFullName's level - faster the search will be.