list.add reporting list as null

Hello,my AddFolder function reports that folders.Add(folder) is null it is the folders that is null and I don't understand why.in some other code I create an instance of FolderManager and in the constructor the folders.add(new Folder("test"... works finelater in the other code I call the AddFolder fuction on the created instance and it reports that folders is null.Why? it is the same instance being called; folders is in the same class and access is not restricted; Many thanks class FolderManager { List<Folder> folders = new List<Folder>(); public FolderManager() { folders.Add(new Folder("test", "test")); } public Folder AddFolder(string strFolderFullPath, string strFolderName) { Folder folder = new Folder(strFolderFullPath, strFolderName); if (FindFolder(s ...