We can use the TypedContentAtRoot() member function in order to get a list of all the root content nodes.
var rootNodes = Umbraco.TypedContentAtRoot();
The previous line of code works in a View but if we want to get the tree nodes in a class then we have to use the following code snippet.
var umbracoHelper = new Umbraco.Web.UmbracoHelper(Umbraco.Web.UmbracoContext.Current);
var rootNodes = umbracoHelper.TypedContentAtRoot();