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ControlledApplication.GetLibraryPaths

Method
Description:
Returns path information identifying where Revit searches for content.
Remarks:
The map that is returned contains a key that is purpose of the path, such as Material Libraries and the value in the map is the fully qualified path that is used for that search path.
Syntax
Examples
string strLibPaths = "";
foreach (String path in application.GetLibraryPaths().Values)
{
    strLibPaths += path + "\n";
}