Linked Files

ExternalFileUtils

As its name implies, this utility class provides information about external file references.  The ExternalFileUtils.GetAllExternalFileReferences() method returns a collection of ElementIds of all elements that are external file references in the document.   (Note that it will not return the ids of nested Revit links; it only returns top-level references.)   This utility class has two other methods, IsExternalFileReference() and GetExternalFileReference() which perform the same function as the similarly named methods of the Element class, but can be used when you have an ElementId rather than first obtaining the Element.

ModelPath

ModelPaths are paths to another file. They can refer to Revit models, or to any of Revit's external file references such as DWG links.  Paths can be relative or absolute, but they must include an extension indicating the file type.  Relative paths are generally relative to the currently opened document.  If the current document is workshared, paths will be treated as relative to the central model.  To create a ModelPath, use one of the derived classes FilePath or ServerPath. 

The class ModelPathUtils contains utility functions for converting ModelPaths to and from user-visible path strings, as well as to determine if a string is a valid server path.

Transmission Data

TransmissionData stores information on both the previous state and requested state of an external file reference.  This means that it stores the load state and path of the reference from the most recent time this TransmissionData's document was opened.  It also stores load state and path information for what Revit should do the next time the document is opened.

As such, TransmissionData can be used to perform operations on external file references without having to open the entire associated Revit document.  The methods ReadTransmissionData and WriteTransmissionData can be used to obtain information about external references, or to change that information.  For example, calling WriteTransmissionData with a TransmissionData object which has had all references set to LinkedFileStatus.Unloaded would cause no references to be loaded upon next opening the document.

TransmissionData cannot add or remove references to external files. If AddExternalFileReference is called using an ElementId which does not correspond to an element which is an external file reference, the information will be ignored on file load.

The following example reads the TransmissionData for a file at the given location and sets all Revit links to be unloaded the next time the document is opened.

void UnloadRevitLinks(ModelPath location)
/// This method will set all Revit links to be unloaded the next time the document at the given location is opened. 
/// The TransmissionData for a given document only contains top-level Revit links, not nested links.
/// However, nested links will be unloaded if their parent links are unloaded, so this function only needs to look at the document's immediate links. 
{
    // access transmission data in the given Revit file
    TransmissionData transData = TransmissionData.ReadTransmissionData(location);
    if (transData != null)
    {
        // collect all (immediate) external references in the model
        ICollection<ElementId> externalReferences = transData.GetAllExternalFileReferenceIds();
        // find every reference that is a link
        foreach (ElementId refId in externalReferences)
        {
            ExternalFileReference extRef = transData.GetLastSavedReferenceData(refId);
            if (extRef.ExternalFileReferenceType == ExternalFileReferenceType.RevitLink)
            {
                // we do not want to change neither the path nor the path-type
                // we only want the links to be unloaded (shouldLoad = false)
                transData.SetDesiredReferenceData(refId, extRef.GetPath(), extRef.PathType, false);
            }
        }

        // make sure the IsTransmitted property is set 
        transData.IsTransmitted = true;
        // modified transmission data must be saved back to the model
        TransmissionData.WriteTransmissionData(location, transData);
    }
    else
    {
        Autodesk.Revit.UI.TaskDialog.Show("Unload Links", "The document does not have any transmission data");
    }
}