BrepToStl.cs

protected override void SolveInstance(IGH_DataAccess DA)
{
    List<Brep> InputBreps = new List<Brep>();
    string SavePath = null;
    bool On = false;

    DA.GetDataList(0, InputBreps);
    DA.GetData(1, ref SavePath);
    DA.GetData(2, ref On);

    if(On)
    {
        ExportStl(InputBreps, SavePath);
    }

    IEnumerable<string> files = GetStlFiles(SavePath);
    DA.SetDataList(0, files);
}

protected void ExportStl(List<Brep> breps, string path)
{
    List<Guid> guidList = new List<Guid>();
    Rhino.DocObjects.Tables.ObjectTable ot = Rhino.RhinoDoc.ActiveDoc.Objects;
    for (int i = 0; i < breps.Count; i++)
    {
        if (breps[i] == null || !breps[i].IsValid)
        {
            this.AddRuntimeMessage(GH_RuntimeMessageLevel.Error, "No object to bake, or brep not valid, aborted.");
            return;
        }
        System.Guid guid = ot.AddBrep(breps[i]);
        guidList.Add(guid);
    }
    int nSelected = ot.Select(guidList);
    if (nSelected != guidList.Count)
    {
        this.AddRuntimeMessage(GH_RuntimeMessageLevel.Error, "Not all objects could be selected, aborted.");
        return;
    }

    string cmd = "-_Export " + path + "file.stl" + " _Enter";
    Rhino.RhinoApp.RunScript(cmd, true);

    ot.Delete(guidList, true);

}

protected IEnumerable<string> GetStlFiles(string path)
{
    System.IO.DirectoryInfo di = new System.IO.DirectoryInfo(path);
    System.IO.FileInfo[] files = di.GetFiles("*.stl", System.IO.SearchOption.AllDirectories);

    IEnumerable<string> fullNames = files.Select(file => file.FullName);

    return fullNames;
}