public static void AddNextBlock( string blockName )
{
  Editor ed = Autodesk.AutoCAD.ApplicationServices.
      Application.DocumentManager.MdiActiveDocument.Editor;
  Database db = HostApplicationServices.WorkingDatabase; 
  TypedValue[] filter = new TypedValue[2]
 {
  new TypedValue((int)DxfCode.Start, "insert"),
  new TypedValue((int)DxfCode.BlockName, blockName + "*"),
 };

  SelectionFilter filterSS = new SelectionFilter(filter);
  PromptSelectionResult psr = ed.SelectAll(filterSS);

  if (psr.Status != PromptStatus.OK) return;

  using (Transaction tr =
      db.TransactionManager.StartTransaction())
  {
    SelectionSet brSS = null;
    brSS = psr.Value;
    ArrayList blockList = new ArrayList();

    for (int i = 0; i < brSS.Count; i++)
    {
      BlockReference oEnt;
      oEnt = (BlockReference)tr.GetObject(brSS[i].ObjectId, OpenMode.ForWrite);
      blockList.Add(oEnt.Name);
    }

    blockList.Sort();
    string lastBlock;
    lastBlock = blockList[blockList.Count - 1].ToString();
    //insert lastBlock + 1

  }
}