AutoCAD 2021 .NET API:获取数据库中的所有块引用？

using Autodesk.AutoCAD.ApplicationServices;
using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.EditorInput;

Document doc = Application.DocumentManager.MdiActiveDocument;
Database db = doc.Database;

using (Transaction tr = db.TransactionManager.StartTransaction())
{
    // 在事务中执行操作
}

BlockTable bt = tr.GetObject(db.BlockTableId, OpenMode.ForRead) as BlockTable;
if (bt != null)
{
    foreach (ObjectId btrId in bt)
    {
        BlockTableRecord btr = tr.GetObject(btrId, OpenMode.ForRead) as BlockTableRecord;
        if (btr != null && !btr.IsLayout)
        {
            foreach (ObjectId entId in btr)
            {
                Entity ent = tr.GetObject(entId, OpenMode.ForRead) as Entity;
                if (ent != null && ent is BlockReference)
                {
                    BlockReference blkRef = ent as BlockReference;
                    // 处理块引用
                }
            }
        }
    }
}

tr.Commit();