这是获取GetBlockImage接口的替代方案，可以测试一下能不能满足需求

        [CommandMethod("GetBlkImg")]
        public void GetBlkImg4()
        {
            var doc = Application.DocumentManager.MdiActiveDocument;
            var db = doc.Database;
            var ed = doc.Editor;
            var dwgPath = (string)Application.GetSystemVariable("DWGPREFIX");

            var promptRes = ed.GetInteger("\nOutput image height:");
            var nImgHeight = promptRes.Value;
            promptRes = ed.GetInteger("\nOutput image width:");
            var nImgWidth = promptRes.Value;
            using (var tr = doc.TransactionManager.StartTransaction())
            {
                var blkTbl = tr.GetObject(db.BlockTableId, OpenMode.ForRead) as BlockTable;
                foreach (var blkId in blkTbl)
                {
                    var blkTblRec = tr.GetObject(blkId, OpenMode.ForRead) as BlockTableRecord;
                    if (blkTblRec == null || blkTblRec.IsLayout || blkTblRec.IsAnonymous) continue;

                    var blkName = blkTblRec.Name;

                    var backgroundColor = new ZwSoft.ZwCAD.Colors.Color();
                    backgroundColor.SetRgb(0, 0, 0);

                    Bitmap bitmap = GetBlockImage(blkId, nImgWidth, nImgHeight, backgroundColor);
                    if (bitmap != null)
                    {
                        var savePath = dwgPath + blkName + ".png";
                        if (File.Exists(savePath))
                        {
                            File.Delete(savePath);
                        }
                        bitmap.Save(savePath);
                        bitmap.Dispose();
                        ed.WriteMessage("Save block \"" + blkName + "\" to path : " + savePath);
                    }
                    else
                    {
                        ed.WriteMessage("block icon didn't generate.\n");
                    }
                }
            }
        }

        private bool SetView(View m_View, ObjectId id, double nImgWidth, double nImgHeight)
        {
            var doc = Application.DocumentManager.MdiActiveDocument;

            using (var tr = doc.TransactionManager.StartTransaction())
            {
                try
                {
                    var blkTblRec = tr.GetObject(id, OpenMode.ForRead) as BlockTableRecord;
                    if (blkTblRec.IsLayout || blkTblRec.IsAnonymous)
                    {
                        doc.Editor.WriteMessage("id not a block!");
                        tr.Commit();
                        return false;
                    }

                    Extents3d ext = new Extents3d();
                    var ents = blkTblRec.GetEnumerator();
                    while (ents.MoveNext())
                    {
                        var ent = tr.GetObject(ents.Current, OpenMode.ForRead) as Entity;
                        ext.AddExtents(ent.GeometricExtents);
                    }
                    ents.Dispose();

                    var minPt = ext.MinPoint;
                    var maxPt = ext.MaxPoint;
                    var center = minPt + 0.5 * (maxPt - minPt);

                    m_View.EraseAll();
                    m_View.SetView(center + Vector3d.ZAxis, center, Vector3d.YAxis, nImgWidth, nImgHeight);
                    m_View.ZoomExtents(minPt, maxPt);
                    m_View.Zoom(0.9);

                    tr.Commit();
                }
                catch (ZwSoft.ZwCAD.Runtime.Exception ex)
                {
                    doc.Editor.WriteMessage(ex.Message);
                    return false;
                }
            }

            return true;
        }

        private void Display(View m_View, Model m_Model, ObjectId id)
        {
            var doc = Application.DocumentManager.MdiActiveDocument;

            using (var tr = doc.TransactionManager.StartTransaction())
            {
                var blkTblRec = tr.GetObject(id, OpenMode.ForRead) as BlockTableRecord;
                m_View.Add(blkTblRec, m_Model);
                m_View.Invalidate();
                m_View.Update();
                tr.Commit();
            }
        }

        public Bitmap GetBlockImage(ObjectId id, double nImgWidth, double nImgHeight, ZwSoft.ZwCAD.Colors.Color backgroundColor)
        {
            Bitmap bmp = null;
            var doc = Application.DocumentManager.MdiActiveDocument;
            var m_Manager = doc.GraphicsManager;
            var gk = new GraphicsKernel();

            var m_Device = m_Manager.CreateZWCADOffScreenDevice(gk);
            var m_Model = m_Manager.CreateZWCADModel(gk);

            var m_View = new View();
            if (m_View == null)
            {
                doc.Editor.WriteMessage("\nView create error!\n");
                goto _FIN;
            }

            m_Device.BackgroundColor = backgroundColor.ColorValue;
            m_Device.OnSize(new Size(Convert.ToInt32(nImgWidth), Convert.ToInt32(nImgHeight)));
            m_Device.Add(m_View);

            if (!SetView(m_View, id, nImgWidth, nImgHeight))
            {
                goto _FIN;
            }

            Display(m_View, m_Model, id);

            bmp = m_Device.GetSnapshot(m_View.Viewport);
            if (bmp == null)
            {
                doc.Editor.WriteMessage("\nGetSnapshot error!\n");
            }

            _FIN:
            if (m_Device != null) m_Device.Erase(m_View);
            if (m_View != null)
            {
                m_View.EraseAll();
                m_View.Dispose();
                m_View = null;
            }
            if (m_Manager != null)
            {
                if (m_Model != null)
                {
                    m_Model.Dispose();
                    m_Model = null;
                }
                if (m_Device != null)
                {
                    m_Device.Dispose();
                    m_Device = null;
                }
            }
            return bmp;
        }