斜体部分报错：未将对象引用设置到对象的实例

OpenFileDialog fileDlg = new OpenFileDialog();

            fileDlg.Filter = "DWG File | *.dwg";

            fileDlg.Multiselect = false;

            if (fileDlg.ShowDialog() == System.Windows.Forms.DialogResult.OK)

            {

                try

                {

                    string fullname = fileDlg.FileName;

                    int index = fileDlg.FileName.LastIndexOf("\\");

                    string path = fullname.Substring(0,index);

                    string filename = fullname.Substring(index+1);

                    IWorkspaceFactory pWorkSpaceFactory = new ShapefileWorkspaceFactory();

                    IWorkspace pWorkSpace = pWorkSpaceFactory.OpenFromFile(System.IO.Path.GetDirectoryName(fileDlg.FileName), 0);

                    //IFeatureWorkspace pFeatureWorkspace = pWorkSpace as IFeatureWorkspace;

                    //IFeatureClass layPolygon = pFeatureWorkspace.OpenFeatureClass(System.IO.Path.GetFileNameWithoutExtension(fileDlg.FileName));//获取FeatureClass

                    //_rowCount = (layPolygon as ITable).RowCount(null);//获取FeatureClass数量

                    ICadDrawingWorkspace cadDrawingWorkspace = pWorkSpace as ICadDrawingWorkspace;

                    [i]ICadDrawingDataset cadDrawingDataset = cadDrawingWorkspace.OpenCadDrawingDataset(filename);[/i]

                    ICadLayer cadlayer = new CadLayerClass();

                    cadlayer.CadDrawingDataset = cadDrawingDataset;

                    axMapControl1.AddLayer(cadlayer, 0);

                }

                catch (Exception exp)

                {

                    MessageBox.Show(exp.Message);

                }

            }