ArcObject必须使用反射创建实例对象

在ArcObject开发时，有时非常让人无语的是使用new创建对象时，报"无法将类型为Systerm._ComObject的对象强制转换为类型******"，官方知道这是个坑，早就讲得非常清楚和透彻了。

Interacting with singleton objects (ArcObjects .NET 10 SDK) (arcgis.com)

大体原因：当使用New关键字创建实例化COM对象时，默认会创建一个强类型的RCW，如果这个对象已存在，则会创建为弱类型的RCW，即System.__ComObject，而.NET Framework 无法将new的强类型RCW接口转换为单实例弱类型RCW。

错误写法：

ISpatialReferenceFactory spatialReferenceFactory = new SpatialReferenceEnvironment();
ISpatialReference spatialReference = spatialReferenceFactory.CreateGeographicCoordinateSystem(4490);

正确写法：

Type t = Type.GetTypeFromProgID("esriGeometry.SpatialReferenceEnvironment");
object obj = Activator.CreateInstance(t);
ISpatialReferenceFactory spatialReferenceFactory = obj as ISpatialReferenceFactory;
ISpatialReference spatialReference = spatialReferenceFactory.CreateGeographicCoordinateSystem(4490);

那么，不禁要问了，ArcObject中哪些对象是单实例呢？下面根据官方的文档整理了一下：

类	ProgID	备注
AccessWorkspaceFactory	esriDataSourcesGDB.AccessWorkspaceFactory	常用
ActionEnvironment*	esriTrackingAnalyst.ActionEnvironment
AGAnimationUtils	esriAnimation.AGAnimationUtils
AGSServerConnectionFactory	esriGISClient.AGSServerConnectionFactory
AMSWorkspaceFactory	esriTrackingAnalyst.AMSWorkspaceFactory
AnimationControlsDialog	esriAnimationUI.AnimationControlsDialog	仅Desktop
AnimationCreateKeyframeDialog	esriAnimationUI.AnimationCreateKeyframeDialog	仅Desktop
AnimationFromPathDialog	esriAnimationUI.AnimationFromPathDialog	仅Desktop
AnimationLayerGroupDialog	esriAnimationUI.AnimationLayerGroupDialog	仅Desktop
AnimationManagerDialog	esriAnimationUI.AnimationManagerDialog	仅Desktop
AnimationTypeCamera	esri3DAnalyst.AnimationTypeCamera
AnimationTypeGlobeCamera	esriGlobeCore.AnimationTypeGlobeCamera
AnimationTypeGlobeLayer	esriGlobeCore.AnimationTypeGlobeLayer
AnimationTypeLayer	esri3DAnalyst.AnimationTypeLayer
AnimationTypeMapLayer	esriAnimation.AnimationTypeMapLayer
AnimationTypeMapView	esriAnimation.AnimationTypeMapView
AnimationTypeScene	esri3DAnalyst.AnimationTypeScene
AnimationTypeTimeLayer	esriAnimation.AnimationTypeTimeLayer
AppRef*	esriFramework.AppRef	仅Desktop
ArcInfoWorkspaceFactory	esriDataSourcesFile.ArcInfoWorkspaceFactory
CadWorkspaceFactory	esriDataSourcesFile.CadWorkspaceFactory	常用
CieLabConversion	esriDisplay.CieLabConversion
CommandsEnvironment	esriControls.CommandsEnvironment
DddEnvironment	esri3DAnalystUI.DddEnvironment	仅Desktop
DddServerEnvironment	esriGeodatabase.DddServerEnvironment
DDDToolbarEnvironment	esri3DAnalystUI.DDDToolbarEnvironment	仅Desktop
DEGdbUtilities	esriGeodatabase.DEGdbUtilities
DEUtilities	esriGeoprocessing.DEUtilities
DummyGraduatedMarkerLayer	esriCarto.DummyGraduatedMarkerLayer
DummyLayer	esriCarto.DummyLayer
EngineEditor	esriControls.EngineEditor	常用，仅Engine
EngineNetworkAnalystEnvironment	esriControls.EngineNetworkAnalystEnvironment	仅Engine
EnvironmentManager	esriSystem.EnvironmentManager	常用
ErrorDialog	esriSystemUtility.ErrorDialog	仅Engine (C++)
ErrorMessage	esriSystemUtility.ErrorMessage	仅Engine (C++)
ExcelWorkspaceFactory	esriDataSourcesOleDB.ExcelWorkspaceFactory	常用
ExtensionManager	esriSystem.ExtensionManager
FileGDBWorkspaceFactory	esriDataSourcesGDB.FileGDBWorkspaceFactory	常用
FindDialog	esriCatalogUI.FindDialog	仅Desktop
FormatList	esriDataSourcesRaster.FormatList
GeoDatabaseHelper	esriGeodatabase.GeoDatabaseHelper
GMxEffectsToolbarEnvironment	esriArcGlobe.GMxEffectsToolbarEnvironment	仅Desktop
GPGlobeFunctionFactory	esriGlobeCore.GPGlobeFunctionFactory
GPHolder	esriGeoprocessing.GPHolder
GPSAUtil	esriSpatialAnalyst.GPSAUtil
GPServerFunctionFactory	esriGeoprocessing.GPServerFunctionFactory
GpsPositionDialog	esriArcMapUI.GpsPositionDialog	仅Desktop
GPUtilities	esriGeoprocessing.GPUtilities
GxCatalogROT	esriCatalogUI.GxCatalogROT	仅Desktop
GxDatabaseExtensions	esriCatalog.GxDatabaseExtensions	仅Desktop
IdentifyDialog	esriCartoUI.IdentifyDialog	仅Desktop
IMSWorkspaceFactory	esriGISClient.IMSWorkspaceFactory
InMemoryWorkspaceFactory	esriDataSourcesGDB.InMemoryWorkspaceFactory
LabelEnvironment	esriCarto.LabelEnvironment
LayerEffectsEnvironment	esriArcMapUI.LayerEffectsEnvironment	仅Desktop
LocatorManager	esriLocation.LocatorManager
MemoryRelationshipClassFactory	esriGeodatabase.MemoryRelationshipClassFactory
MessageLogger	esriSystem.MessageLogger
MetadataSynchronizer	esriGeodatabase.MetadataSynchronizer
MonitorSettings	esriDisplay.MonitorSettings
MyPlaceCollection	esriControls.MyPlaceCollection	仅Engine
NADirectionsWindowManager	esriNetworkAnalystUI.NADirectionsWindowManager	仅Desktop
NetCDFWorkspaceFactory	esriDataSourcesNetCDF.NetCDFWorkspaceFactory
NetworkIdentifyDialog	esriNetworkAnalystUI.NetworkIdentifyDialog	仅Desktop
OLEDBWorkspaceFactory	esriDataSourcesOleDB.OLEDBWorkspaceFactory	常用
PCCoverageWorkspaceFactory	esriDataSourcesFile.PCCoverageWorkspaceFactory
RasterAnalysisToolbarEnvironment	esriSpatialAnalystUI.RasterAnalysisToolbarEnvironment	仅Desktop
RasterAttributeTableManager	esriDataSourcesRaster.RasterAttributeTableManager
RasterDefaultsEnv	esriCarto.RasterDefaultsEnv
RasterSettings	esriSpatialAnalystUI.RasterSettings	仅Desktop
RasterStatus	esriDataSourcesRaster.RasterStatus
RasterWorkspaceFactory	esriDataSourcesRaster.RasterWorkspaceFactory	常用
RelQueryTableFactory	esriGeodatabase.RelQueryTableFactory	常用
SAExtension	esriSpatialAnalystUI.SAExtension	仅Desktop
SchematicProjectMgr	esriSchematic.SchematicProjectMgr
SchematicWorkspaceFactory	esriSchematic.SchematicWorkspaceFactory
ScratchWorkspaceFactory	esriDataSourcesGDB.ScratchWorkspaceFactory
SDCWorkspaceFactory	esriDataSourcesFile.SDCWorkspaceFactory
SdeWorkspaceFactory	esriDataSourcesGDB.SdeWorkspaceFactory	常用
SelectionEnvironment	esriCarto.SelectionEnvironment
ServerStyleGallery	esriDisplay.ServerStyleGallery
ShapefileWorkspaceFactory	esriDataSourcesFile.ShapefileWorkspaceFactory	常用
SpatialReferenceEnvironment	esriGeometry.SpatialReferenceEnvironment	常用
StreetMapWorkspaceFactory	esriDataSourcesFile.StreetMapWorkspaceFactory
StyleGallery	esriFramework.StyleGallery	仅Desktop
SxAnimationEnvironment	esri3DAnalyst.SxAnimationEnvironment
SxEffectsToolbarEnvironment	esriArcScene.SxEffectsToolbarEnvironment	仅Desktop
SymbologyEnvironment	esriDisplay.SymbologyEnvironment
SystemHelper	esriSystem.SystemHelper
TAExtension	esriTrackingAnalystUI.TAExtension	仅Desktop
TextFileWorkspaceFactory	esriDataSourcesOleDB.TextFileWorkspaceFactory	常用
TexturePersistenceProperties	esriGeometry.TexturePersistenceProperties
TinWorkspaceFactory	esriDataSourcesFile.TinWorkspaceFactory	常用
TMS_MarshalHelper	esriTrackingAnalyst.TMS_MarshalHelper
ToolboxWorkspaceFactory	esriGeoprocessing.ToolboxWorkspaceFactory
TrackingEnvironment	esriTrackingAnalyst.TrackingEnvironment
VpfWorkspaceFactory	esriDataSourcesFile.VpfWorkspaceFactory
WMSConnectionFactory	esriGISClient.WMSConnectionFactory
WorkspaceFactory	esriGeodatabase.WorkspaceFactory
XMLTypeMapper	esriSystem.XMLTypeMapper