Microsoft ASP.NET QuickStarts Tutorial

COM Interoperability

The common language runtime enables .NET objects to interoperate seamlessly with traditional COM components. ASP.NET exposes the familiar Server.CreateObject(ProgId) API to developers for creating late-bound references to COM.

Dim myConn
myConn = Server.CreateObject("ADODB.Connection");

You can also use early-bound, traditional COM components by creating runtime callable wrappers (RCWs), which optimize the performance of calls between unmanaged and managed code. You can create an RCW using the Tlbimp.exe utility included in the .NET Framework SDK. For more information on Tlbimp.exe, see the Interoperability section of the Common Tasks QuickStart. The ASP.NET Performance section contains more information comparing late binding with early binding.

Like ASP, you can also create traditional COM components using the <object> tag with either a progid or a classid attribute. In addition to using the <object> tag in pages, you can also use it in the Global.asax file for the application. In this case, the object is added to the Page.Application.StaticObjects collection and can be accessed programmatically by simply using its id attribute. Note that you cannot create single-threaded apartment (STA) objects statically in the Global.asax file because doing so generates a runtime error, as it does in ASP.

ASP.NET also continues to support the existing ASP intrinsic interfaces ObjectContext Intrinsic Flow, OnStartPage, and OnEndPage. Supporting these interfaces means that you can use existing components (Commerce Server, Exchange, and so on) in ASP.NET pages. These interfaces are not enabled by default but are explicitly turned on using the following page directive:

<%@ Page ASPCompat="true" %>

This directive causes ASP.NET to create unmanaged ASP intrinsic objects and pass them to COM components used in the page. It also runs the page in an STA thread pool. See the following section for information.

Performance Considerations

In ASP.NET, the thread pool is a multithreaded apartment (MTA) by default, which can affect the performance of traditional apartment-threaded Visual Basic 5 and Visual Basic 6 components. The ASPCompat="true" attribute enables an STA thread pool to address performance with existing Visual Basic components on a per-page basis.

Calling between managed and unmanaged components also incurs a marshaling cost, which can impede the performance of your pages. Every scenario yields different performance characteristics, so it is important to test adequately before deciding whether interoperability is the right choice for your application. However, in nearly all scenarios, rewriting your COM components in managed code provides performance benefits. See the ASP.NET Performance section for more information and important tips.

Section Summary

ASP.NET exposes the familiar Server.CreateObject API to developers for creating late-bound references to COM.
You can also use early-bound, traditional COM components by creating runtime callable wrappers, which optimize the performance of calls between unmanaged and managed code.
ASP.NET continues to support the existing ASP intrinsic interfaces ObjectContext Intrinsic Flow, OnStartPage, and OnEndPage. These interfaces are explicitly enabled using the page directive <%@ Page ASPCompat="true" %>.
The ASPCompat="true" attribute enables STA thread pools on a per-page basis to address performance with existing Visual Basic components.
In nearly all scenarios, rewriting your COM components in managed code provides performance benefits.