1. Native / Managed Interop
Jim Fawcett
CSE775 – Distributed Objects
Spring 2012

2. References
.Net and COM, the Complete Interoperability Guide, Adam Nathan, SAMS, 2002
The title is accurate

3. Interop Interop happens in four ways:
Platform Invoke Services (Pinvoke)
Mixed mode programmin with C++/CLR
COM Interoperability
Java / Native Interop (JNI)

4. Platform Invocation Services
The Win32 API and other programs expose static entry points in libraries:
Kernel32.dll, user32.dll, gdi32.dll, and advapi32.dll
User defined DLLs
PInvoke supports
Calling entry points in unmanaged dlls with LoadLibrary and GetProcAddress
Passing function pointers to unmanaged code for callbacks

5. InteropServices DllImportAttribute
One required string parameter holds name of the DLL containing entry point
Optional CallingConvention
Optional CharSet
Optional EntryPoint if different from managed method
Optional ExactSpelling for W or A
Optional PreserveSig
Optional SetLastError

6. DllImportAttribute Arguments
CallingConvention Enum
Cdecl used for methods with variable number of parameters
FastCall not used
StdCall is default
ThisCall used to call class methods
CharSet Enum
Ansi for 8 bit chars
Auto uses the OS type
Unicode for 16 bit chars
C# and C++ default to Ansi

7. Mixed Mode Programming with C++
Managed and unmanaged class instances can directly call each other’s functions
Managed code can hold references to unmanaged class instances and vice versa
See ManagedCpp folder for examples

8. COM Interop COM Interop is made up of three parts: Core services
Interop APIs in System.Runtime.InteropServices
Tool support

9. Example: Using Speech API
using System;
using System.Collections.Generic;
using System.Text;
using SpeechLib;
namespace SpeakToMe {
class hello
static void Main(string[] args)
SpVoice voice = new SpVoice();
voice.Speak(
"Hello CSE775 - Distributed Objects",
SpeechVoiceSpeakFlags.SVSFDefault ); }
Navigate to:
C: \program files \common files \Microsoft shared \speech
tlbimp sapi.dll -->SpeechLib.dll

10. Tool Support COM Tools .Net Tools OleView displays type libraries
MIDL compiles IDL into COM source code and type libraries
Regsvr32 registers and unregisters COM components
RegEdit32 edits Windows registry
.Net Tools
TlbImp creates an interop assembly from a COM type library
TlbExp creates a type library from a .Net assembly
Regasm registers and unregisters assemblies
AxImp converts an ActiveX type library to an interop assembly and an activeX assembly
Regsvcs installs and uninstalls assemblies in COM+ catalog

11. Callable Wrappers Runtime-callable and COM-callable wrappers
They handle:
Data marshaling
Exception handling
Object lifetime

12. Proxies between Managed and COM

13. Runtime-Callable Wrappers
Runtime Callable Wrapper (RCW)
A .Net class that is a COM client, using IUnknown and IDispatch.
You could hand craft an RCW using managed C++, which can directly call the COM API.
However, you don’t need to. Just use tlbimp.exe

14. COM-Callable Wrapper COM-Callable Wrapper (CCW)
A COM component that provides a standard COM interface and holds a reference to the .Net component it wraps.
You could build one with managed C++ using reflection-based invocation.
You don’t have to. Instead install the .net component in the GAC and use tlbexp and regasm.

15. Primary Interop Assembly
An interop assembly is a RCW.
It contains definitions of a COM type described in metadata.
Authors of COM components should create a primary interop assembly.
Digitally signed, installed in the GAC

16. Primary Interop Assembly
.Net treats the same type defined in different assemblies as different types.
Use of a Primary Interop Assembly instructs .Net that each of these are different instances of the same type.
Thus we get a single managed identity for the COM type.

17. Making a PIA Use tlbimp /primary
That inserts the attribute: System.Runtime.InteropServices. PrimaryInteropAssembly
Use tlbimp /keyfile:FileName

18. Interop Services Marshal Class has many static methods
QueryInterface, AddRef, Release
AllocCoTaskMem, AllocHGlobal
FreeBSTR, FreeCoTaskMem, FreeHGlobal
Many helper functions

19. Interop Marshaling Interop implies two possible marshalings
COM marshaling from COM component to COM client inside a RCW
Only needed if the RCW and COM component reside in different apartments
Interop marshaling that converts a COM type to a .Net type.

20. Interop Marshaling Example
If the COM component interface has an [out] *BSTR, then the RCW will convert the BSTR to a System.String.
Both are unicode and length counted, but String has some additional information stored in its length prefix.
This results in a copy of the characters.
Blittable types (same memory representations) are pinned and no copy is needed.
Sbyte, Intxx, IntPtr, Byte, UIntxx, UIntPtr, Single, Double, structs with these types, C arrays with these types

21. Calling Methods on RCW
Method calls on the wrapper result in vtable-based calls on the COM object
Reflection with Type.InvokeMember results in an invocation through the COM object’s IDispatch interface.
See LateBinding in code folder.
HRESULT errors are mapped into .Net exceptions

22. Type Correspondences COM .Net variant IUnknown IDispatch SCODE BSTR
Blittable
.Net
Object
UnKnownWrapper
DispatchWrapper
ErrorWrapper
String
Blittable

23. Guidelines – Designing COM Components for Interop
Use dual interfaces to support early and late binding
Expose events with connection points
Expose enumerations with IEnumVARIANT
Use single-dimensional SAFEARRAYs for all arrays
Avoid variant parameters (ambiguous conversion)
Return only S_OK for success
Implement the IErrorInfo interface so exception messages are meaningful
Use threading model Both, as all .Net objects but UIs run in an MTA
If you use ApartmentThreading then you guarantee COM marshaling

24. End of Presentation