COM Interop & P/Invoke

.NET and the Old World

Objectives

Introduction to interoperation between .NET and COM COM and .NET .NET and platform services

Contents

Section 1: Overview

Section 2: Calling COM Services from .NET

Section 3: Calling .NET Services from COM

Section 4: Calling Platform Services from .NET

Existing Stuff Must Coexist

Transition to .NET will be a lasting process

Portions of systems will remain as they are now ...and even maintained in their current form

Interoperability increases the momentum of .NET

Goal is to achieve total transparency

Mere presence of .NET must not interfere with COM

Performance should not be hit

What is different in the two Worlds?

Managed Code vs. Unmanaged Code Lifetime management Metadata

Common Type System

Inheritance concept

Threading model

Error handling mechanisms

Registration

Calling COM Services from .NET

Metadata for COM class must be available Can be generated from Type Library Can declare Custom Wrapper

Use COM class like .NET class

Early and late bound activation

COM component must be registered locally

Early Bound Activation

Instantiate like creating a .NET object

Metadata must be available at compile time Must be referenced in development environment

At runtime, Metadata and COM library must be available

COMLib.COMClass aCOMObject;

aCOMObject = new COMLib.COMClass;

Runtime Callable Wrappers Preserving object identity

Maintaining object lifetime

Proxying interfaces

Marshalling method calls

Consuming selected interfaces

.NET Object

COM Object RCW

IUnknown

IDispatch

IIfc

IIfc

Consuming Selected Interfaces

ISupportErrorInfo Additional information with exceptions

IProvideClassInfo Typed wrapper

ITypeInfo TlbImp converts references to System.Type

Identity and Lifetime Control

COM lifetime control is wrapped by RCW

COM object released when RCW garbage collected

Non-deterministic finalization issue

COMLib.COMClass aCOMObject;

aCOMObject = new COMLib.COMClass();

aCOMObject.SomeFunc();

Marshal.ReleaseComObject(aCOMObject);

Converting Type Library to Metadata Use SDK tool TlbImp

Or just add reference to server in Visual Studio

Type library can be imported from executable

Reference resulting metadata file in project

Custom IDL attributes are preserved

IDL:

[custom(„FE42746F-...-AC...84178”, “Value”)]

.NET:

[IdlAttribute(„FE42746F-...-AC...84178”, “Value”)]

Conversions by TlbImp 1/2

Library level Library to namespace

Module is converted to class Constants within module to constants within that class Module functions to static members of the class

library COMLib { interface Widget {}; coclass Slingshot {};

}

namespace COMLib { interface Widget {};

class Slingshot {}; }

custom(0F21F359-AB84-41e8-9A78-36D110E6D2F9, „MyNamespace.Class")

Conversions by TlbImp 2/2

Interfaces Base interfaces IUnknown and IDispatch are stripped

Type definitions are not imported Imported as the underlying types

Properties Generates corresponding get and set

Events

namespace COMLib { interface Widget {}; class Slingshot {}; }

Marshalling Isomorphic Types

Integer, Float Values

Non-isomorphic Types Booleans, Chars, Strings, Arrays, Objects

Copying vs. Pinning Isomorphic Types are pinned Non-Isomorphic Types are copied Watch out with Unicode Strings!

Passing by value actually passes a reference!

Passing by reference creates new String

Marshalling a Union Use attributed struct in .NET

StructLayout attribute for Sequential layout – is a regular structure Default Union layout Explicit layout

[StructLayout(Layout.Explicit)] public class SYSTEM_INFO { [FieldOffset(0)] ulong OemId; [FieldOffset(4)] ulong PageSize; [FieldOffset(16)] ulong ActiveProcessorMask; [FieldOffset(20)] ulong NumberOfProcessors; [FieldOffset(24)] ulong ProcessorType; }

Inheritance

COM uses containment/aggregation

.NET adds implementation inheritance

Mixed-mode objects Managed objects can derive from COM objects

Must have metadata available

Must be instantiatable

Must be aggregatable

Interfaces can be inherited IUnknown, IDispatch derivation is removed

Calling an COM Servernamespace CallingCOMServer

{

using System;

using COMServerLib;

public class DotNET_COMClient

{...

public static int Main(string[] args)

{

MyCOMServer myS = new MyCOMServer();

return (myS.Add (17,4));

}

}

};

Late Bound Activation

Accomplished by Reflection API No difference whether COM or .NET object

Type can be obtained from ProgID or ClsID

InvokeMember to access methods and properties

Metadata is not needed, but useful COM object is wrapped by __ComObject

Late Bound Call to COM Servernamespace LateBoundClient

{

using System.Reflection; ... Type typ; Object obj; Object[] prms = new Object[2]; int r;

typ = Type.GetTypeFromProgID(„MyLib.MyServer"); obj = Activator.CreateInstance(typ); prms[0] = 10; prms[1] = 150;

r = (int)typ.InvokeMember(„aMethod", BindingFlags.InvokeMethod, null, obj,

prms); ...

}

Calling an Automation Servernamespace AutomationClient{ using EmotionalTulip; using System.Reflection; ... TulipApplication tulipApp = new TulipApplication();

Type t = tulipApp.GetType(); Object[] prms = new Object[1]; prms[0] = true;

t.InvokeMember("Visible", BindingFlags.SetProperty, null, tulipApp, prms);

t.InvokeMember("Exit", BindingFlags.InvokeMethod, null, tulipApp, null);

...}

Threading

Most COM objects are single threaded

.NET thread must initialize COM apartment Deferred initialization Initialized either as MTA or STA

Runtime provides transparent use of COM threads Access apartment threaded objects through proxy Can avoid proxy by setting apartment state of thread

Let the runtime initialize the default MTA

Setting ApartmentState explicitly eliminates proxy Do before the object is created!

Using ApartmentState

using System.Threading;using APTOBJLib;

AptSimple obj = new AptSimple ();

obj.Counter = 1;

using System.Threading; using APTOBJLib;

Thread.CurrentThread.ApartmentState = ApartmentState.STA;

AptSimple obj = new AptSimple();

obj.Counter = 1;

Results and Exceptions COM reports errors via result code

.NET methods throws exceptions Runtime manages transition

HRESULT specifies exception class

Extended information via IErrorInfo COM object should implement interface ErrorCode, HelpLink, Message Source, StackTrace, TargetSite

PreserveSigAttribute in custom wrapper Suppresses translation to exception

Design for Interoperability Provide and register Type Libraries

Use Automation Compliant Data Types

Use Chunky Calls Marshalling may be costly

Explicitly Free External Resources

Avoid Using Members of System.Object Object, Equals, Finalize, GetHashCode, GetType MemberwiseClone and ToString

Restrictions and Limitations

void* parameters need marshalling

Success HRESULTS cannot be distinguished Failure HRESULT do raise exceptions Success code don‘t

Typedef do not get through

Variable length arrays cannot be imported

is imported as

HRESULT DoSomething(int cb, [in] byte buf[]);

public void DoSomething(int cb, ref Byte buf);

Calling .NET Services from COM

Use .NET class like COM class Type and methods must be public

Early and late bound activation

Convert Metadata to Type Library

Wrapper of .NET component must be registered Can use RegAsm tool

COM Callable Wrappers Responsibilities of the COM Callable Wrapper

Preserving Object Identity Maintaining object lifetime Proxying custom interfaces Marshalling method calls Synthezising selected interfaces

COM Object CCW

.NET Object

IUnknown

IDispatch

IIfc

IIfc

Identity and Lifetime Control

Single CCW for .NET class insures identity No matter how many COM classes call to it No matter how object is created

Each interface of the CCW is referenced counted No reference counting on the managed object

Managed objects lives at least while the CCW lives But finalization is undeterministic Managed objects may occasionally leak

Should be explicitly freed using CoEEShutDown

Synthesizing Selected Interfaces myClass implementing myItf derived from myBase

_myClass: all methods of myItf, myBase, myClass _myBase: all methods of myBase myItf: all methods of myItf _Object

Additional interfaces implemented on CCW IUnknown

Standard implementation for lifetime management

IDispatch Automation compatible or internal

ITypeInfo, IProvideClassInfo, IErrorInfo

Exporting a Type Library

Use SDK tool TlbExp Type library exported from assembly

Use the TypeLibConverter Class

Leave the conversion to RegAsm utility

Export process involves a lot of conversions Name collisions are solved using name mangling

tlbexp AssemblyName.dll /out:TlbFile

Sample Conversionpublic class MyClass { public void Write(string p, ref string q) {...}; public int GetInt() {...}; }

[uuid(…), hidden, dual, odl, nonextensible, oleautomation] interface _MyClass : IDispatch { [propget] HRESULT ToString([out, retval] BSTR* pRetVal);

HRESULT Equals([in] VARIANT obj, [out, retval] VARIANT_BOOL* pRetVal); HRESULT GetHashCode([out, retval] long* pRetVal); HRESULT GetType([out, retval] _Type** pRetVal); HRESULT Write([in] BSTR p, [in, out] BSTR* q); HRESULT GetInt([out, retval] long* pRetVal);}

[ uuid(...) ] coclass MyClass { [default] interface _MyClass; interface _Object;}

Special Attributes

Attributes control the conversion process [In], [Out], [MarshalAs(...)]

Parameters, Fields

[GuidAttribute("...")] Assemblies, classes, interfaces, structures, enumerations,

or delegates

Uses Marshal.GenerateGuidForType function

[ComVisibleAttribute(...)] Individual type or assembly level

Registration (RegAsm) Adds assembly information to the registry

regasm assembly /tlb:tlbfile /regfile:regfile

[HKCR\Namespace.Class] @=„Namespace.Class"

[HKCR\Namespace.Class\CLSID] @="{...}"

[HKCR\CLSID\{...}] @="Namespace.Class"

[HKCR\CLSID\{...}\InprocServer32]

@="C:\WINNT\System32\MSCorEE.dll"

"ThreadingModel"="Both"

"Class"="Namespace.Class"

"Assembly"=„AssemblyName, Ver=1.0.0.0, Loc="""

[HKCR\CLSID\{...}\ProgId] @="Namespace.Class"

Results and Exceptions You know: COM uses HRESULTs, .NET exceptions

Can‘t propagate exceptions to unmanaged code User defined exception classes define HRESULT

Default is HRESULT of base class

Class NoAccessException : public ApplicationException{ NoAccessException() { HResult = E_ACCESSDENIED; } } CMyClass::MethodThatThrows{ NoAccessException e = new NoAccessException(); throw e; }

Aggregation and Containment

Simple requirements to be met Class must be public Class must have default constructor

Containment: Simply create instance and delegate calls to it

Aggregation Just use CoCreateInstance

COM object‘s IUnknown as outer IUnknown

Queries for unsupported interfaces are delegated back Reference counting is done on outer IUnknown

Exposing Inheritance

Managed interfaces provide inheritance Always derive from IUnknown or IDispatch Interface inheritance is flattened

Advantages with versioning Users of interfaces not effected when base changes Interfaces may be separately attributed

Disadvantage Interfaces cannot be used polymorphically

Converted Inheritance Tree Original managed codeinterface IBase { void m1(); }

interface IDrvd : IBase { void m2(); }

class CDrvdImpl : IDrvd { void m1(); void m2(); }

Converted type libraryinterface IBase : IDispatch { void m1(); }

interface IDrvd : IDispatch { void m2(); }

interface _CDrvdImpl : IDispatch{ boolean Equals(); // and other methods of object void m1(); void m2(); }

coclass CDrvdImpl { interface IBase; interface IDerived; interface _CDerivedImpl; interface _Object; }

Restrictions and Limitations

Parameterized constructors are not exposed Always uses default constructor

Static methods are not exposed Wrapping with instance method required

Calling Platform Services from .NET

Calling static functions in DLLs

P/Invoke provides services Locates implementing DLL Loads DLL Finds function address

Fuzzy name matching algorithm

Pushes arguments on stack Performs marshalling Enables pre-emptive garbage collection Transfers control to unmanaged code

Code Sample

namespace HelloWorld{ using System;

class MyClass { [dllimport(„user32.dll“, CharSet=CharSet.Ansi)] static extern int MessageBox(int h, string m,

string c, int t); public static int Main() { return MessageBox(0, "Hello World!", "Caption", 0); } }}

Marshalling

Strings are copied Converted to platform format: ANSI or Unicode Strings are never copied back – use StringBuilder

Interfaces supported Cannot be used as out parameter

Arrays of primitive types can be used

Custom marshalling using attributes

Showcase for the Standard Marshaller

int SomeFunc( int sel, void* buf, int size);

[DllImport("some.dll")]static extern int SomeFunc(int sel, [MarshalAs(UnmanagedType.LPArray)] byte[] buf,

int size);

[DllImport("some.dll", EntryPoint="SomeFunc")]static extern int SomeFunc_String(int sel,

StringBuilder sb, int size);

[DllImport("some.dll", EntryPoint="SomeFunc")]static extern int SomeFunc_Int(int sel,

ref int i, int size);

Given a method in some DLL

Standard declaration may need special decoding

May use taylored versions

Callbacks Unmanaged code can call back to managed code

Unmanaged parameter is function pointer Must supply parameter as delegate P/Invoke creates callback thunk

Passes address of thunk as callback parameter

Managed Code

.NET Application

Call passes pointer to callback function

Implementation of callback function

Unmanaged Code

DLL

DLL function

Code Samplepublic class EnumReport { public bool Report(int hwnd, int lParam) { // report the window handle Console.Write("Window handle is "); Console.WriteLine(hwnd); return true; }};

public class SampleClass{ delegate bool CallBack(int hwnd, int lParam);

[DllImport("user32")] static extern int EnumWindows(CallBack x, int y);

public static void Main() { EnumReport er = new EnumReport(); CallBack myCallBack = new CallBack(er.Report); EnumWindows(myCallBack, 0); }}

Security

Suppressing demands for unmanaged code permission

Calling class must be fully trusted Unmanaged code runs with no runtime checks

Any code calling that class must be fully trusted

[SuppressUnmanagedCodeSecurityAttribute()] [dllimport("some.dll")] private static extern int EntryPoint(args);

Component Services Fundamental building blocks

Just-In-Time activation, synchronization, pooling Transaction processing, shared properties, etc.

Mostly restricted to Windows 2000

COM+ generates and services context object

.NET Client

Context

Object A

Context

Object B

.NET Framework

COM+ Services

Object A

(TX required)

Object B

(TX supported)

Using COM+ Services

Derive class from System.ServicedComponent

Add attributes to parameterize services Use attributes from Microsoft.ComServices namespace Helper classes: ContextUtil, RegistrationHelper

Use regsvcs to register assembly as server Or rely on lazy registration

Transactions Sampleusing System;using System.Runtime.CompilerServices;using Microsoft.ComServices;

[assembly:ApplicationName(“TestApp”)]

[Transaction(TransactionOption.Required)]public class Account : ServicedComponent {

[AutoComplete] public void Debit(int amount) {

// Any exception thrown here aborts transaction // otherwise transaction commits }

}

class client {static int Main() {

Account accountX = new Account(); accountX.Debit(100); return 0;

}}

Summary

Preserving investments and moving on Keep COM components and enhance using .NET Add managed components to COM projects Use Windows Component Services

Prepare COM components for use with .NET

.NET offers maximum transparency

Questions?