Interoperating With Existing Code

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InteroperatingWith Existing Code

• Peter Ty
• MCDBA, MCSE, MCPSBTechnology Specialist
• Microsoft Hong Kong Ltd.

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Common Language Runtime Type System

• Compilers use the runtime type system to produce
  type compatible components

Runtime Components
PlatformCompilers

C
VB
C
Runtime Type System
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Interoperability Services
Type SystemStandard
BinaryStandard
Runtime Execution Sys
COM Binary Std
C
VB
MFC/ATL
MSVCRT
C
Runtime Type Sys
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Why Interop

• Preserve/Utilize your investment
• No need to start over
• Continue to use exist code
• Incremental migration path
• Migrate your application step by step
• Reality some things never change
• Need to interop with code that cant change

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Interop

• COM/DLL
• Binary standard
• Type Libraries
• Immutable
• Type unsafe
• Interface based
• HResults
• Guids

• .NET Framework
• Type Standard
• Assemblies
• Resilient Bind
• Type Safe
• Class and Interface based
• Exceptions
• Strong Names

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Model Consistency

• Programming model remains consistent
• COM developers use COM model
• .NET Framework developers use .NET Framework
  model
• Model Transparency

• .NET Framework
• new operator
• Cast operator
• Memory mgmt
• Exceptions
• Static Methods

• COM/DLL Model
• CoCreateInstance
• QueryInterface
• Reference Counting
• Hresults
• GetProcAddress

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Interop Services

• COM Interop
• Use COM components from the runtime
• Use .NET Framework components from COM
• Platform Invoke
• Call static Entry Points in unmanaged DLLs
• Pass function pointers to unmanaged code for
  callback

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COM Interoperability
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COM Interop

• Provides a bridge between .NET Framework and COM
  and vice versa
• Maintains programming model consistency on both
  sides
• Abstracts the inconsistencies between the two
  models
• Different data types
• Method signatures
• Exception/Hresults
• Interop is for backward compatibility, build .NET
  clients for real simplicity

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Bi-Directional COM Interop

• .Net to COM
• Allows .Net types to access COM types
• COM to .Net
• Allows COM types to access .Net types

.Net
COM
COM
.Net
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.Net To COM Interop
Common Language Runtime
COM Server
Object
Runtime CallableWrapper
ReferenceCounted
Client
Traced Reference
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Using COM Types From The .NET Framework

• Create type definitions for COM types
• By adding references in VS
• By using type library importer (tlbimp)
• By defining types manually
• Use the types as managed types
• Create instance with new
• Cast between interfaces
• Extend them
• No reference counting
• Catch Exceptions

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Helpful Information

• Dont think of the types as COM types
• Catch exceptions
• Use reflection
• Use delegates based events
• COM types must be registered (of course)
• Assembly with COM type definitions must be
  resolvable at runtime
• Late bind to COM types using reflection
• Threading model must be compatible
• Application must have full trust in order to use
  COM types

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Under the Covers

• The runtime wraps all COM objects with a Runtime
  Callable Wrapper (RCW)
• The wrapper handles
• Reference counting
• Signature conversion
• Object identity
• Exception mapping
• More
• Runtime marshals calls between the wrapper and
  the COM object

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Using COM Types From .NET Framework Code Walk
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COM To .Net Interop
Traced Reference
Common Language Runtime
COM Client
COM CallableWrapper
IFoo
Object
ReferenceCounted
Client
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Using .NET Framework Types From COM

• Create manage type in the usual way
• Only public are exposed to com
• Classes must have default constructor to be
  creatable
• Use TlbExp to create COM type library
• Register the assembly with RegAsm
• Only needed for interop
• Reference the typelib as necessary
• Use the types as normal COM types
• Create instance with CoCreateInstance
• Use QueryInterface to type coercion
• Use AddRef/Release to reference count
• Examine Hresults on return

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Helpful Information

• Guids generated automatically at export time.
• Based on assembly and type name
• Based on complete interface definition
• Typelib file may be needed at runtime
• Runtime will produce one automatically
• Prepackage along with assembly
• Assembly must be resolvable at runtime to
  activate from COM
• Installed in application directory or
• Installed in global assembly store

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Under The Covers

• The runtime wraps all .NET Framework objects with
  a COM Callable Wrapper (CCW)
• The wrapper implements
• IUnknown
• IDispatch
• ITypeInfo
• IProvideClassInfo
• IConnectionPoint, etc.
• The runtime marshals calls between the COM client
  and the wrapper
• Wrapper also manages identity, handles
  exceptions, signature conversion, etc

CCW
.Net Class
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Using .NET Types From COM Code Walk Thru
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Building COM Compatible Types
.NET Framework
COM
IFooBaz
IFooBaz
CCW
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Building COM Compatible Types

• COM Compatible types provide compatible
  implementations of existing COM interfaces
• Interfaces must have compatible layout, IID,
  DispID, method signature
• Start by importing interface definitions with
  TlbImp
• Implement necessary interfaces, export with
  TlbExp and register with RegAsm

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Signature TranslationPerformed by TlbImp/TlbExp

• .NET Framework Method Signature
• int FormatDate(String s, DateTime d)
• HRESULT FormatDate(BSTR s, DATE d, out, retval
  int retval)
• COM Method Signature

TlbImp
TlbExp
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Custom Attributes
Custom Attributes can be applied to types,
methods, properties, fields or parameters to
effect the COM type definitions produced by TlbExp

• Using System.Runtime.InteropServices
• Guid(), InterfaceType(ComInterfaceType.IsIUnknow
  n)
• Interface IFooBar
• DispId(64) int Format( MarshalAs(LPSTR)
  String s)

Custom Attributes are also used for other
purposes
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Platform Invoke (P/Invoke)

• Provides access to static entry points in
  unmanaged DLLs
• Similar to
• VB Declare statement
• Load library / GetProcAddress
• Requires method definition with custom attribute
• Uses same underlying marshaling service as COM
  Interop

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