Assembly

1
Assembly

• Where it all gets physical

2
Objectives

• Introduce concepts of assemblies
• Discuss elements of assemblies
• Show how to build assemblies
• Runtime aspects

3
Contents

• Section 1 Overview
• Section 2 Concepts and Elements
• Section 3 Assemblies at Buildtime
• Section 4 Assemblies at Runtime

4
Section 1 Overview

• Versioning and DLL conflicts must be resolved
• Windows 2000 partially fixed DLL conflicts
• New shared version still replaces the old
• Physical units instead of logical
• Easy installation/deinstallation procedures
  required
• xcopy installation
• Just delete the files!

5
Whats an Assembly

• Runtime executable code IL
• Single point of entry
• Fundamental unit
• Version control
• Reuse
• Scoping
• Identity
• Security permissions
• Runtime metadata

6
Static and Dynamic Assemblies

• Static
• Generated by compilers
• Loaded from disk or downloaded from the net
• Dynamic
• Built "on-the-fly"
• From scripts or via Reflection.Emit
• Can be transient

7
Assembly vs. Module

• Module is compiled unit
• Modules contain types and global methods
• Assemblies contain modules
• An assembly consists of modules and resources
• Assembly manifest references files

8
Dependencies

• Viewed as collection of exported resources
• Independent of implementation
• Assembly may depend on other assemblies
• May import any public resource of other assembly
• Types
• Resource files
• Etc.
• Dependencies are recorded in the manifest
• Resolved at runtime no static linking

9
Type Referencing
External Refs
Module1
ref null, TypeA, module1
ref null, TypeB, module2
AssemblyB
ref AssemblyB, TypeC, module3
AssemblyA
10
Assembly vs. Namespace

• Namespaces are used to group names
• Assemblies can contain several namespaces
• Namespaces can be partitioned across assemblies
• Types implemented twice are different!
• Both must be included into project independently
• Namespaces are imported in the source code
• Assemblies are referenced by compiler switch

using System.Runtime.Remoting.Services
csc /rSystem.Runtime.Remoting.DLL ...
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Section 2 Concepts and Elements

• Elements of an assembly
• Manifest
• Versioning
• Security
• Physical representation
• What's in it's name

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Manifest Standard Elements

• Manifest is table with info records
• Manifest contains info about
• Assembly name
• Version information
• Strong name information
• Culture information
• Processor and OS
• Files that make up this assembly
• References to types and resources
• Exported and local types

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Manifest Custom Elements

• AssemblyCompany
• AssemblyConfiguration
• AssemblyCopyright
• AssemblyDefaultAlias
• AssemblyDescription
• AssemblyInformationalVersion
• AssemblyProduct
• AssemblyTitle
• AssemblyTrademark

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Multi-File Assemblies

• Association based on metadata within assembly
• Not linked by the file system

Module1
Module2
Graph
Assembly Manifest
Module Manifest
Assembly.exe
Module2.dll
Graph.jpg
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Versioning

• Manifest carries version information
• Compatibility version
• Major, minor, build, revision 2.1.1254.0
• Informational version
• String stored in AssemblyInformationalVersion
• References to other assemblies carry version info

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Side-by-side execution

• Run different versions simultaneously
• Per machine
• Or even per process
• Requires special coding considerations
• Issues with dependencies on machine resources
• Process-wide resources

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Security Considerations

• Integrity of files is guaranteed by hash
  verification
• Assembly carries permission requests
• Security policy is applied to requests at load
  time
• AuthentiCode digital signing
• Strong names

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Strong Names

• Simple name accompanied by
• Public key
• Digital signature
• Generated from assembly and private key
• Prevent others from taking over your namespace
• Protect version lineage
• Assemblies with same strong name are identical
• Versioning only works with strong named assemblies

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Strong Name Utility

• sn.exe provides options for
• Signature generation
• Generate public/private key pair
• Key management
• Signature verification
• Setting Cryptographic Service Provider (CSP)

sn k keyFile.snk
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Assigning a Strong Name

• Need to have a public-private key pair
• Using attributes
• Using al (assembly linker)

assemblyAssemblyKeyFile("sgKey.snk")
assemblyAssemblyKeyName("AContainer")
al myModule.dll /keyfilesgKey.snk ...
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Delaying Strong Name Assignment

• Access to private key might be restricted
• Delayed (or partial) signing reserves space in
  file
• Actual signing is deferred
• Process works as follows
• Developer works with public key file only
• Verification must be switched off
• Full signing must be applied later

assemblyAssemblyKeyFile(pubKey.snk")
assemblyAssemblyDelaySign(true)
sn Vr myAssembly.dll
sn R myAssembly.dll fullKey.snk
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Using Strong Named Assemblies

• Consumer of strong named assembly uses token
• Token is portion of public key
• Runtime verifies strong name signature
• Referencing usually transparent
• Compiler inserts token of referenced assembly
• Dynamic loading requires explicit notion

gt sn t myDll.DLL Assembly.Load(myDll,Version1.
0.0.1,Cultureneutral,PublicKeyToken9b35aa32c18
d4fb1)
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Section 3 More Tools and Deployment

• Assembler ilasm
• Disassembler ildasm
• Global Assembly Cache
• Installation

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Assembler ilasm

• "Assembles" IL streams into loadable files
• Generates the metadata
• Output can be disassembled by ildasm
• No optimizations made

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Dis-Assembly ildasm

• "Disassembles" assemblies (or modules) into IL
• Output can be reassembled by ilasm
• GUI for examining an assembly
• Manifest
• Metadata
• IL code

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Global Assembly Cache Advantages

• Using the GAC has advantages
• Performance improvements
• Integrity checking
• File security
• Versioning
• Automatic pickup of Quick Fixes
• Additional search location

27
Installation

• Private vs. shared assemblies
• Private assemblies deployed in local directory
• Shared assemblies stored in Global Assembly Cache
• Cache viewer as shell extension (shfusion.dll)
• Snap-In for Management Console (mscorcfg.msc)

gacutil i myAssembly.DLL
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Section 4 Assemblies at Runtime

• Loading an assembly
• Concept of Application Domain
• JITting an assembly
• PreJITting an assembly

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Loading an Assembly

• Assembly is Portable Executable (PE) file ...
• ... with CLR related information added
• Runtime aware environment loads assembly directly
• Unaware operating system loads assembly as PE
• Entry point stub that loads and calls CLR
• CLR examines addtional header information

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Application Domain

• Concept for application isolation
• Provide isolation at lower cost than processes
• AppDomains are created by the runtime host
• AppDomain is created for each application
• Direct references between AppDomains disallowed
• Requires proxies or copied objects

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Loader Optimization

• Assembly is SingleDomain by default
• Each AppDomain loads and compiles assembly
• Assembly can be marked for MultiDomain use
• Assembly is compiled once
• Mapped into all referencing AppDomains
• A copy is available for each process
• References to static data is indirected
• Assembly is unloaded when process ends
• MultiDomainHost
• Copy of code is hosted in each AppDomain

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Just-In-Time Compilation

• MSIL is made for compilation
• Needs some per-method analysis
• Code is compiled when needed
• Compilation on a per-method base
• Code that is not called is not compiled
• Loader creates stub for each method
• First step Verification of type safety
• JITted code is not persisted

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PreJITting with ngen

• Complete compile at installation time
• PreJITted assembly is installed in GAC
• Speeds up loading time significantly
• Both IL and native image are loaded
• No verification needed
• Native image is not used...
• ...When module version ID of IL is different
• ...If the same applies to any dependencies
• ...Assembly binding policy has changed
• Fallback to normal JIT process

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Summary

• Assemblies as logical DLLs
• Anatomy of an assembly
• Units of versioning
• Strong names
• Installation in Global Assembly Cache
• Loading and Compiling

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Questions?