Assemblies in the .NET CLR

1
Assemblies in the .NET CLR

• Presented Monday, 21 May 2001
• by Donal Lafferty

2
References

• Online conversation with
• Pransih Kumar, Prgroam Manager on Visual C
  libratries team (ATL, MFC, ATL Server)
• Ronald Laeremans, Program Manager for C
  compiler
• Assemblies
• Closest thing .NET has to COM objects
• versioning
• contain multiple classes and interface
  definitions
• Framework SDK
• Notes from Understanding Managed Execution
• .NET Framework Developers Guide

3
Assembly Concepts

• WRT creation time, there are two flavours
• Dynamic and Static
• Dynamic are defined at runtime, static are
  defined at development time.
• Dynamic need not be persisted on disk, but static
  always will be.
• Dynamic created using System.Reflection.Emit.
• Dynamic assemblies correspond to compiled
  scripts.
• Assemblies are a logical unit of distribution
• Single-file and multi-file assemblies both exist.
• Ex. Person class starts off as one file, but a
  later version includes three files

Person V1.2.0.0
Person V1.0.0.0
Person.DLL
People.MCL
Address.DLL
Person.MCL (Manifest)
Manifest
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Assembly Concepts

• Assembly Manifest Contents
• Assembly name
• Textual string
• Version info
• Major.Minor.Build.Revision (x.x.x.x)
  (compatibility version)
• Shared name
• public key from publsher
• hash of file containing the manifest signed w/
  publishers private key
• Target
• Target culture, processor and O/S
• Assembly file list
• hash of each file in assembly
• Relative path to file from manifest.
• Docs say files must be in same directory, but I
  think this includes subdirectories.
• Type reference
• Identifies supported types
• Dependency list
• other assemblies statically referenced by the
  assembly.
• Assemblys name, metadata (version, culture,
  O/S), public key (shared name)

5
Assembly Concepts

• Assembly design guidelines
• Assemblies are the unit of modularity, so group
  classes with similar needs in the following
  categories
• Version
• Reuse
• Security
• Scoping

6
Creating Assemblies

• Creation tools
• al.exe
• IDE
• probably calls al.exe anyways
• Configuration parameters
• Name
• simple ? string
• shared (I.e. Strong) ? string, public key,
  encrypted hash
• Deployment location
• Applications root directory or subdirectory
• Global assembly cache
• Loader Optimization
• Assembly creation time
• Single Domain
• More memory for multiple versions, but smaller
  JIT-code
• Multi-Domain, same code in each domain
• Multi-Domain, different code in each domain
• Multi-Domain cannot be unloaded.
• Load time

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Shared Name

• Key points
• Guaranteed uniqueness
• Key, Hash and string name
• Ownership over name
• via private key
• Does not imply Trust!
• Creation
• sn.exe
• Usage
• Strong name assemblies stored in Global Assembly
  Cache (GAC)
• Strong name assembly referenced by name and token
• token is shorter version of public key.
• Binding occurs with assembly whose key matches
• Binding results in integrity check.

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Global Assembly Cache

• Runtime cache has two parts
• Download cache
• Global assembly cache
• Deploying to cache
• al.exe, WindowsInstaller, WindowsExplorer
• Why use for assemblies?
• Performance
• No verfication at load time, load is faster,
  locating assembly much faster.
• Integrity checking
• At install time
• File delection security
• Only admin can delete
• Versioning integrated with loader
• Questionable.
• QFE easier
• Single assembly needs to be updated.

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Locating an Assembly

• 1. Assembly load intiated when assembly is
  referenced
• Static reference recorded in an assemblies
  dependency list.
• Dynamic reference via System.Reflection.Assembly.L
  oad
• Assembly Reference consists of
• name
• version
• culture
• originator
• (I.e. reference to the publishers public key)
• If reference is missing originator, then
  versioning is not adherred to.

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Locating an Assembly

• 2. Apply version policy specified by config file
• Determine configuration file
• For an exe, the configuration file has the same
  name as the .exe, but ends with a .cfg.
• For an applet, the HTML specifies the
  configuration file.
• File is written in XML
• Determine revisions to version requirements
• BindingRedir tag can explicitly remap a version,
  all versions and change the QFE policy.
• QFE is Quick Fix Engineering
• File is written XML
• Ex.
• ltBindingPolicygt
• ltBindingRedir NamePerson
• Originator32ab4ba45e0a69a1
• Version VersionNew2.1.0.0
• UseLatestBuildRevisionyes/gt
• lt/BindingPolicygt
• Determine revisions to version requirements
• BindingMode tag can explicitly select safe mode
  which forces the desired assembly version to be
  that record at compile time.
• Ex

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Locating an Assembly

• Locating assembly
• No originator, no versioning
• Codebase approach
• Config file can explicity select location using a
  URL (file, http, ftp, etc.)
• Ex
• ltAssembliesgt
• ltCodeBaseHint NamePerson
• Originator8e47bf1a5ed0ec84
• Version1.0.0.1
• CodeBasehttp//www.dsg.cs.tcd.ie/1ba2/person.
  dll/gt
• lt/Assembliesgt
• Current binding context second alternative
• Corresponds to location from which referencing
  assembly was obtained.
• Probing approach
• Probing stops when 1st acceptable version found.
• Search for file with assembly name and extension
  of .MCL, .DLL or .EXE
• Start at AppBase
• Location of 1st assembly loaded in AppDomain
• URL, not file name!

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Locating an Assembly

• 4. Global Assembly Cache
• QFE policy will dictate GAC look up even if
  probing finds a version.
• If probing/codebase failes, Global Assembly Cache
  (GAC) look up occurs.

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Misc Points

• Namespace support
• Runtime type name consists of two logical parts
• Assembly name
• type name in assembly
• Namespace not supported natively
• Implemented at language level
• Ex.TCD.Reification and TCD.Terminal can both be
  in 1BA2.DLL
• Side by side execution
• Ex. two versions of assembly can be loaded into
  separate processes
• Must make sure no conflicts between system
  resources captured by each.
• Ex. two versions of assembly can be loaded into
  separate AppDomains
• Must make sure no conflicts between process
  resources captured by each.
• Security
• AuthentiCode signature implies trust, but is
  separate from strong naming
• Prob. a DLL standard.
• Assembly makes permission request to security
  system for required access.
• Policy set by adminstrator.