COM Dos and Donts Krishna Kumar Parthasarathy Sulabh Shrivastava Support Engineer Developer Support

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COM Dos and DontsKrishna Kumar
ParthasarathySulabh ShrivastavaSupport
EngineerDeveloper Support COMMicrosoft
Corporation
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Best PracticesWhat to do

• Unattended Execution and Retain in Memory
  should be enabled for all Visual Basic
  components
• Object marked as both threading model should be a
  thread-safe component
• Design your test environment so that it is close
  to your production environment
• Use object pooling if components are expensive to
  create

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Best Practices (2)What to do

• Make sure Identity is set to THIS USER in
  production environment
• COM talking to Oracle database should follow the
  information in KB article Q193893http//support.
  microsoft.com/support/kb/articles/q193/8/93.asp
• Close and release your ADO objects as soon as
  possible

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Best Practices (3)What to do

• Use XML or disconnected recordset to send data
  back to client
• If multiple DTCs involved make sure there is
  proper RPC connectivity between all DTCs in both
  directions
• If a firewall is involved, make sure that proper
  ports are opened in both directions
• Run Web sites as Medium or High Isolation level

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Best Practices (4)What to do

• Estimate the stress for the COM component in
  production and run a similar level of stress in
  test environment
• When installing application in cluster make sure
  DTC IP belongs to public IP and not to private
  one
• If you want to use same CLSID and IID for the
  component, enable binary compatibility for VB
  components

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Best Practices (5)What to do

• To enhance scalability of COM application,
  divide into time-dependent and time-independent
  services
• Creating and managing transaction is expensive
  use transaction in an efficient way
• Authenticate as close to the user as you can for
  efficiency and performance

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Best Practices (6)What to do

• When creating a DSN, use object constructor
  strings specified on the component instead of
  creating a file DSN
• Make sure any objects created inside COM have
  been properly released

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Practices to AvoidWhat to avoid

• Avoid storing objects in Shared Property Manager
• Avoid storing STA objects in Application or
  Session scope in Web applications
• Avoid marshaling ADO Connection object
• Microsoft does not recommend storing state in
  COM components

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Practices to Avoid (2)What to avoid

• Microsoft does not recommend using long running
  code in ClassInitialize of VB components
• Avoid calling GetObjectContext from
  ClassInitialize of VB components
• Avoid writing a huge blocking call in STA
  components

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Practices to Avoid (3)What to avoid

• Microsoft does not recommend using callbacks from
  COM components
• Avoid displaying UI on a server-side component
• Avoid using non-threadsafe ODBC or OLEDB providers

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References

• COM Articles on MSDN
• http//msdn.microsoft.com/library/techart/complus
  .htm
• http//msdn.microsoft.com/library/techart/complus
  4vb.htm
• Knowledge Base article
• http//support.microsoft.com/support/kb/articles/
  q252/3/18.asp
• COM Reference in MSDN

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