The SPSite.AllWebs [] and SPWebCollection[] index operator returns a new SPWeb instance each time it is accessed. An object is created during the indexing operation even if that object was already accessed. If not properly closed, the following code samples leave an SPWeb object in the .NET Framework garbage collector. See sample from MSDN:

Bad Coding Practice

void AllWebsForEachLeak()
{
  using (SPSite siteCollection = new SPSite("http://moss"))
  {
    using (SPWeb outerWeb = siteCollection.OpenWeb())
    {
      foreach (SPWeb innerWeb in siteCollection.AllWebs)
      {
         // Explicitly dispose here to avoid out of memory leaks with large number of SPWeb objects.
      }
    } // SPWeb object outerWeb.Dispose() automatically called.
  }  // SPSite object siteCollection.Dispose() automatically called.
}

Good Coding Practice

void AllWebsForEachNoLeakOrMemoryOOM()
{
  using (SPSite siteCollection = new SPSite("http://moss"))
  {
    using (SPWeb outerWeb = siteCollection.OpenWeb())
    {
      foreach (SPWeb innerWeb in siteCollection.AllWebs)
      {
        try
        {
          // ...
        }
        finally
        {
          if(innerWeb != null)
            innerWeb.Dispose();
        }
      }
    } // SPWeb object outerWeb.Dispose() automatically called.
  }  // SPSite object siteCollection.Dispose() automatically called.
}

Remarks

Rule relates to SPDisposeCheckId 'SPDisposeCheckID_130'. To ignore this rule add the attribute '[SPDisposeCheckIgnore(SPDisposeCheckID.SPDisposeCheckID_130, "Caller will dispose")]' to your method.

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