*** May 25^th 2009: Revised post after important community feedback! Thanks.

By now it is a well known fact that it is not entirely trivial working with SPSite and SPWeb objects in the WSS object model when it comes to memory management. Forgetting to call the Dispose() method on objects you own, can result in a memory leak that might eventually bring down your SharePoint server. Fortunately, Microsoft has published a best practice to help SharePoint developers avoid this phenomenon. It basically says that you are responsible for disposing all the SPSite and SPWeb objects you own.

However, I have by some reverse engineering learned that the SPSite object has a built-in safety value for disposing all SPWeb objects that were not disposed as prescribed by the best practice. But it only works if the parent SPSite object is disposed as prescribed. Consider the following example where we correctly dispose of all the objects we create.

using (SPSite site = new SPSite("http://server/sites/asite")) 
{ 
  using (SPWeb web = site.OpenWeb()) 
  { 
    foreach (SPWeb subWeb in web.Webs)
    { 
      // Perform operations... 
      subWeb.Dispose(); 
    } 
  } // web.Dispose() automatically called. 
} // site.Dispose() automatically called.

No memory leaks there - except if an exception is thrown just before subWeb.Dispose()! But a proper exception safe try/finally construct has been omitted not to clutter the example. Now, consider a scenario where someone forgot to dispose the SPWeb objects:

using (SPSite site = new SPSite("http://server/sites/asite")) 
{
  SPWeb web = site.OpenWeb();   // Oops! Forgot to wrap it in a using block
  foreach (SPWeb subWeb in web.Webs)
  {
    // Perform operations...
    // Oops! Forgot to call subWeb.Dispose() 
  } 
} // site.Dispose() automatically called.