title: Should I use GUID or DefaultExtendedPropertySet.PublicStrings while constructing ExtendedPropertyDefinition?
body: <p>I am developing C# .NET Framework 4.5 Windows Form application using EWS Managed API 1.2 with Exchange Server 2007 which performs some sort of syncing of mails.</p>
  
  <p>Now that I am dealing with Extended Properties, I want to be clear some things:</p>
  
  <p><strong>Q1.</strong> What is the purpose of <code>DefaultExtendedPropertySet</code> class? MSDN <a href="http://msdn.microsoft.com/en-us/library/microsoft.exchange.webservices.data.defaultextendedpropertyset(v=exchg.80).aspx" rel="noreferrer">says</a> "Defines the default sets of extended properties." </p>
  
  <ul>
  <li>Is it just to group the extended properties? </li>
  <li>If yes, why is the grouping there at first place? </li>
  <li>Do we have any Ews API method which can fetch values of all extended properties belonging to the same group on an item? </li>
  </ul>
  
  <p><strong>Q2.</strong> I am unable to decide whether should I use custom GUID or <code>DefaultExtendedPropertySet.PublicStrings</code> while constructing <code>ExtendedPropertyDefinition</code>:</p>
  
  <pre><code>var MyXProp = new ExtendedPropertyDefinition(
               DefaultExtendedPropertySet.PublicStrings, 
              "MyXProp", MapiPropertyType.String);
  </code></pre>
  
  <p>OR</p>
  
  <pre><code>Guid MyPropertySetId = new Guid("{C11FF724-AA03-4555-9952-FA248A11C3E}");            
  var extendedPropertyDefinition = new ExtendedPropertyDefinition(
               MyPropertySetId, "MyXProp", MapiPropertyType.String);
  </code></pre>
  
  <ul>
  <li>What are the factors that should dictate the above decision?</li>
  <li>Also what difference it makes by above two approaches?</li>
  </ul>