[dotNetRDF-Develop] Issue CORE-327 has been added to a project you are monitoring.

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<p>The following issue has been added to a project that you are monitoring.</p>
<table border="0">
  <tr>
    <td width="90px" valign="top"><b>Title:</b></td>
    <td>Use null instead of RdfQueryException to propogate type errors in SPARQL Expression Evaluation</td>
  </tr>
  <tr>
    <td><b>Project:</b></td>
    <td>Core Library (dotNetRDF.dll)</td>
  </tr>
  <tr>
    <td><b>Created By:</b></td>
    <td>Rob Vesse</td>
  </tr>
  <tr>
    <td><b>Milestone:</b></td>
    <td>1.1.0</td>
  </tr>
  <tr>
    <td><b>Category:</b></td>
    <td>Query</td>
  </tr>
  <tr>
    <td><b>Priority:</b></td>
    <td>High</td>
  </tr>
  <tr>
    <td><b>Type:</b></td>
    <td>Refactor</td>
  </tr>
  <tr>
    <td><b>Description:</b></td>
  </tr>
  <tr>
    <td colspan="2"><p>
	Currently we use RdfQueryException to propogate type errors through expression evaluation. &nbsp;However using exceptions to control flow is always more expensive than other alternatives.</p>
<p>
	&nbsp;</p>
<p>
	We should instead be using null to denote an error/unbound. &nbsp;We will need to carefully comb the expressions codebase in order to make this refactor to ensure that code treats nulls correctly as errors. &nbsp;There may also be some places where it is still legitimate to throw an error.</p></td>
  </tr>
</table>
<p>
            More information on this issue can be found at
            <a href="http://www.dotnetrdf.org/tracker/Issues/IssueDetail.aspx?id=327" target="_blank">http://www.dotnetrdf.org/tracker/Issues/IssueDetail.aspx?id=327</a></p>
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