Re: [Dotnetrdf-support] SPARQL Handlers error when no accept header is specified

[Rob V. <rv...@do...>]

Hi Bob

The issue is partly poor behaviour on the part of the script and partly a 
bug in dotNetRDF.  The script you are using isn't sending any accept header 
which means the array received by the 
MimeTypesHelper.GetSparqlWriter(String[] ctypes, String, &contentType)  
function is null and therefore you get your null reference exception.

This has now been fixed so that when no accept header is specified SPARQL 
Handlers default to returning SPARQL XML Results Format.

Hopefully your example should now work as desired, let me know if it 
doesn't.

Regards,

Rob Vesse

----------------------------------------
From: "Bob DuCharme" <bo...@sn...>
Sent: Wednesday, June 16, 2010 12:23 AM
To: dot...@li...
Subject: [Dotnetrdf-support] "Object reference not set to an instance of an 
object." 

When I paste the following URL (which issues a SPARQL query against some 
RDFa to find the URL of the FOAF file listed there) into a browser, it 
works great:

http://www.dotnetrdf.org/demos/leviathan/?query=PREFIX+foaf%3A+%3Chttp%3A%2F
%2Fxmlns.com%2Ffoaf%2F0.1%2F%3E%0D%0ASELECT+%3Fpage+WHERE+{%0D%0A++%3Fs+foaf
%3Ahomepage+%3Chttp%3A%2F%2Fwww.snee.com%2Fbob%3E+%3B%0D%0A+++++foaf%3Apage+
%3Fpage+.}%0D%0A%0D%0A&default-graph-uri=http%3A%2F%2Fwww.w3.org%2F2007%2F08
%2FpyRdfa%2Fextract%3Furi%3Dhttp%253A%252F%252Fwww.snee.com%252Fbob%26format
%3Dpretty-xml%26warnings%3Dfalse%26parser%3Dlax%26space-preserve%3Dtrue%0A&t
imeout=10000

Using the  LWP::Simple perl library, though, the get() command in my 
perl script gets the following:

  Error
  -----

  1: PREFIX foaf: 
  2: SELECT ?page WHERE {
  3:   ?s foaf:homepage  ;
  4:      foaf:page ?page .}
  5:

  Object reference not set to an instance of an object.
     at VDS.RDF.MimeTypesHelper.GetSparqlWriter(String[] ctypes, String& 
contentType)
     at VDS.RDF.Web.BaseSparqlHandler.ProcessResults(HttpContext 
context, Object result)
     at VDS.RDF.Web.FileSparqlHandler.ProcessQuery(HttpContext context, 
String query, List`1 userDefaultGraphs, List`1 userNamedGraphs, Int64 
timeout, Boolean partialResults)

I get the same if I specify the URL for the file with the RDFa in a FROM 
clause instead of in the &default-graph-uri parameter.

This get() function has successfully retrieved query results from 
similar services.

Does the error message above look familiar to anyone?

thanks,

Bob

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