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.NET Forum / Languages / C# / July 2007Writing from an XmlReader
Is there an easy way to (efficiently) walk the entire contents of an XmlReader? For instance, I have an XmlReader returned from a "FOR XML" SQL query (ExecuteXmlReader), and I want to return this data to the caller through a stream. One option is to use: XmlDocument doc = new XmlDocument(); doc.Load(reader); doc.Save(stream); but obviously this unnecessarily loads the DOM; likewise, reader.ReadOuterXml() forces the entire xml to load into a string at once... but the xml in question could be quite large. I was hoping for a stream-like way to walk the nodes, writing to the stream (either directly, or via an XmlWriter). Does anybody have any hints here? Marc > Is there an easy way to (efficiently) walk the entire contents of an > XmlReader? For instance, I have an XmlReader returned from a "FOR XML" [quoted text clipped - 11 lines] > a stream-like way to walk the nodes, writing to the stream (either > directly, or via an XmlWriter). XmlWriter has a method WriteNode that takes an XmlReader <http://msdn2.microsoft.com/en-us/library/System.Xml.XmlWriter.WriteNode.aspx> That might be what you are looking for although I am not sure I have understood that correctly.  SignatureMartin Honnen --- MVP XML http://JavaScript.FAQTs.com/ Perfect; from the name, I presumed it would write a single node - but it does write the subtree. Cheers - I knew it would be there somewhere... Marc Marc, If you want to write it as a stream, then why are you putting it into an XmlReader in the first place? Do you have to do some processing on it before you send it back? If not, then I would just get it as a string or byte array and then place that into a MemoryStream. Signature- Nicholas Paldino [.NET/C# MVP] - mvp@spam.guard.caspershouse.com > Perfect; from the name, I presumed it would write a single node - but it > does write the subtree. Cheers - I knew it would be there somewhere... > > Marc > If you want to write it as a stream, then why are you putting it into an > XmlReader in the first place? Very simple; since the data might be quite large, I was hoping that this might allow the ADO provider to stream the data from the database, rather than loading it in a lump (as would be the case with a string or byte[]). A SqlDataReader might allow me to read chunks of binary, but I've only managed to do this with "image" data-types before, not a FOR XML output. I haven't looked in detail, but a brief peek with "Reflector" leads me to believe that ExecuteXmlReader will stream... if you know of a way to get a FOR XML directly streaming as byte[] chunks, I'd be interested? Marc Marc, You can do this by passing the SequentialAccess value from the CommandBehavior enumeration to the ExecuteReader method on your command. Once you do that, you can call GetBytes or GetChars to access the data for the field as a progressive stream (you have to access the columns in order, btw), as opposed to all at once. Signature- Nicholas Paldino [.NET/C# MVP] - mvp@spam.guard.caspershouse.com >> If you want to write it as a stream, then why are you putting it into an >> XmlReader in the first place? [quoted text clipped - 10 lines] > > Marc Interesting - I'll give that a go. I've used that approach with BLOBs (and (less often) CLOBs), but I haven't (to date) used it with the xml data-type. Cheers. Marc (for the list's benefit) Nicholas is quite correct; I've changed my code to use a fixed char[] buffer, ExecuteReader(), and GetChars(), writing the appropriate buffer-portion to the Stream via a StreamWriter and the .Write(char[], int, int) overload. All is now well (and a little quicker I think, presumably due to ExecuteXmlReader() having to parse the stream). Thanks. Marc Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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