The situation is that I am making a WCF call to a remote server which is returns an XML document as a string.

Most of the time this return value is a few K, sometimes a few dozen K, very occasionally a few hundred K, but very rarely it could be several megabytes (first problem is that there is no way for me to know).

It's these rare occasions that are causing grief. I get a stack trace that starts:

System.OutOfMemoryException: Exception of type 'System.OutOfMemoryException' was thrown.
   at System.Xml.BufferBuilder.AddBuffer()
   at System.Xml.BufferBuilder.AppendHelper(Char* pSource, Int32 count)
   at System.Xml.BufferBuilder.Append(Char[] value, Int32 start, Int32 count)
   at System.Xml.XmlTextReaderImpl.ParseText()
   at System.Xml.XmlTextReaderImpl.ParseElementContent()
   at System.Xml.XmlTextReaderImpl.Read()
   at System.Xml.XmlTextReader.Read()
   at System.Xml.XmlReader.ReadElementString()
   at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationReaderMDRQuery.Read2_getMarketDataResponse()
   at Microsoft.Xml.Serialization.GeneratedAssembly.ArrayOfObjectSerializer2.Deserialize(XmlSerializationReader reader)
   at System.Xml.Serialization.XmlSerializer.Deserialize(XmlReader xmlReader, String encodingStyle, XmlDeserializationEvents events)
   at System.Xml.Serialization.XmlSerializer.Deserialize(XmlReader xmlReader, String encodingStyle)
   at System.Web.Services.Protocols.SoapHttpClientProtocol.ReadResponse(SoapClientMessage message, WebResponse response, Stream responseStream, Boolean asyncCall)
   at System.Web.Services.Protocols.SoapHttpClientProtocol.Invoke(String methodName, Object[] parameters)

I've read around and it is because the Large Object Heap is just getting too fragmented, so even preceding the call with a quick check to StringBuilder.EnsureCapacity just causes the OutOfMemoryException to be thrown earlier (and because I'm guessing at what's needed, it might not actually need that much so my check is causing more problems than it is solving). Some opinions are that there's not much I can do about it.

Some of the questions I've asked myself:

• Use less memory - have you checked for leaks? Yes. The memory usage goes up and down, but there's no fundamental growth that guarantees this to happen. Some of the times it fails, it succeeded at that stage previously.
• Transfer smaller amounts Not an option, this is a third party web service over which I have no control (or at least it would take a long time to resolve, in the meantime I still have a problem)
• Can you do something to the LOH to make it less likely to fail? ... now this is most fruitful course. It's a 32-bit process (it has to be for various political, technical and boring reasons) but there's normally hundreds of meg free (multiples of the largest amount for which we've seen failures).
• Can we monitor the LOH? Using perfmon I can track the size of the heaps, but I don't think there's a way to monitor the largest available contiguous block of memory.

Question is: any advice or suggestions for things to try?