I have a workflow that has a form where datafields would be hard to manage (80 of them) and the data is not really normalized for a smart object. Instead we used xmldatafields.

I don't have time to pretty this up but wanted to post my notes.  I hope they help you.

Things to know, use client connection not management connection.

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•  SourceCode.Workflow.Client.ProcessInstance npi = this.clientConnection.CreateProcessInstance(K2processName);

When you get an XmlDataField it comes out as a string, you need to make it an xmldocument or reader/write (your choice).

After you make the string an XmlDocument , get the root node, then do your work on it.

When you write it back to the process, it's not a string as it came to you, but an XmlDocument.

UPDATE YOUR PROCESS! or when you close your connection your changes will be gone.

----Code not tested in this format, I had a bunch of classes so some pieces may be missing.

SourceCode.Workflow.Client.

Connection clientConnection = new SourceCode.Workflow.Client.Connection();
clientConnection.Open(_serverName);
String K2processName = "MyHelloWorld";
SourceCode.Workflow.Client.ProcessInstance npi = tclientConnection.CreateProcessInstance(K2processName);
npi.Folio = "Development Test";
clientConnection.StartProcessInstance(npi, true);
Int new_process_id = npi.ID;
npi.update();
clientconnection.Close(); //Just to show you that you should close it.

XmlDocument wut = new XmlDocument();
XmlElement root;

SourceCode.Workflow.Client.Connection clientConnection = new SourceCode.Workflow.Client.Connection();
clientConnection.Open(_serverName);
SourceCode.Workflow.Client.ProcessInstance cpi = clientConnection.OpenProcessInstance(new_process_id);
String xml_as_string = cpi.XmlFieldsl"WU"].Value.ToString();
wut.LoadXml(xml_as_string);
root = this.wut.DocumentElement;
XmlNamespaceManager ns = new XmlNamespaceManager(wut.NameTable);
ns = ns;
ns.AddNamespace("", http://schema.example.com/employee/employee.xsd); // set default
ns.AddNamespace("wu", http://schema.example.com/employee/employee.xsd);

//There are a million different ways to get data from an xml Document, this is how I did it this time, could not get xpath expressions to work properly with selectSingleNode so  

//Get some values

String xpparent = "//wu:employee";
String xpchild = "wu:employee_name";
XmlNode parent = this.root.SelectSingleNode(xpparent, this.ns);
XmlNode child = parent.SelectSingleNode(xpchild, this.ns);
String EmployeeName =  child.InnerText.ToString();

//set the value
child.InnerText = "Charlie Brown";

String xpparent = "//wu:employee";
String xpchild = "wu:employee_id";
XmlNode parent = this.root.SelectSingleNode(xpparent, this.ns);
XmlNode child = parent.SelectSingleNode(xpchild, this.ns);
String EmployeeID = child.InnerText.ToString();

child.InnerText ="123456";

cpi.XmlFieldsm"employees"].value = wut.OuterXml;
cpi.update();
clientConnection.close();