How to programatically set parameters for collect feedback workflow?

SPWeb myweb = SPControl.GetContextWeb(this.Context);

SPWorkflowTemplateCollection wtColl = myweb.WorkflowTemplates;

SPWorkflowTemplate wt = wtColl.GetTemplateByName("Collect Feedback", myweb.Locale);

SPDocumentLibrary docLib = myweb.Lists["WFDocLib"] as SPDocumentLibrary;

SPListItem item = docLib.Items[1];

SPWorkflowManager mgr = mysite.WorkflowManager;

StreamReader reader = null;

try
{
reader = new StreamReader(
@"C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\template\layouts\Samples\newdata.xml"
);

string xml = reader.ReadToEnd();

myweb.AllowUnsafeUpdates = true;

mgr.StartWorkflow(item, assn, xml, true);

item.Update();
myweb.Update();
}
catch (Exception ex)
{
Response.Write(ex.Message); //i am not getting any error here. But the workflow instance description shows Error Occurred
}
finally
{
myweb.AllowUnsafeUpdates = false;
}