InfoPath Dev

I have an InfoPath Form. I have this code (c#):

public void IdNumber_OnAfterChange(DataDOMEvent e)
{

if (e.IsUndoRedo)
{ return;
}
if (e.Operation=="Delete")
{ return;
}


XmlDocument xmlDoc = new System.Xml.XmlDocument();
XmlNode ndQuery = xmlDoc.CreateNode
(XmlNodeType.Element,"Query","");
XmlNode ndViewFields = xmlDoc.CreateNode (XmlNodeType.Element,"ViewFields","");
XmlNode ndQueryOptions = xmlDoc.CreateNode(XmlNodeType.Element,"QueryOptions","");


string str=e.NewValue.ToString();

ndQueryOptions.InnerXml
= "<IncludeMandatoryColumns>FALSE</IncludeMandatoryColumns>" +
"<DateInUtc>TRUE</DateInUtc>";



ndQuery.InnerXml="<Where><Eq><FieldRef Name='IdNumber'/><Value Type='Text'>"+str+"</Value></Eq></Where>";



try
{
XmlNode returnXml = WS.GetListItems("FormsFolder",null,ndQuery,null,null,ndQueryOptions);
}
}
This is the code. I get error that the field with this name not found!!
That's mean that the field name "IdNumber" not fount in the SharePoint list.
Why it not found it? I know that I nedd to replace the field name with somthing else. How to do it?

Is there a namespace on the IdNumber field?

Make sure that you're using the name that SharePoint uses. If you create a new column and then change the name of it later, SharePoint will still use the original name as it's "identifier". To get the name SharePoint uses, go to Modify Settings and Columns, click Edit Columns, and then click on the idNumber column. Now, look at the URL in the address bar and there should be a Name field in the query string at the end of the URL. This is what you want to use in the CAML.