The limitation doesn’t apply to distiguished fields.

Thanks to another blogger, I didn’t waste a lot of time on this, and as he points out, it’s documented.  Still, it’s one of those things that can trip you up.  The error you get is:

There was a failure executing the receive pipeline: “Microsoft.Biztalk.DefaultPipelines.XMLReceive”. Source: “XML Disassembler” Receive Location: “C:\ReceiveLocation” The property ‘propertyname’ has a value with length greater than 256 characters

According to the help,


    Property field promotion is subject to the following limitations:

      • Promoting a string value that is longer than 256 characters in length may cause overflow exceptions at run-time

If you try to assign a string >256 characters in the midst of an orchestration, you get this, instead:

Uncaught exception terminated service Project.Orchestration(GUID1), instance GUID2

The value assigned to property ‘http://namespace:propertyname’ is not valid: ‘longstring‘.
Exception type: InvalidPropertyValueException
Source: Microsoft.XLANGs.BizTalk.Engine
Target Site: Microsoft.BizTalk.Agent.Interop.IBTMessage PrepareMessage(Microsoft.XLANGs.BaseTypes.XLANGMessage, System.Collections.IList, System.Collections.IList)

Help Link:
Additional error information:

The property ‘propertyname’ has a value with length greater than 256 characters.
Exception type: COMException
Target Site: Void Promote(System.String, System.String, System.Object)