Thursday, November 13, 2008

Inconsistent "soap:mustUnderstand" in .Net 2.0


It seems that .Net 2.0 web services do not handle soap:mustUnderstand attribute in a consistent manner. This attribute can appear over soap headers in order to require the processing party to fail itself if it cannot process this specific header. Example:

  <SomeHeader soap:mustUnderstand="1">1234</SomeHeader>

Now if our server does not understand SomeHeader an exception should be thrown.

To begin with, when a SoapHeader is defined in the service/proxy, .Net will not allow us to mark it as "required" since it is considered obsolete:

This is not really related to mustUnderstand: When the proxy contains required=true it means (or should have at least) that the proxy will fail itself if this value is not supplied by the user. This is different than omitting a mustUnderstand header which will cause the server to fail.

But the real inconsistency comes with mustUnderstand: When a .Net 2.0 web service gets soap:mustUnderstand header it will always ignore it even if it was not understood. On the other hand, when a .Net 2.0 web service client gets such an header in a response from the server it will throw the following exception:

SOAP header SomeHeader was not understood.

Be aware of this inconsistency when you build or consume .Net 2.0 web services.


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Anonymous said...

And I can't tell you how frustrating it is sometimes when these little issuse pop out from nowhere. It took me a while to figure this out.

I haven't tried an implementation in 3.0 or 3.5 yet, but I'm hopeful it's fixed.

Yaron Naveh (MVP) said...

And all of these small bits are usually interoperability show stoppers...

Anonymous said...


I trying to call a webservice hosted on Websphere 6.1 from a axis(1.3) based client. web service implements a oasis 2004 standard. while calling from a axis client i am getting following error

Did not understand "MustUnderstand" header(s)
at org.apache.axis.handlers.soap.MustUnderstandChecker.invoke(MustUnderstan=

Could you please help on this issue.

whereas when i see the soap response it does not show me any errors

Yaron Naveh (MVP) said...


please publish the soap response.

Anonymous said...


Thanks a lot for your answer!
here's the soap response:



Yaron Naveh (MVP) said...

xml is not displayed well in these comments.

most probably the response contains a soap header with the must understand attribute.

this thread can be useful:

Anonymous said...

i finally figured it out: I needed a handler to process my soap header as the mustunderstand flag was on.

Thanks a lot