>Did you see this http://weblogs.asp.net/rosherove/archive/2004/10/03/DataSet-hell-_2D00_-_2200_Failed-to-enable-constraints.-One-or-more-rows--contain-values_2E002E002E002E002200_.aspx
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>In particular, it sounds like you may try to set .EnforceConstraints to false according to this message.
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>Read also through all the comments if the above would not resolve the issue. There are several other useful suggestions in this blog.

I have read that page. There's a lot of related issues in there. That error is triggered at the dataset fill method. But, what causes it can come from different sources. In most cases, this is related to constraint definitions in the database. But, when you don't have any, this is where it gets really confusing.