The ADO.NET API has some problems when it comes to handling nullable value types (i.e. it simply doesn't work correctly). We've had no end of issues with it, and so have arrived at the conclusion that it's best to manually set the value to null, e.g.
myNewRow.myGuidColumn = myGuid == null ? (object)DBNull.Value : myGuid.Value
It's painful extra work that ADO.NET should handle, but it doesn't seem to do so reliably (even in 3.5 SP1). This at least works correctly.
We've also seen issues with passing nullable value types to SqlParameters where the generated SQL includes the keyword DEFAULT
instead of NULL
for the value so I'd recommend the same approach when building parameters.
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