Update: I found that the issue with similar solution is already discussed in “Setting NullValue property for typed dataset” blog
In generated Typed Datasets attempt to access field with DBNull value by default throw an exception. It is usually not convinient, and I prefer to change nullValue annotation to _null, as it was suggested here
However I didn't find a way to change the default(Anyone knows???).
I have to open each column in desigher and change the nullValue to _null.
Alternatively I have to open .XSD file in XML editor (rigth mouse click,“open with...“,”XML editor” - no XML Vew tab in VS 2005 as it was in VS 2003) and insert ="_null" to each column.
msprop:nullValue
You cah try global replace
type="xs:string" to
msprop:nullValue="_null" type="xs:string"
Note, that you can NOT assign _null to simple types, like int or bool.
It is not very convinient process.
Related request in MS Connect:Allow Null Values to be returned from strongly typed datasets .