Within a configuration item you can set a value to explicitly be null. What that mean means in terms of configuration is dependent upon the context of where it 's is set.
To set a value to null you need to add a special namespace to the config tag, http://www.w3.org/2001/XMLSchema-instance, and then use that to set the value nil to true for the value you want to set to null.
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So in this example the datasourcedataconnection will use the cache definition supplied by existing.cache.config but with the overflowToDisk value reset to its default. The default will be used because that's the normal handling for a cache definition where a value isn't set, other . Other configuration items may have different interpretations of what null means for a specific value and this should be is outlined in the configuration reference for that item.