As I understand it, in Linq the method FirstOrDefault() can return a Default value of something other than null.
No. Or rather, it always returns the default value for the element type… which is either a null reference, the null value of a nullable value type, or the natural “all zeroes” value for a non-nullable value type.
Is there any particular way that this can be set up so that if there is no value for a particular query some predefined value is returned as the default value?
For reference types, you can just use:
var result = query.FirstOrDefault() ?? otherDefaultValue;
Of course this will also give you the “other default value” if the first value is present, but is a null reference…