C# Language Specification

Implicit enumeration conversions

An implicit enumeration conversion permits the decimal-integer-literal 0 to be converted to any enum-type and to any nullable-type whose underlying type is an enum-type. In the latter case the conversion is evaluated by converting to the underlying enum-type and wrapping the result (§4.1.10).

4. Implicit nullable conversions

Predefined implicit conversions that operate on non-nullable value types can also be used with nullable forms of those types. For each of the predefined implicit identity and numeric conversions that convert from a non-nullable value type S to a non-nullable value type T, the following implicit nullable conversions exist:

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  An implicit conversion from S? to T?.
  
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  An implicit conversion from S to T?.
  

Evaluation of an implicit nullable conversion based on an underlying conversion from S to T proceeds as follows:

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  If the nullable conversion is from S? to T?:
  

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  If the source value is null (HasValue property is false), the result is the null value of type T?.
  
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  Otherwise, the conversion is evaluated as an unwrapping from S? to S, followed by the underlying conversion from S to T, followed by a wrapping (§4.1.10) from T to T?.
  

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  If the nullable conversion is from S to T?, the conversion is evaluated as the underlying conversion from S to T followed by a wrapping from T to T?.
  

5.