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When we have two structs, and one is implicitly convertible to the other, then it seems like the System.Nullable<> versions of the two are also implicitly convertible. Like, if struct A has an implicit conversion to struct B, then A? converts to B? as well.
Here is an example:
struct MyNumber
{
public readonly int Inner;
public MyNumber(int i)
{
Inner = i;
}
public static implicit operator int(MyNumber n)
{
return n.Inner;
}
}
Inside some method:
MyNumber? nmn = new MyNumber(42);
int? covariantMagic = nmn; // works!
In the C# Language Specification Version 4.0 we read that a conversion like this shall exist for "the predefined implicit identity and numeric conversions".
But is it safe to assume that it will also work for user-defined implicit conversions?
(This question might be related to this bug: http://connect.microsoft.com/VisualStudio/feedback/details/642227/)
347
In C# a struct is a 'value type', which can't be null.
C# provides a special data types, the nullable types, to which you can assign normal range of values as well as null values. For example, you can store any value from -2,147,483,648 to 2,147,483,647 or null in a Nullable<Int32> variable. Similarly, you can assign true, false, or null in a Nullable<bool> variable.
HasValue indicates whether an instance of a nullable value type has a value of its underlying type. Nullable<T>. Value gets the value of an underlying type if HasValue is true . If HasValue is false , the Value property throws an InvalidOperationException.
As you know, a value type cannot be assigned a null value. For example, int i = null will give you a compile time error. C# 2.0 introduced nullable types that allow you to assign null to value type variables.
But is it safe to assume that it will also work for user-defined implicit conversions?
Yes. From section 6.4.2 of the C# 4 spec:
Given a user-defined conversion operator that convers from a non-nullable value type
Sto a non-nullable value typeT, a lifted conversion operator exists that converts fromS?toT?. This lifted conversion operator performs an unwrapping fromS?toSfollowed by the user-defined conversion fromStoT, followed by a wrapping fromTtoT?, except that a null-valuedS?converts directly to a null-valuedT?.A lifted conversion operator has the same implicit or explicit classification as its underlying user-defined conversion operator. The term "user-defined conversion" applies to the use of both user-defined and lifted conversion operators.
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