Finally I narrowed down the problem.
class Program
{
static void Main(string[] args)
{
int? i = 10;
A a = i;
}
class A
{
A(int i)
{
Value = i;
}
public int Value { get; private set; }
public static implicit operator A(int i)
{
return new A(i);
}
}
}
ReSharper highlights an error on line 7. However as you can imagine on a runtime corresponding implicit operator is called.
I investigated this behaviour and found that actually it is according to the C# spec
Nullable conversions and lifted conversions permit predefined and user-defin ed conversions that operate on non-nullable value types also to be used with nullable forms of those types. Likewise, lifted operators permit predefined and user-defined operators that work for non-nullable value types also to work for nullable forms of those types.
For every predefined conversion from a non-nullable value type S to a non-nullable value type T , a predefined nullable conversion automatically exists from S? to T?. This nullable conversion is a null propagating form of the underlying conversion: It converts a null source value directly to a null target value, but otherwise performs the underlying non-nullable co nversion. Nullable conversions are furthermore provided from S to T? and from S? to T , the latter as an explicit conversion that throws an exception if the source value is null.
So finally I know that it is a ReSharper bug and I will report it on their bug-tracker system.
Potentially it is a good question for juniors on interviews :)
UPD: Raised bug on ReSharper bug tracker
http://youtrack.jetbrains.net/issue/RSRP-287221?projectKey=RSRP&query=lifted
