Question : Check for nulls to stop the error - Nullable object must have a value

In the line of code below I want to check if nothing is being passed to User.longVal to stop the error
"Nullable object must have a value"

User.longVal = (double)CurrentUser.longVal;

So if nothing is in the
   (double)CurrentUser.longVal
I want to set a default value  -    

Do I use the ??  directive

Answer : Check for nulls to stop the error - Nullable object must have a value

I believe it should be something like (C# only):

1:	User.longVal = (double)CurrentUser.longVal ?? default_value;