问题
I have a simple question. if we cast some null variable to a type. I expect compiler to throw Some Exception, but it is not. And I realy want to know the reason why. I mean
string sample1 = null as string;
string sample2 = (string)null;
object t1 = null;
TestClass t2 = (TestClass)t1;
maybe in the first one, as operator handles the exception handling. But others samples must throw exception. How compiler handles these situations , maybe since the variables are null , it does not perform cast operation? Cause if it really cast a null pointer it must be an error.
回答1:
According to the documentation (Explicit conversions) you can cast from a base type to a derived type.
Since null is a valid value for all reference types, as long as the cast route exists you should be fine.
object null → TestClass null works as object is a superclass to all reference types.
However, if you try string null → TestClass null (Assuming TestClass is not a subtype of string), you will find a compilation error as TestClass is not a derived type of string.
来源：https://stackoverflow.com/questions/41219501/cast-null-value-to-a-type