How do actually castings work at the CLR level?
- by devoured elysium
When doing an upcast or downcast, what does really happen behind the scenes? I had the idea that when doing something as:
string myString = "abc";
object myObject = myString;
string myStringBack = (string)myObject;
the cast in the last line would have as only purpose tell the compiler we are safe we are not doing anything wrong. So, I had the idea that actually no casting code would be embedded in the code itself. It seems I was wrong:
.maxstack 1
.locals init (
[0] string myString,
[1] object myObject,
[2] string myStringBack)
L_0000: nop
L_0001: ldstr "abc"
L_0006: stloc.0
L_0007: ldloc.0
L_0008: stloc.1
L_0009: ldloc.1
L_000a: castclass string
L_000f: stloc.2
L_0010: ret
Why does the CLR need something like castclass string?
There are two possible implementations for a downcast:
You require a castclass something. When you get to the line of code that does an castclass, the CLR tries to make the cast. But then, what would happen had I ommited the castclass string line and tried to run the code?
You don't require a castclass. As all reference types have a similar internal structure, if you try to use a string on an Form instance, it will throw an exception of wrong usage (because it detects a Form is not a string or any of its subtypes).
Also, is the following statamente from C# 4.0 in a Nutshell correct?
Upcasting and downcasting between compatible reference types performs reference
conversions: a new reference is created that points to the same object.
Does it really create a new reference? I thought it'd be the same reference, only stored in a different type of variable.
Thanks
