How do castings really work at the CLR level?

Question

When you do upcast or downcast, what really happens behind the scenes? I had the idea that when doing something like:

string myString = "abc";
object myObject = myString;
string myStringBack = (string)myObject;

The listing on the last line will only have the purpose of telling the compiler that we are safe, we are not doing anything wrong. So, I had the idea that the cast code will not be embedded on the code itself. I seem to be 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 options for downcast:

• You require castclass something. When you hit a line of code that executes castclass, the CLR tries to throw. But then, what happens if I overshadow a string-style string and try to run the code?
• castclass. , , ( , - ).

, statamente # 4.0 ?

Upcasting and downcasting between compatible reference types performs reference
conversions: a new reference is created that points to the same object.

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Answer 1

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{
    object x = 123;
    object y = (string)x;
}
catch(InvalidCastException ex)
{ ... }

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Answer 2

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object foo = "bar";
string baz = (string)foo;

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foo = "bim";

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