Why strings behave like ValueType

Question

I was perplexed after executing this piece of code, where strings seems to behave as if they are value types. I am wondering whether the assignment operator is operating on

Answer 1

   a2 = "XYZ";

That's syntax sugar, provided by the compiler. A more accurate representation of this statement would be:

   a2 = CreateStringObjectFromLiteral("XYZ")

which explains how a2 simply gets a reference to a new string object and answers your question. The actual code is highly optimized because it is so common. There's a dedicated opcode available for it in IL:

   IL_0000:  ldstr      "XYZ"

String literals are collected into a table inside the assembly. Which allows the JIT compiler to implement the assignment statement very efficiently:

   00000004  mov         esi,dword ptr ds:[02A02088h] 

A single machine code instruction, can't beat that. More so: one very notable consequence is that the string object doesn't live on the heap. The garbage collector doesn't bother with it since it recognizes that the address of the string reference isn't located in the heap. So you don't even pay for collection overhead. Can't beat that.

Also note that this scheme easily allows for string interning. The compiler simply generates the same LDSTR argument for an identical literal.