c# - Why string pointer position is different?

str_01 holds a reference to constant string. StringBuilder however builds string instances dynamically, so the returned string instance is not referentially the same instance as the constant string with the same content. System.Object.ReferenceEquals() will return false.

Since the str_01 is a reference to a constant string, its data is probably stored in a data section of the executable, which always gets the same address in the application virtual address space.

Edit:

You can see the "my string" text in UTF-8 encoding when you open the compiled .exe file using PE.Explorer or similar software. It is present in the .data section of the file, including a preferred Virtual Address where the section should be loaded in process virtual memory.

I have however not been able to reproduce that str_01 has a same address on multiple runs of the application, probably because my x64 Windows 8.1 performs Address space layout randomization (ASLR). Because of that, all pointers will be different across multiple runs of the application, even those that point directly to loaded PE sections.