Strings in .NET are reference types, so they are always on the heap (even when they are interned). You can verify this using a debugger such as WinDbg.
If you have the class below
class SomeType {
public void Foo() {
string s = "hello world";
Console.WriteLine(s);
Console.WriteLine("press enter");
Console.ReadLine();
}
}
And you call Foo() on an instance, you can use WinDbg to inspect the heap.
The reference will most likely be stored in a register for a small program, so the easiest is to find the reference to the specific string is by doing a !dso. This gives us the address of our string in question:
0:000> !dso
OS Thread Id: 0x1660 (0)
ESP/REG Object Name
002bf0a4 025d4bf8 Microsoft.Win32.SafeHandles.SafeFileHandle
002bf0b4 025d4bf8 Microsoft.Win32.SafeHandles.SafeFileHandle
002bf0e8 025d4e5c System.Byte[]
002bf0ec 025d4c0c System.IO.__ConsoleStream
002bf110 025d4c3c System.IO.StreamReader
002bf114 025d4c3c System.IO.StreamReader
002bf12c 025d5180 System.IO.TextReader+SyncTextReader
002bf130 025d4c3c System.IO.StreamReader
002bf140 025d5180 System.IO.TextReader+SyncTextReader
002bf14c 025d5180 System.IO.TextReader+SyncTextReader
002bf15c 025d2d04 System.String hello world // THIS IS THE ONE
002bf224 025d2ccc System.Object[] (System.String[])
002bf3d0 025d2ccc System.Object[] (System.String[])
002bf3f8 025d2ccc System.Object[] (System.String[])
Now use !gcgen to find out which generation the instance is in:
0:000> !gcgen 025d2d04
Gen 0
It’s in generation zero – i.e. it has just be allocated. Who’s rooting it?
0:000> !gcroot 025d2d04
Note: Roots found on stacks may be false positives. Run "!help gcroot" for
more info.
Scan Thread 0 OSTHread 1660
ESP:2bf15c:Root:025d2d04(System.String)
Scan Thread 2 OSTHread 16b4
DOMAIN(000E4840):HANDLE(Pinned):6513f4:Root:035d2020(System.Object[])->
025d2d04(System.String)
The ESP is the stack for our Foo() method, but notice that we have a object[] as well. That’s the intern table. Let’s take a look.
0:000> !dumparray 035d2020
Name: System.Object[]
MethodTable: 006984c4
EEClass: 00698444
Size: 528(0x210) bytes
Array: Rank 1, Number of elements 128, Type CLASS
Element Methodtable: 00696d3c
[0] 025d1360
[1] 025d137c
[2] 025d139c
[3] 025d13b0
[4] 025d13d0
[5] 025d1400
[6] 025d1424
...
[36] 025d2d04 // THIS IS OUR STRING
...
[126] null
[127] null
I reduced the output somewhat, but you get the idea.
In conclusion: strings are on the heap – even when they are interned. The interned table holds a reference to the instance on the heap. I.e. interned strings are not collected during GC because the interned table roots them.