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copied!<p>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. </p> <p>If you have the class below</p> <pre><code> class SomeType { public void Foo() { string s = "hello world"; Console.WriteLine(s); Console.WriteLine("press enter"); Console.ReadLine(); } } </code></pre> <p>And you call <code>Foo()</code> on an instance, you can use WinDbg to inspect the heap.</p> <p>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 <code>!dso</code>. This gives us the address of our string in question:</p> <pre><code>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[]) </code></pre> <p>Now use <code>!gcgen</code> to find out which generation the instance is in:</p> <pre><code>0:000> !gcgen 025d2d04 Gen 0 </code></pre> <p>It's in generation zero - i.e. it has just be allocated. Who's rooting it?</p> <pre><code>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) </code></pre> <p>The ESP is the stack for our <code>Foo()</code> method, but notice that we have a <code>object[]</code> as well. That's the intern table. Let's take a look.</p> <pre><code>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 </code></pre> <p>I reduced the output somewhat, but you get the idea. </p> <p><strong>In conclusion</strong>: 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.</p>

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