Where does .NET place the String value?

.net, clr

Solution

At m_firstChar. The heap allocation is large enough to fit the entire string, not just the first character. Easy to see in Visual Studio as well:

class Program {
    static void Main(string[] args) {
        string s = "hello" + "world";
    }  // <=== Breakpoint here
}

When the breakpoint hits, use Debug + Windows + Memory + Memory1. In the Address box type s. You'll see:

0x01B3F6BC  e8 0a 67 6e 0b 00 00 00 0a 00 00 00 68 00 65 00  è.gn........h.e.
0x01B3F6CC  6c 00 6c 00 6f 00 77 00 6f 00 72 00 6c 00 64 00  l.l.o.w.o.r.l.d.

- The object starts at the address - 4, the syncblk is stored there (not visible).

- Next 4 bytes is the method table pointer (0x6e670ae8, aka type handle).

- Next 4 bytes is the m_arrayLength member, the allocated size of the string (0x0b).

- Next 4 bytes is the m_stringLength member, the actual number of characters in the string (0x0a).

- Next bytes store the string, starting at m_firstChar.

This is for .NET 3.5 SP1. You won't see the m_arrayLength member in .NET 4.0 and up, the field was removed.

Problem

I am using SOS debug extension dll to check the memory layout of a String type, and below is the result. !dso ``` ESP/REG Object Name 0015EFC0 01c6b9cc System.String hello,world ``` !do 01c6b9cc ``` Name: System.String MethodTable: 6de3f9ac EEClass: 6db78bb0 Size: 36(0x24) bytes File: C:\Windows\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089>\mscorlib.dll String: hello,world Fields: MT Field Offset Type VT Attr Value Name 6de42978 40000ed 4 System.Int32 1 instance 11 m_stringLength 6de41dc8 40000ee 8 System.Char 1 instance 68 m_firstChar 6de3f9ac 40000ef 8 System.String 0 shared static Empty >> Domain:Value 00331488:01c61228 << ``` Now I am wondering, where exactly is the string value "hello world" stored? Thanks.

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