一个字符串在x64中占用多少字节？

Question

For the purpose of learning, I'm trying to understand how C# strings are internally stored in memory.

According to this blog post , C# string size is (x64 with .NET framework 4.0) :

26 + 2 * length

A string with a single character will take (26 + 2 * 1) / 8 * 8 = 32 bytes . This is indeed similar to what I measured.

What puzzle me is what is in that 26 bytes overhead.

I have run the following code and inspected memory :

string abc = "abcdeg";
string aaa = "x";
string ccc = "zzzzz";

AFAIK those blocks are the following :

• Green : Sync block (8 bytes)
• Cyan : Type info (8 bytes)
• Yellow : Length (4 bytes)
• Pink : The actual characters : 2 bytes per char + 2 bytes for NULL terminator.

Look at the "x" string. It is indeed 32 bytes (as calculated).

Anyway it looks like the end of the string if padded with zeroes. The "x" string could end up after the two bytes for NULL terminator and still be memory aligned (thus being 24 bytes). Why do we need an extra 8 bytes ?

I have experimented similar results with other (bigger) string sizes. It looks like there is always an extra 8 bytes.

Answer 1

As Hans Passant suggested, there is an extra field added at the end of string object which is 4 bytes (in x64, it might require another 4 bytes extra, for padding).

So in the end we have :

= 8 (sync) + 8 (type) + 4 (length) + 4(extra field) + 2 (null terminator) + 2 * length 
= 26 + 2 * length

So Jon Skeet's blog post was right (how could it be wrong ?)