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C# String - things you may not know

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We use string very often in programming. "A" + "B", such things like this.
But have you ever think about that is string maybe not value type?

Value Type or Reference Type?

Let us see an example:

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int intA = 1;
int intB = intA;
Console.WriteLine(int.ReferenceEquals(intA, intA)); // False
Console.WriteLine(int.ReferenceEquals(intA, intB)); // False

int is clearly value type, they are being boxed when you call object.ReferenceEquals.
Each integer is boxed inside an object instance.
Thus, this is actually comparing references between two boxed values, which clearly aren’t equal.
And how about string?

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string strA = "aa";
string strB = strA;
Console.WriteLine(string.ReferenceEquals(strA, strA)); // True
Console.WriteLine(string.ReferenceEquals(strA, strB)); // True

We can see that strA and strB share same reference.
Now you may be considering string as some sort of char[].
But in reality, modifying strA does not affect strB, right?

Mutable or Immutable?

Lets see another example:

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string strA = "AA";
string strB = strA;
Console.WriteLine(strA); // AA
Console.WriteLine(strB); // AA
Console.WriteLine(string.ReferenceEquals(strA, strB)); //True

strA += "BB";
Console.WriteLine(strA); // AABB
Console.WriteLine(strB); // AA
Console.WriteLine(string.ReferenceEquals(strA, strB)); //False

char[] is mutable, which means the value can be modified after assignment.
Instead, string is immutable, you cannot change the value since it was been initialized.
So what strA += "BB" actually does is creating a new string object "AABB" re-assigned to strA.

String Pool

string is very commonly used in C# programming, there is a mechanism called string pool.
Which collects new created string instances for reusing to minimize memory allocations.

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string strA = "AA";
string strB = "AA";
Console.WriteLine(string.ReferenceEquals(strA, strB)); //True

strB points to the same "AA" instance as strA, since "AA" has been created and collected in string pool.

Reference

https://github.com/microsoft/referencesource/blob/master/mscorlib/system/string.cs
https://docs.microsoft.com/en-us/windows/communitytoolkit/high-performance/stringpool
https://wiztone.github.io/2019/02/18/string-YTYKBYD/