Question

In the code below, I’m checking the equality of object references.

string x = "Some Text";
string y = "Some Other Text";
string z = "Some Text";

Console.WriteLine(object.ReferenceEquals(x, y)); // False
Console.WriteLine(object.ReferenceEquals(x, z)); // True
Console.WriteLine(object.ReferenceEquals(y, z)); // False

y = "Some Text";

Console.WriteLine(object.ReferenceEquals(x, y)); // True
Console.WriteLine(object.ReferenceEquals(x, z)); // True
Console.WriteLine(object.ReferenceEquals(y, z)); // True

Here:

• x and z refers to same object; I can say that x is interned and z is used taht version. Well, I’m not sure about this; Please correct me, if I am wrong.
• I changed the value of y by assigning it the same value as x. I thought it is going to create a new object here; but I was wrong, it used the same reference.

My questions are:

• Does .net uses string interns for every string that I use?
• If so, isn’t it hurts the performance?
• If not, how the references became same in above example?

Answer 1

Yes, constant string expressions in the compiler are treated with ldstr, which guarantees interning (via MSDN):

The Common Language Infrastructure (CLI) guarantees that the result of two ldstr instructions referring to two metadata tokens that have the same sequence of characters return precisely the same string object (a process known as “string interning”).

This isn’t every string; it is constant string expressions in your code. For example:

string s = "abc" + "def";

is only 1 string expression – the IL will be a ldstr on “abcdef” (the compiler can compute the composed expression).

This does not hurt performance.

Strings generated at runtime are not interned automatically, for example:

int i = GetValue();
string s = "abc" + i;

Here, “abc” is interned, but “abc8” is not. Also note that:

char[] chars = {'a','b','c'};
string s = new string(chars);
string t = "abc";

note that s and t are different references (the literal (assigned to t) is interned, but the new string (assigned to s) is not).