Does string comparison == only work because strings are immutable?

Question

I had a thought before when comparing two strings with their variables:

string str1 = "foofoo";
string strFoo = "foo";
string str2 = strFoo + strFoo;

// Even thought str1 and str2 reference 2 different
//objects the following assertion is true.

Debug.Assert(str1 == str2);

Is this purely because the .NET runtime recognises the string's value is the same and because strings are immutable makes the reference of str2 equal to that of str1?

So when we do str1 == str2 we are actually comparing references and not the values? I originally thought this was the product of syntactic sugar, but was I being incorrect?

Any inaccuracies with what I've written?

Answer 1

The answer is in the C# Spec §7.10.7

The string equality operators compare string values rather than string references. When two separate string instances contain the exact same sequence of characters, the values of the strings are equal, but the references are different. As described in §7.10.6, the reference type equality operators can be used to compare string references instead of string values.

Answer 2

No.

== works because the String class overloads the == operator to be equivalent to the Equals method.

From Reflector:

[TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries")]
public static bool operator ==(string a, string b)
{
    return Equals(a, b);
}