For a long time I, had a block of code on my web site, that for many was hard to understand:

static unsafe void Main()
    string a = "a";

    fixed (char *p = a)
        p[0] = 'b';

    Console.WriteLine("a"); //"b"

What happens here is a little bit of optimization magic in the .Net CLR. When having two equal strings hardcoded, the address in memory will be the same. This is because .Net uses a string table, aka. an intern pool.

By default, a new string created in runtime does not reference the same address, but you can put (or get the old) string on the pool by using the string.Intern keyword.

Se example:

string a = "A", b = "B";

Object.ReferenceEquals(a, "A").Dump(); //True
var ab = a + b; //"AB"
Object.ReferenceEquals(ab, "AB").Dump(); //False
Object.ReferenceEquals("A" + "B", "AB").Dump(); //True
var internString = string.Intern(a + b).Dump(); //"AB"
Object.ReferenceEquals(internString, "AB").Dump(); //True

String are immutable, but by using the unsafe keyword, you can actually manually change the content of it. Be aware, as this opens for some attacks from libraries (or a colleague that want to mess with you) when it comes to things like reflection and connection strings.