class string is immutable. It is impossible implementing an immutable struct type as you can't disable or override assignment operation (=). Even you define a readonly struct, you can always change the value of the struct type by using assignment operator.
class string
=
For example, x,y are values of a readonly struct. x = y; will change the content of x. So, the readonly struct section in C# language reference is not correct. Is this right?
x,y
x = y;
x