December 29. 2009 15:29

It isn't that all structs are immutable, you can create a mutable struct. But it is a very bad idea, and all structs in the standard library are immutable.

Here is one reason. Assume for a moment that DateTimeM is a mutable struct equivalent to DateTime, with a mutating method void AddSeconds(int seconds) available. And consider these two cases:

class Foo {
DateTime dt;
DateTimeM dtm;

void AdvanceTime() {
dt = dt.AddSeconds(1);
dtm.AddSeconds(1);
}
}

class Foo1 {
DateTime DT {get; set;};
DateTimeM DTM {get; set;};

void AdvanceTime() {
DT = DT.AddSeconds(1);
DTM.AddSeconds(1);
}
}

AdvanceTime will change dt, DT and dtm correctly; but it won't change DTM! Do you understand why?

Alexey Romanov