// Simple mutable structure.
// Method IncrementI mutates current state.
struct Mutable
{
public Mutable(int i) : this()
{
I = i;
}
public void IncrementI() { I++; }
public int I { get; private set; }
}
// Simple class that contains Mutable structure
// as readonly field
class SomeClass
{
public readonly Mutable mutable = new Mutable(5);
}
// Simple class that contains Mutable structure
// as ordinary (non-readonly) field
class AnotherClass
{
public Mutable mutable = new Mutable(5);
}
class Program
{
void Main()
{
// Case 1. Mutable readonly field
var someClass = new SomeClass();
someClass.mutable.IncrementI();
// still 5, not 6, because SomeClass.mutable field is readonly
// and compiler creates temporary copy every time when you trying to
// access this field
Console.WriteLine(someClass.mutable.I);
// Case 2. Mutable ordinary field
var anotherClass = new AnotherClass();
anotherClass.mutable.IncrementI();
// Prints 6, because AnotherClass.mutable field is not readonly
Console.WriteLine(anotherClass.mutable.I);
}
}
Mutable[] arrayOfMutables = new Mutable[1];
arrayOfMutables[0] = new Mutable(5);
// Now we actually accessing reference to the first element
// without making any additional copy
arrayOfMutables[0].IncrementI();
// Prints 6!!
Console.WriteLine(arrayOfMutables[0].I);
// Every array implements IList<T> interface
IList<Mutable> listOfMutables = arrayOfMutables;
// But accessing values through this interface lead
// to different behavior: IList indexer returns a copy
// instead of an managed reference
listOfMutables[0].IncrementI(); // Should change I to 7
// Nope! we still have 6, because previous line of code
// mutate a copy instead of a list value
Console.WriteLine(listOfMutables[0].I);
struct Person {
public string name; // mutable
public Point position = new Point(0, 0); // mutable
public Person(string name, Point position) { ... }
}
Person eric = new Person("Eric Lippert", new Point(4, 2));
struct Mutable {
public int x;
}
class Test {
private Mutable m = new Mutable();
public int mutate()
{
m.x = m.x + 1;
return m.x;
}
}
static void Main(string[] args) {
Test t = new Test();
System.Console.WriteLine(t.mutate());
System.Console.WriteLine(t.mutate());
System.Console.WriteLine(t.mutate());
}
