Why do mainstream OO languages not have immutability on class-level built-in? [duplicate]

Question

I regularly write classes which can only have immutable instances, much like string.

I am wondering why Java or C# or VB.NET don't have immutability built-in into the language? That way, I can communicate immutability better to my fellow programmers and also make assumptions about state in the class.

For instance: immutable class Foo { public string Name { get; set; } //compile error: mutable property on immutable class }

Answer 1

C# does have immutability built in. So does Java, and VB.NET.

Mutable:

public class Counter
{
   private int count;

   public Counter(int initialCount)
   {
       count = initialCount;
   }

   public void Increment()
   {
       count++;
   }
}

Immutable:

public class Counter
{
   private readonly int count;  // `final` for Java

   public Counter(int finalCount)
   {
       // set at construction time, but readonly, so can't modify.
       count = finalCount;
   }

   public void Increment()
   {
       // Invalid operation
       count++;
   }
}

Note that I'm using "immutability" hear to mean "set once during object construction." Constants are not very useful in this context, because they can only be set at compile-time.

If you dig deep enough, every programming language mutates state under the hood, because that's how the processor instructions are designed.