In C#, why is String a reference type that behaves

Question

A String is a reference type even though it has most of the characteristics of a value type such as being immutable and having == overloaded to compare the text rather than making sure they reference the same object.

Why isn't string just a value type then?

Answer 1

It is mainly a performance issue.

Having strings behave LIKE value type helps when writing code, but having it BE a value type would make a huge performance hit.

For an in-depth look, take a peek at a nice article on strings in the .net framework.

Answer 2

It is not a value type because performance (space and time!) would be terrible if it were a value type and its value had to be copied every time it were passed to and returned from methods, etc.

It has value semantics to keep the world sane. Can you imagine how difficult it would be to code if

string s = "hello";
string t = "hello";
bool b = (s == t);

set b to be false? Imagine how difficult coding just about any application would be.

Answer 3

Not only strings are immutable reference types. Multi-cast delegates too. That is why it is safe to write

protected void OnMyEventHandler()
{
     delegate handler = this.MyEventHandler;
     if (null != handler)
     {
        handler(this, new EventArgs());
     }
}

I suppose that strings are immutable because this is the most safe method to work with them and allocate memory. Why they are not Value types? Previous authors are right about stack size etc. I would also add that making strings a reference types allow to save on assembly size when you use the same constant string in the program. If you define

string s1 = "my string";
//some code here
string s2 = "my string";

Chances are that both instances of "my string" constant will be allocated in your assembly only once.

If you would like to manage strings like usual reference type, put the string inside a new StringBuilder(string s). Or use MemoryStreams.

If you are to create a library, where you expect a huge strings to be passed in your functions, either define a parameter as a StringBuilder or as a Stream.

Answer 4

At the risk of getting yet another mysterious down-vote...the fact that many mention the stack and memory with respect to value types and primitive types is because they must fit into a register in the microprocessor. You cannot push or pop something to/from the stack if it takes more bits than a register has....the instructions are, for example "pop eax" -- because eax is 32 bits wide on a 32-bit system.

Floating-point primitive types are handled by the FPU, which is 80 bits wide.

This was all decided long before there was an OOP language to obfuscate the definition of primitive type and I assume that value type is a term that has been created specifically for OOP languages.

Answer 5

In a very simple words any value who has a definite size can be treated as a value type.

Answer 6

Actually strings have very few resemblances to value types. For starters, not all value types are immutable, you can change the value of an Int32 all you want and it it would still be the same address on the stack.

Strings are immutable for a very good reason, it has nothing to do with it being a reference type, but has a lot to do with memory management. It's just more efficient to create a new object when string size changes than to shift things around on the managed heap. I think you're mixing together value/reference types and immutable objects concepts.

As far as "==": Like you said "==" is an operator overload, and again it was implemented for a very good reason to make framework more useful when working with strings.