Why is string a reference type?

Question

Why is string a reference type, even though it's normally primitive data type such as int, float, or double.

Answer 1

In addition to the reasons posted by Dan:

Value types are, by definition those types which store their values in themselves, rather than referring to a value somewhere else. That's why value types are called "value types" and reference types are called "reference types". So your question is really "why does a string refer to its contents rather than simply containing its contents?"

It's because value types have the nice property that every instance of a given value type is of the same size in memory.

So what? Why is this a nice property? Well, suppose strings were value types that could be of any size and consider the following:

string[] mystrings = new string[3];

What are the initial contents of that array of three strings? There is no "null" for value types, so the only sensible thing to do is to create an array of three empty strings. How would that be laid out in memory? Think about that for a bit. How would you do it?

Now suppose you say

string[] mystrings = new string[3];
mystrings[1] = "hello";

Now we have "", "hello" and "" in the array. Where in memory does the "hello" go? How large is the slot that was allocated for mystrings[1] anyway? The memory for the array and its elements has to go somewhere.

This leaves the CLR with the following choices:

• resize the array every time you change one of its elements, copying the entire thing, which could be megabytes in size
• disallow creating arrays of value types of unknown size
• disallow creating value types of unknown size

The CLR team chose the latter one. Making strings into reference types means that you can create arrays of them efficiently.

Answer 2

Yikes, this answer got accepted and then I changed it. I should probably include the original answer at the bottom since that's what was accepted by the OP.

New Answer

Update: Here's the thing. string absolutely needs to behave like a reference type. The reasons for this have been touched on by all answers so far: the string type does not have a constant size, it makes no sense to copy the entire contents of a string from one method to another, string[] arrays would otherwise have to resize themelves -- just to name a few.

But you could still define string as a struct that internally points to a char[] array or even a char* pointer and an int for its length, make it immutable, and voila!, you'd have a type that behaves like a reference type but is technically a value type.

This would seem quite silly, honestly. As Eric Lippert has pointed out in a few of the comments to other answers, defining a value type like this is basically the same as defining a reference type. In nearly every sense, it would be indistinguishable from a reference type defined the same way.

So the answer to the question "Why is string a reference type?" is, basically: "To make it a value type would just be silly." But if that's the only reason, then really, the logical conclusion is that string could actually have been defined as a struct as described above and there would be no particularly good argument against that choice.

However, there are reasons that it's better to make string a class than a struct that are more than purely intellectual. Here are a couple I was able to think of:

To prevent boxing

If string were a value type, then every time you passed it to some method expecting an object it would have to be boxed, which would create a new object, which would bloat the heap and cause pointless GC pressure. Since strings are basically everywhere, having them cause boxing all the time would be a big problem.

For intuitive equality comparison

Yes, string could override Equals regardless of whether it's a reference type or value type. But if it were a value type, then ReferenceEquals("a", "a") would return false! This is because both arguments would get boxed, and boxed arguments never have equal references (as far as I know).

So, even though it's true that you could define a value type to act just like a reference type by having it consist of a single reference type field, it would still not be exactly the same. So I maintain this as the more complete reason why string is a reference type: you could make it a value type, but this would only burden it with unnecessary weaknesses.

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Original Answer

It's a reference type because only references to it are passed around.

If it were a value type then every time you passed a string from one method to another the entire string would be copied*.

Since it is a reference type, instead of string values like "Hello world!" being passed around -- "Hello world!" is 12 characters, by the way, which means it requires (at least) 24 bytes of storage -- only references to those strings are passed around. Passing around a reference is much cheaper than passing every single character in a string.

Also, it's really not a normal primitive data type. Who told you that?

_{*Actually, this isn't stricly true. If the string internally held a char[] array, then as long as the array type is a reference type, the contents of the string would actually not be passed by value -- only the reference to the array would be. I still think this is basically right answer, though.}