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

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?

Solution 1

Strings aren't value types since they can be huge, and need to be stored on the heap. Value types are (in all implementations of the CLR as of yet) stored on the stack. Stack allocating strings would break all sorts of things: the stack is only 1MB for 32-bit and 4MB for 64-bit, you'd have to box each string, incurring a copy penalty, you couldn't intern strings, and memory usage would balloon, etc...

(Edit: Added clarification about value type storage being an implementation detail, which leads to this situation where we have a type with value sematics not inheriting from System.ValueType. Thanks Ben.)

Solution 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.

Solution 3

A string is a reference type with value semantics. This design is a tradeoff which allows certain performance optimizations.

The distinction between reference types and value types are basically a performance tradeoff in the design of the language. Reference types have some overhead on construction and destruction and garbage collection, because they are created on the heap. Value types on the other hand have overhead on assignments and method calls (if the data size is larger than a pointer), because the whole object is copied in memory rather than just a pointer. Because strings can be (and typically are) much larger than the size of a pointer, they are designed as reference types. Furthermore the size of a value type must be known at compile time, which is not always the case for strings.

But strings have value semantics which means they are immutable and compared by value (i.e. character by character for a string), not by comparing references. This allows certain optimizations:

Interning means that if multiple strings are known to be equal, the compiler can just use a single string, thereby saving memory. This optimization only works if strings are immutable, otherwise changing one string would have unpredictable results on other strings.

String literals (which are known at compile time) can be interned and stored in a special static area of memory by the compiler. This saves time at runtime since they don't need to be allocated and garbage collected.

Immutable strings does increase the cost for certain operations. For example you can't replace a single character in-place, you have to allocate a new string for any change. But this is a small cost compared to the benefit of the optimizations.

Value semantics effectively hides the distinction between reference type and value types for the user. If a type has value semantics, it doesn't matter for the user if the type is a value type or reference type - it can be considered an implementation detail.

Solution 4

This is a late answer to an old question, but all other answers are missing the point, which is that .NET did not have generics until .NET 2.0 in 2005.

String is a reference type instead of a value type because it was of crucial importance for Microsoft to ensure that strings could be stored in the most efficient way in non-generic collections, such as System.Collections.ArrayList.

Storing a value-type in a non-generic collection requires a special conversion to the type object which is called boxing. When the CLR boxes a value type, it wraps the value inside a System.Object and stores it on the managed heap.

Reading the value from the collection requires the inverse operation which is called unboxing.

Both boxing and unboxing have non-negligible cost: boxing requires an additional allocation, unboxing requires type checking.

Some answers claim incorrectly that string could never have been implemented as a value type because its size is variable. Actually it is easy to implement string as a fixed-length data structure containing two fields: an integer for the length of the string, and a pointer to a char array. You can also use a Small String Optimization strategy on top of that.

If generics had existed from day one I guess having string as a value type would probably have been a better solution, with simpler semantics, better memory usage and better cache locality. A List<string> containing only small strings could have been a single contiguous block of memory.

Solution 5

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.

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Comments

[jason]

Java is not known for being pithy.

[Davy8]

Good information, but I think a misinterpretation of the question

[jason]

@WebMatrix, @Davy8: The primitive types (int, double, bool, ...) are immutable.

[WebMatrix]

@Jason, I thought immutable term mostly apply to objects (reference types) which can not change after initialization, like strings when strings value changes, internally a new instance of a string is created, and original object remains unchanged. How does this apply to value types?

[nikolas]

I'm nitpicking here, but only because it gives me an opportunity to link to an blog post relevant to the question: value types are not necessarily stored on the stack. It's most often true in ms.net, but not at all specified by the CLI specification. The main difference between value and reference types is, that reference types follow copy-by-value semantics. See docs.microsoft.com/en-us/archive/blogs/ericlippert/… and docs.microsoft.com/en-us/archive/blogs/ericlippert/…

[Marc Gravell]

There are plenty of examples of immutable reference-types. And re the string example, that is indeed pretty-much guaranteed under the current implementations - technically it is is per module (not per-assembly) - but that is almost always the same thing...

[Marc Gravell]

Re the last point: StringBuilder doesn't help if you trying to pass a large string (since it is actually implemented as a string anyway) - StringBuilder is useful for manipulating a string multiple times.

[Prasanth Kumar]

Somehow, in "int n = 4; n = 9;", it's not that your int variable is "immutable", in the sense of "constant"; it's that the value 4 is immutable, it doesn't change to 9. Your int variable "n" first has a value of 4 and then a different value, 9; but the values themselves are immutable. Frankly, to me this is very close to wtf.

[abhijeet nigoskar]

@Matt: exactly. When I switched over to C# this was kind of confusing, since I always used (an do still sometimes) .equals(..) for comparing strings while my teammates just used "==". I never understood why they didn't leave the "==" to compare the references, although if you think, 90% of the time you'll probably want to compare the content not the references for strings.

[Qwertie]

Not to mention, strings are variable-size, so they can't be value types (as value types are stored directly wherever you declare them). When you declare a string inside a class, how could the class hold the string directly, given that one can change the string to another string of different length at any time? No, there would have to be a REFERENCE to the string because it is variable-size.

[codekaizen]

@Qwertie: String is not variable size. When you add to it, you are actually creating another String object, allocating new memory for it.

[Qwertie]

That said, a string could, in theory, have been a value type (a struct), but the "value" would have been nothing more than a reference to the string. The .NET designers naturally decided to cut out the middleman (struct handling was inefficient in .NET 1.0, and it was natural to follow Java, in which strings were already defined as a reference, rather than primitive, type. Plus, if string were a value type then converting it to object would require it to be boxed, a needless inefficiency).

[Qwertie]

@codekaizen: String variables are mutable and therefore variable-size.

[codekaizen]

@Qwertie: A variable doesn't have a size (except if you are talking about the size of the reference, but even if you are, it is always the same). What actually takes up the memory is the object.

[Jon Hanna]

+1. I'm sick of hearing this "strings are like value types" when they quite simply aren't.

[Michael]

@Juri: Actually i think it's never desirable to check the references, since sometimes new String("foo"); and another new String("foo") can evaluate in the same reference, which kind of is not what you would expect a new operator to do. (Or can you tell me a case where I would want to compare the references?)

[Kevin Brock]

@codekaizen Qwertie is right but I think the wording was confusing. One string may be a different size than another string and thus, unlike a true value type, the compiler could not know beforehand how much space to allocate to store the string value. For instance, an Int32 is always 4 bytes, thus the compiler allocates 4 bytes any time you define a string variable. How much memory should the compiler allocate when it encounters an int variable (if it were a value type)? Understand that the value has not been assigned yet at that time.

[Kevin Brock]

Sorry, a typo in my comment that I cannot fix now; that should have been.... For instance, an Int32 is always 4 bytes, thus the compiler allocates 4 bytes any time you define an int variable. How much memory should the compiler allocate when it encounters a string variable (if it were a value type)? Understand that the value has not been assigned yet at that time.

[codekaizen]

@KevinBrock - first, the compiler doesn't manage stack space, the runtime does. This means you can dynamically allocate stack space. You can stackallocate arrays with a size not known until runtime for example. Given this, and that string instances are immutable and the size doesn't change after allocation (even though the size may not be known until runtime), it is conceivable that strings could be stack allocated and therefore be value types.

[Kevin Brock]

@codekaizen As far as I know you can do this only in an unsafe context and the stack allocated array is fixed size. Yes the runtime can choose to rearrange things but the compiler defines how big the allocation on the stack is, thus a value type string must have a known size or use unsafe pointer management (for dynamically sized stack allocation). Are you proposing then that strings should be unsafe?

[Kevin Brock]

@codekaizon Of course the compiler could do special things for such a string and then it would not have to be unsafe, but then you are proposing a special kind of value type for string (not what struct is today) which would be another reason for defining string as a class - it can use one of the two main types of definition (struct or class) already defined in the language with out more "magic" (already plenty of that in the compiler for the existing string class).

[codekaizen]

@KevinBrock - my point is that the language doesn't even need to expose the workings in our hypothetical case, which you seem to realize in your second post. The compiler does special things with special types all the time, and strings would just be another case if they were value types. However, if they were value types, the runtime would need to change drastically, as my answer above enumerates. My point in debating this is to show that Qwertie's statements about strings continue to be inaccurate, even under your interpretation.

[supercat]

@Davy8: Value types inherit mutability, or lack thereof, from the location in which they're stored. If a value type has any fields--public or private--that can ever take on a non-default value, those fields will be mutable for instances stored in mutable locations, and immutable for instances stored in immutable locations. Some so-called "immutable" value types may require one to rewrite all fields whenever one rewrites any, but that doesn't make them immutable.

[Jon Hanna]

@Michael Well, you have to include a reference comparison in all comparisons to catch comparison with null. Another good place to compare references with strings, is when comparing rather than equality-comparing. Two equivalent strings, when compared should return 0. Checking for this case though takes as long as running through the whole comparison anyway, so is not a useful short-cut. Checking for ReferenceEquals(x, y) is a fast test and you can return 0 immediately, and when mixed in with your null-test doesn't even add any more work.

[supercat]

@Jason: If string were implemented as a value type with a single field of type char[] or PrivateStringData (the latter being a class type which was private to the module which defined the structure), most things would work as they do now; the difference would be that unless strings had special boxing rules, a boxed string would be mutable (note that all boxed structs, even supposedly "immutable" structs--are nullable*), though mutating a boxed string would cause it to reference a different heap object internally, rather than mutating the heap object itself). On the other hand, ...

[supercat]

...having strings be a value type of of that style rather than being a class type would mean the default value of a string could behave as an empty string (as it was in pre-.net systems) rather than as a null reference. Actually, my own preference would be to have a value type String which contained a reference-type NullableString, with the former having a default value equivalent to String.Empty and the latter having a default of null, and with special boxing/unboxing rules (such that boxing a default-valued NullableString would yield a reference to String.Empty).

[codekaizen]

"string size is not known before it is allocated " - this is incorrect in the CLR.

[Servy]

The distinction between value types and reference types isn't really about performance at all. It's about whether a variable contains an actual object or a reference to an object. A string could never possibly be a value type because the size of a string is variable; it would need to be constant to be a value type; performance has almost nothing to do with it. Reference types are also not expensive to create at all.

[JacquesB]

@Sevy: The size of a string is constant.

[Servy]

Because it just contains a reference to a character array, which is of variable size. Having a value type who's only real "value" was a reference type would just be all the more confusing, as it would still have reference semantics for all intensive purposes.

[JacquesB]

@Sevy: The size of an array is constant.

[Servy]

The size of a reference to an array is constant. The size of an array itself is dependent on the number of items in the array and the size of the type the array holds.

[Servy]

Once you have created an array it's size is constant, but all arrays in the entire world are not all of exactly the same size. That's my point. For a string to be a value type all strings in existence would need to all be exactly the same size, because that's how value types are designed in .NET. It needs to be able to reserve storage space for such value types before actually having a value, so the size must be know at compile time. Such a string type would need to have a char buffer of some fixed size, which would be both restrictive and highly inefficient.

[JacquesB]

Ah, now I get what you are saying. Yes, the size of a string is not necessarily known at compile time. And .net does not support dynamically typed arrays on the stack.

[symbiont]

@Jon Hanna a reference comparison speeds up the case where the strings are equal and happen to be the same object (so it is an improvement). i expect the .net guys to be smart enough to have used this, when implementing the "==" operator for strings.

[Jon Hanna]

@symbiont it does, and strings to tend generally to be a case where this benefits. More generally, the benefit depends on how likely comparison with self is to happen, though the fact that most more detailed comparisons would fail on null meaning a check for the possibility of x == null && y == null has to be in there somewhere if test for ReferenceEquals(x, y) has not already been done, means there's little downside to doing a reference-equals test for all such types. I was talking about the generalisation of this, which is that for ordered comparisons (.CompareTo() and .Compare()...

[Jon Hanna]

@symbiont ... then the shortcut if(ReferenceEquals(x, y)) return 0; is also always valid, and sometimes useful, not only does identity entail equality (why if(ReferenceEquals(x, y)) return true; works for .Equals()) but also equality entails equivalence for most orderings, and identity entails equivalence for all of them. The built-in string comparisons will use this short-cut some of the time, but not all.

[Asad Saeeduddin]

@codekaizen The stack allocation for value-type variables is independent of their assignment. Space is allocated for a stack frame when the method begins, not when the method is actually running, so I'm not sure how this dynamic allocation idea would pan out.

[siride]

@codekaizen: and how would this work for strings that are members of classes? If a string member was reassigned, would the entire object be resized? It's unmanageable.

[ρяσѕρєя K]

This should be a comment

[Abou-Emish]

do you mean that b evauates "=" to false ? b is true because the reference of the two variables are the same

[Lucas]

"You couldn't intern strings" , String.Intern() ??

[codekaizen]

@I'mBlueDaBaDee - right; this method would not work if System.String were a System.ValueType, since it would not be possible to track a single instance, as any reference to the instance would copy it.

[LONG]

easier to understand for ppl new to c#

[CodingYoshi]

@BenSchwehn You say: reference types follow copy-by-value semantics. Are you sure about that or do you mean value types follow copy-by-value semantics?

[jrandomuser]

Literals are interned so that should be true.

[V0ldek]

My, thanks for this answer! I've been looking at all the other answers saying things about heap and stack allocations, while stack is an implementation detail. After all, string contains only its size and a pointer to the char array anyway, so it wouldn't be a "huge value type". But this is a simple, relevant reason for this design decision. Thanks!

[JacquesB]

@V0ldek: This is not true though, a string object in .net does not contain a pointer to a separately allocated character array. The size and the characters are stored in the same place.

[V0ldek]

@JacquesB I was judging that by the type definition in the BCL. It just has the size and the first char. I might be wrong though, that entire class is just some magic native interop.

[JacquesB]

@V0ldek: Notice the _firstChar field is not a pointer, it is a char. The rest of the chars (if any) are located directly after. But yes, lots of magic going on.

[jmjohnson85]

@BenSchwehn According to the article you linked: Surely the most relevant fact about value types is not the implementation detail of how they are allocated, but rather the by-design semantic meaning of “value type”, namely that they are always copied “by value”.