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Why is String a value type that behaves like a reference type in C#?


Even though it has most of the features of a value type, such as being immutable and having == overloaded to compare the text rather than ensuring they reference the same object, a String is a reference type.

Why isn’t string treated as a value type?

Asked by Davy8

Solution #1

Strings aren’t value types since they can be large and require heap storage. Value types are saved on the stack (in all CLR implementations so far). Strings allocated from the stack would break a lot 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 wouldn’t be able to intern strings, and memory use would skyrocket, and so on…

(Edit: Clarified that value type storage is an implementation detail, leading to the circumstance where we have a type with value semantics that do not inherit from System.) ValueType. Thank you, Ben.)

Answered by codekaizen

Solution #2

It isn’t a value type because it would have poor performance (in terms of both space and time!) if it were, and its value would have to be copied every time it was supplied to and returned from methods, etc.

To maintain the world sane, it has value semantics. Can you imagine how tough it would be to code if you had to do it all over again?

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

set the value of b to false? Consider how tough it would be to code any application.

Answered by jason

Solution #3

A string is a value-oriented reference type. This design is a compromise that allows for some performance improvements.

In the design of the language, the distinction between reference types and value types is essentially a performance tradeoff. Because reference types are formed on the heap, they have some overhead in terms of construction, destruction, and garbage collection. Because the entire object is copied in memory rather than just a pointer, value types have overhead on assignments and method calls (if the data size is larger than a pointer). Strings are built as reference types because they can (and usually do) exceed the size of a pointer. A value type’s size must also be known at compile time, which is not always the case with strings.

Strings, on the other hand, have value semantics, which implies they are immutable and are compared by value (in the case of a string, character by character), not by comparing references. Certain optimizations are possible as a result of this:

If numerous strings are known to be equivalent, interning allows the compiler to utilize just one string, conserving memory. This optimization only works provided the strings are immutable; otherwise, modifying one string would have unknown consequences for the others.

The compiler stores the data in a particular static section of memory. Because they don’t need to be allocated and trash collected during runtime, this saves time.

Certain actions cost more when using immutable strings. You can’t, for example, replace a single character in situ; instead, you must allocate a new string for each modification. However, when compared to the benefits of the optimizations, this is a minor expense.

For the user, value semantics essentially hides the distinction between reference and value types. It doesn’t matter to the user whether a type is a value type or a reference type if it has value semantics; it can be considered an implementation detail.

Answered by JacquesB

Solution #4

This is a late response to an old question, but all other responses miss the point, which is that generics were not introduced in.NET until version 2.0 in 2005.

Because it was critical for Microsoft to ensure that strings could be kept in non-generic collections like System in the most efficient way possible, String is a reference type rather than a value type. Collections.ArrayList.

Boxing is a particular conversion to the type object that is required when storing a value-type in a non-generic collection. The CLR wraps a value type in a System.Object and stores it on the managed heap when it boxes it.

Unboxing is the inverse operation that is required to read the value from the collection.

Both boxing and unpacking have a significant cost: boxing necessitates a separate allocation, while unboxing necessitates type verification.

Because string’s size is variable, some replies assert wrongly that it could never have been implemented as a value type. Actually, string can be implemented as a fixed-length data structure with two fields: an integer for the string length and a pointer to a char array. On top of that, you can apply a Small String Optimization approach.

If generics had been available from the start, I believe string as a value type would have been a better choice, with simpler semantics, better memory use, and greater cache locality. A single contiguous block of memory could have been a Liststring> containing only short strings.

Answered by ZunTzu

Solution #5

Immutable reference types include more than just strings. There are also multi-cast delegates. As a result, it is safe to write.

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

I assume strings are immutable because working with them and allocating memory in this manner is the safest technique. Why aren’t they Value types? The previous authors are correct in their assessments of stack size and other factors. I’d also add that having strings reference types allows you to reduce the size of your assembly by using the same constant string throughout the application. If you set a goal for yourself,

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

Both occurrences of the “my string” constant are likely to be allocated just once in your assembly.

Put the string inside a new StringBuilder if you want to manage it like a regular reference type (string s). Alternatively, MemoryStreams can be used.

If you’re writing a library that expects large strings to be supplied to its functions, use a StringBuilder or a Stream as a parameter type.

Answered by Bogdan_Ch

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