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I understand the difference of assigning a value or not, what I would like to understand is how the assignment is handle in memory.
What will be stored in the HEAP and in the STACK? Which one is the most efficient?
For example is more efficient to have a method signature like
private Item GetItem(pageModel page, string clickableText = null);
Or
private Item GetItem(pageModel page, string clickableText = "");
Note:
The question is not about which one to use. It is about how their differ in memory.
The proposed method might be called a few hundred times - therefore a different variable assignment might/could have an impact?
There's no difference. The compiler interns string literals, so you're not creating a new string with the call, just referencing an existing string.
The heap and the stack are implementation details in C#. There is some behaviour that depends on the runtime, but the only real contract is that the runtime provides as much memory as you ask for, and guarantees the memory is still there if you access it in the future.
If you do care about the implementation details of the current desktop .NET runtimes, reference types are never passed on the stack. String is a reference type, so it is always passed by reference, and never by value. However, arguments aren't even required to be on the stack in the first place - the reference can also be passed in a register.
In general, in a managed language like C#, you should only care about what exactly happens in memory if you have a good reason it affects the characteristics of your program. The default case should always be thinking about the semantics. Should an empty string mean "no value"? Should a null string mean "no value"? That depends on the semantics of your program. Until you have a good reason to believe the decision is e.g. performance critical, just go with the most clear option, least prone to mistakes, and easiest to read and modify.
A null string is a string that has not been initialized. It is a string variable that hasn't even been given some memory to store data. This will create a null string:
string myString; //Without initializing it, will create a null string.
An empty string is a string that has been initialized and given some memory, but it just doesn't contain any characters (except a null terminator at the end, but you don't see that) so as far as the compiler and you are concerned, it is a string with a length of 0.
string myString = String.Empty; //Will create an empty string.
In terms of efficiency, there shouldn't be a difference at all, but it would good to keep in mind that NULL's can cause projects to crash more than empty strings, unless you are using the NULL pattern in your code.
We have four main types of things we'll be putting in the Stack and Heap as our code is executing: Value Types, Reference Types, Pointers, and Instructions.
Rules
A Reference Type always goes on the Heap.
Value Types and Pointers always go where they were declared. This is a little more complex and needs a bit more understanding of how the Stack works to figure out where "things" are declared.
The Stack, as we mentioned earlier, is responsible for keeping track of where each thread is during the execution of our code (or what's been called).
You can think of it as a thread "state" and each thread has its own stack. When our code makes a call to execute a method the thread starts executing the instructions that have been JIT compiled and and live on the method table, it also puts the method's parameters on the thread stack. Then, as we go through the code and run into variables within the method they are placed on top of the stack.
String.Empty and "" are almost the same, both refer to an existing string that has no content.
Said almost because, "" creates a temporary string in memory (to have something to compare against) while String.Empty is a language constant.
On the other hand, null means nothing, no object at all.
In more familiar terms, String.Empty is like having an empty drawer while null means no drawer at all!
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?
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.)
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.
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.
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.
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.
In a very simple words any value which has a definite size can be treated as a value type.
Also, the way strings are implemented (different for each platform) and when you start stitching them together. Like using a StringBuilder. It allocats a buffer for you to copy into, once you reach the end, it allocates even more memory for you, in the hopes that if you do a large concatenation performance won't be hindered.
Maybe Jon Skeet can help up out here?
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.
How can you tell string is a reference type? I'm not sure that it matters how it is implemented. Strings in C# are immutable precisely so that you don't have to worry about this issue.
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.
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.
Isn't just as simple as Strings are made up of characters arrays. I look at strings as character arrays[]. Therefore they are on the heap because the reference memory location is stored on the stack and points to the beginning of the array's memory location on the heap. The string size is not known before it is allocated ...perfect for the heap.
That is why a string is really immutable because when you change it even if it is of the same size the compiler doesn't know that and has to allocate a new array and assign characters to the positions in the array. It makes sense if you think of strings as a way that languages protect you from having to allocate memory on the fly (read C like programming)
Seeing as new instances of value types are created every time they are passed as arguments, I started thinking about scenarios where using the ref or out keywords can show a substantial performance improvement.
After a while it hit me that while I see the deficit of using value types I didn't know of any advantages.
So my question is rather straight forward - what is the purpose of having value types? what do we gain by copying a structure instead of just creating a new reference to it?
It seems to me that it would be a lot easier to only have reference types like in Java.
Edit: Just to clear this up, I am not referring to value types smaller than 8 bytes (max size of a reference), but rather value types that are 8 bytes or more.
For example - the Rectangle struct that contains four int values.
An instance of a one-byte value type takes up one byte. A reference type takes up the space for the reference plus the sync block and the virtual function table and ...
To copy a reference, you copy a four (or eight) byte reference. To copy a four-byte integer, you copy a four byte integer. Copying small value types is no more expensive than copying references.
Value types that contain no references need not be examined by the garbage collector at all. Every reference must be tracked by the garbage collector.
Value types are usually more performant than reference types:
A reference type costs extra memory for the reference and performance when dereferencing
A value type does not need extra garbage collection. It gets garbage collected together with the instance it lives in. Local variables in methods get cleaned up upon method leave.
Value type arrays are efficient in combination with caches. (Think of an array of ints compared with an array of instances of type Integer)
"Creating a reference" is not the problem. This is just a copy of 32/64 bits. Creating the object is what is costly. Actually creating the object is cheap but collecting it isn't.
Value types are good for performance when they are small and discarded often. They can be used in huge arrays very efficiently. A struct has no object header. There are a lot of other performance differences.
Edit: Eric Lippert posed a great example in the comments: "How many bytes does an array of one million bytes take up if they are value types? How many does it take up if they are reference types?"
I will answer: If struct packing is set to 1 such an array will take 1 million and 16 bytes (on 32 bit system). Using reference types it will take:
array, object header: 12
array, length: 4
array, data: 4*(1 million) = 4m
1 million objects, headers = 12 * (1 million)
1 million objects, data padded to 4 bytes: 4 * (1 million)
And that is why using value types in large arrays can be a good idea.
The gain is visible if your data is small (<16 bytes), you have lots of instances and/or you manipulate them a lot, especially passing to functions. This is because creating an object is relatively expensive compared to creating a small value type instance. And as someone else pointed out, objects need to be collected and that is even more expensive. Plus, very small value types take less memory than their reference type equivalents.
Example of non-primitive value type in .NET is Point structure (System.Drawing).
Every variable has a lifecycle. but not every variable need the flexibility for your variable to perform high but not managed in heap.
Value types (Struct) contain their data allocate in stack or allocated in-line in a structure. Reference types (Class) store a reference to the value's memory address, and are allocated on the heap.
what is the purpose of having value types?
Value types are quite efficient to handle simple data, (It should be use to represent immutable types to represent value)
Value type objects cannot be allocated on the garbage-collected heap, and the variable representing the object does not contain a pointer to an object; the variable contains the object itself.
what do we gain by copying a structure instead of just creating a new reference to it?
If you copy a struct, C# creates a new copy of the object and assigns the copy of the object to a separate struct instance. However, if you copy a class, C# creates a new copy of the reference to the object and assigns the copy of the reference to the separate class instance. Structs can't have destructors, but classes can have destructors.
One major advantage of value types like Rectangle is that if one has n storage locations of type Rectangle, one can be certain that one has n distinct instances of type Rectangle. If one has an array MyArray of type Rectangle, of length at least two, a statement like MyArray[0] = MyArray[1] will copy the fields of MyArray[1] into those of MyArray[0], but they will continue to refer to distinct Rectangle instances. If one then performs a statement line MyArray[0].X += 4 that will modify field X of one instance, without modifying the X value of any other array slot or Rectangle instance. Note, by the way, that creating the array instantly populates it with writable Rectangle instances.
Imagine if Rectangle were a mutable class type. Creating an array of mutable Rectangle instances would require that one first dimension the array, and then assign to each element in the array a new Rectangle instance. If one wanted to copy the value of one rectangle instance to another, one would have to say something like MyArray[0].CopyValuesFrom(MyArray[1]) [which would, of course, fail if MyArray[0] had not been populated with a reference to a new instance). If one were to accidentally say MyArray[0] = MyArray[1], then writing to MyArray[0].X would also affect MyArray[1].X. Nasty stuff.
It's important to note that there are a few places in C# and vb.net where the compiler will implicitly copy a value type and then act upon a copy as though it was the original. This is a really unfortunate language design, and has prompted some people to put forth the proposition that value types should be immutable (since most situations involving implicit copying only cause problems with mutable value types). Back when compilers were very bad at warning of cases where semantically-dubious copies would yield broken behavior, such a notion might have been reasonable. It should be considered obsolete today, though, given that any decent modern compiler will flag errors in most scenarios where implicit copying would yield broken semantics, including all scenarios where structs are only mutated via constructors, property setters, or external assignments to public mutable fields. A statement like MyArray[0].X += 5 is far more readable than MyArray[0] = new Rectangle(MyArray[0].X + 5, MyArray[0].Y, MyArray[0].Width, MyArray[0].Height).
I'm just learning C# and working with some examples of strings and StringBuilder. From my reading, I understand that if I do this:
string greeting = "Hello";
greeting += " my good friends";
that I get a new string called greeting with the concatenated value. I understand that the run-time(or compiler, or whatever) is actually getting rid of the reference to the original string greeting and replacing it with a new concatenated one of the same name.
I was just wondering what practical application/ramification this has. Why does it matter to me how C# shuffles strings around in the background when the effect to me is simply that my initial variable changed value.
I was wondering if someone could give me a scenario where a programmer would need to know the difference. * a simple example would be nice, as I'm a relative beginner to this.
Thanks in advance..
Strings, again, are a good example. A very common error is:
string greeting = "Hello Foo!";
greeting.Replace("Foo", "World");
Instead of the proper:
string greeting = "Hello Foo!";
greeting = greeting.Replace("Foo", "World");
Unless you knew that string was an immutable class, you could suspect the first method would be appropriate.
Why does it matter to me how C# shuffles strings around in the background when the effect to me is simply that my initial variable changed value.
The other major place where this has huge advantages is when concurrency is introduced. Immutable types are much easier to deal with in a concurrent situation, as you don't have to worry about whether another thread is modifying the same value within the same reference. Using an immutable type often allows you to avoid the potentially significant cost of synchronization (ie: locking).
I understand that the run-time(or compiler, or whatever) is actually getting rid of the reference to the original string greeting and replacing it with a new concatenated one of the same name.
Pedantic intro: No. Objects do not have names -- variables do. It is storing a new object in the same variable. Thus, the name (variable) used to access the object is the same, even though it (the variable) now refers to another object. An object may also be stored in multiple variables and have multiple "names" at the same time or it might not be accessible directly by any variable.
The other parts of the question have already been succinctly answered for the case of strings -- however, the mutable/immutable ramifications are much larger. Here are some questions which may widen the scope of the issue in context.
What happens if you set a property of an object passed into a method? (There are these pesky "value-types" in C#, so it depends...)
What happens if a sequence of actions leaves an object in an inconsistent state? (E.g. property A was set and an error occurred before property B was set?)
What happens if multiple parts of code expect to be modifying the same object, but are not because the object was cloned/duplicated somewhere?
What happens if multiple parts of code do not expect the object to be modified elsewhere, but it is? (This applies in both threading and non-threading situations)
In general, the contract of an object (API and usage patterns/scope/limitations) must be known and correctly adhered to in order to ensure program validity. I generally find that immutable objects make life easier (as then only one of the above "issues" -- a meager 25% -- even applies).
Happy coding.
C# isn't doing any "shuffling", you are! Your statement assigns a new value to the variable, the referenced object itself did not change, you just dropped the reference.
The major reason immutability is useful is this:
String greeting = "Hello";
// who knows what foo does
foo(greeting);
// always prints "Hello" since String is immutable
System.Console.WriteLine(greeting);
You can share references to immutable objects without worrying about other code changing the object--it can't happen. Therefore immutable objects are easier to reason about.
Most of the time, very little effect. However, in the situation of concatenating many strings, the performance hit of garbage collecting all those strings becomes problematic. Do too many string manipulations with just a string, and the performance of your application can take a nosedive.
This is the reason why StringBuilder is more effective when you have a lot of string manipulation to do; leaving all those 'orphaned' strings out there makes a bigger problem for the Garbage Collector than simply modifying an in memory buffer.
I think the main benefit of immutable strings lies in make memory management easier.
C# allocates memory byte by byte for each object. If you create a string "Tom" it takes up three bytes. You may then allocate an integer and that would be four bytes. If you then tried to change the string "Tom" to "Tomas" it would require moving all the other memory to make room for the two new characters a and s.
To eliminate this pain, it's easier (and quicker) to just allocate five new bytes for the string "Tomas".
Does that help?
In performance terms, the advantage of immutuable is copying an object is cheap in terms of both CPU and memory since it only involves making a copy of a pointer. The downside is that writing to the object becomes more expensive since it must make a copy of the object in the process.
Why is string a reference type, even though it's normally primitive data type such as int, float, or double.
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.
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.
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.
String is a reference type, not a value type. In many cases, you know the length of the string and the content of the string, in such cases, it is easy to allocate the memory for the string. but consider something like this.
string s = Console.ReadLine();
is it not possible to know the allocation details for "s" in compilation time. User enters the values and all the entered string/line is stored in the s. So, strings are stored on heap so that memory is reallocated to fit the content for the string s. And reference to this string is stored on stack.
To learn more please read: .net zero by petzold
Read: Garbage collection from CLR Via C# for allocation details on stack.
Edit: Console.WriteLine(); to Console.ReadLine();