Some guy asked me this question couple of months ago and I couldn't explain it in detail. What is the difference between a reference type and a value type in C#?
I know that value types are int, bool, float, etc and reference types are delegate, interface, etc. Or is this wrong, too?
Can you explain it to me in a professional way?
On
This is probably wrong in esoterical ways, but, to make it simple:
Value types are values that are passed normally "by value" (so copying them). Reference types are passed "by reference" (so giving a pointer to the original value). There isn't any guarantee by the .NET ECMA standard of where these "things" are saved. You could build an implementation of .NET that is stackless, or one that is heapless (the second would be very complex, but you probably could, using fibers and many stacks)
Structs are value type (int, bool... are structs, or at least are simulated as...), classes are reference type.
Value types descend from System.ValueType. Reference type descend from System.Object.
Now.. In the end you have Value Type, "referenced objects" and references (in C++ they would be called pointers to objects. In .NET they are opaque. We don't know what they are. From our point of view they are "handles" to the object). These lasts are similar to Value Types (they are passed by copy). So an object is composed by the object (a reference type) and zero or more references to it (that are similar to value types). When there are zero references the GC will probably collect it.
In general (in the "default" implementation of .NET), Value type can go on the stack (if they are local fields) or on the heap (if they are fields of a class, if they are variables in an iterator function, if they are variables referenced by a closure, if they are variable in an async function (using the newer Async CTP)...). Referenced value can only go to the heap. References use the same rules as Value types.
In the cases of Value Type that go on the heap because they are in an iterator function, an async function, or are referenced by a closure, if you watch the compiled file you'll see that the compiler created a class to put these variables, and the class is built when you call the function.
Now, I don't know how to write long things, and I have better things to do in my life. If you want a "precise" "academic" "correct" version, read THIS:
http://blogs.msdn.com/b/ericlippert/archive/2010/09/30/the-truth-about-value-types.aspx
It's 15 minutes I'm looking for it! It's better than the msdn versions, because it's a condensed "ready to use" article.
On
I found it easier to understand the difference of the two if you know how computer allocate stuffs in memory and know what a pointer is.
Reference is usually associated with a pointer. Meaning the memory address where your variable reside is actually holding another memory address of the actual object in a different memory location.
The example I am about to give is grossly over simplified, so take it with a grain of salt.
Imagine computer memory is a bunch of PO boxes in a row (starting w/ PO Box 0001 to PO Box n) that can hold something inside it. If PO boxes doesn't do it for you, try a hashtable or dictionary or an array or something similar.
Thus, when you do something like:
var a = "Hello";
the computer will do the following:
Value type will hold the actual thing in its memory location.
Thus, when you do something like:
var a = 1;
the computer will do the following:
On
This is from a post of mine from a different forum, about two years ago. While the language is vb.net (as opposed to C#), the Value Type vs. Reference type concepts are uniform throughout .net, and the examples still hold.
It is also important to remember that within .net, ALL types technically derive from the base type Object. The value types are designed to behave as such, but in the end they also inherit the functionality of base type Object.
A. Value Types are just that- they represent a distinct area in memory where a discrete VALUE is stored. Value types are of fixed memory size and are stored in the stack, which is a collection of addresses of fixed size.
When you make a statement like such:
Dim A as Integer
DIm B as Integer
A = 3
B = A
You have done the following:
The Value of each variable exists discretely in each memory location.
B. Reference Types can be of various sizes. Therefore, they can't be stored in the "Stack" (remember, the stack is a collection of memory allocations of fixed size?). They are stored in the "Managed Heap". Pointers (or "references") to each item on the managed heap are maintained in the stack (Like an Address). Your code uses these pointers in the stack to access objects stored in the managed heap. So when your code uses a reference variable, it is actually using a pointer (or "address" to an memory location in the managed heap).
Say you have created a Class named clsPerson, with a string Property Person.Name
In this case, when you make a statement such as this:
Dim p1 As clsPerson
p1 = New clsPerson
p1.Name = "Jim Morrison"
Dim p2 As Person
p2 = p1
In the case above, the p1.Name Property will Return "Jim Morrison", as you would expect. The p2.Name property will ALSO return "Jim Morrison", as you would Iintuitively expect. I believe that both p1 and p2 represent distinct addresses on the Stack. However, now that you have assigned p2 the value of p1, both p1 and p2 point to the SAME LOCATION on the managed heap.
Now COnsider THIS situation:
Dim p1 As clsPerson
Dim p2 As clsPerson
p1 = New clsPerson
p1.Name = "Jim Morrison"
p2 = p1
p2.Name = "Janis Joplin"
In this situation, You have created one new instance of the person Class on the Managed Heap with a pointer p1 on the Stack which references the object, and assigned the Name Property of the object instance a value of "Jim Morrison" again. Next, you created another pointer p2 in the Stack, and pointed it at the same address on the managed heap as that referenced by p1 (when you made the assignement p2 = p1).
Here comes the twist. When you the Assign the Name property of p2 the value "Janis Joplin" you are changing the Name property for the object REFERENCED by Both p1 and p2, such that, if you ran the following code:
MsgBox(P1.Name)
'Will return "Janis Joplin"
MsgBox(p2.Name)
'will ALSO return "Janis Joplin"Because both variables (Pointers on the Stack) reference the SAME OBJECT in memory (an Address on the Managed Heap).
Did that make sense?
Last. If you do THIS:
DIm p1 As New clsPerson
Dim p2 As New clsPerson
p1.Name = "Jim Morrison"
p2.Name = "Janis Joplin"
You now have two distinct Person Objects. However, the minute you do THIS again:
p2 = p1
You have now pointed both back to "Jim Morrison". (I am not exactly sure what happened to the Object on the Heap referenced by p2 . . . I THINK it has now gone out of scope. This is one of those areas where hopefullly someone can set me straight . . .). -EDIT: I BELIEVE this is why you would Set p2 = Nothing OR p2 = New clsPerson before making the new assignment.
Once again, if you now do THIS:
p2.Name = "Jimi Hendrix"
MsgBox(p1.Name)
MsgBox(p2.Name)
Both msgBoxes will now return "Jimi Hendrix"
This can be pretty confusing for a bit, and I will say one last time, I may have some of the details wrong.
Good Luck, and hopefully others who know better than me will come along to help clarify some of this . . .
On
There are many little details of the differences between value types and reference types that are stated explicitly by the standard and some of them are not easy to understand, especially for beginners.
See ECMA standard 33, Common Language Infrastructure (CLI). The CLI is also standardized by the ISO. I would provide a reference but for ECMA we must download a PDF and that link depends on the version number. ISO standards cost money.
One difference is that value types can be boxed but reference types generally cannot be. There are exceptions but they are quite technical.
Value types cannot have parameter-less instance constructors or finalizers and they cannot refer to themselves. Referring to themselves means for example that if there is a value type Node then a member of Node cannot be a Node. I think there are other requirements/limitations in the specifications but if so then they are not gathered together in one place.
On
value data type and reference data type
1) value( contain the data directly ) but reference ( refers to the data )
2) in value( every variable has its own copy)
but
in reference (more than variable can refer to some objects)
3) in value (operation variable can`t effect on other variable ) but in reference (variable can affect other )
4) value types are(int, bool, float) but reference type are (array , class objects , string )
On
Variable types and Reference Value are easy to apply and well applied to the domain model, facilitate the development process.
To remove any myth around the amount of "value type", I will comment on how this is handled on the platform. NET, specifically in C # (CSharp) when called APIS and send parameters by value, by reference, in our methods, and functions and how to make the correct treatment of the passages of these values.
Read this article Variable Type Value and Reference in C #
On
The simplest way to think of reference types is to consider them as being "object-IDs"; the only things one can do with an object ID are create one, copy one, inquire or manipulate the type of one, or compare two for equality. An attempt to do anything else with an object-ID will be regarded as shorthand for doing the indicated action with the object referred to by that id.
Suppose I have two variables X and Y of type Car--a reference type. Y happens to hold "object ID #19531". If I say "X=Y", that will cause X to hold "object ID #19531". Note that neither X nor Y holds a car. The car, otherwise known as "object ID #19531", is stored elsewhere. When I copied Y into X, all I did was copy the ID number. Now suppose I say X.Color=Colors.Blue. Such a statement will be regarded as an instruction to go find "object ID#19531" and paint it blue. Note that even though X and Y now refer to a blue car rather than a yellow one, the statement doesn't actually affect X or Y, because both still refer to "object ID #19531", which is still the same car as it always has been.
On
Let me tell this with an example.
using System;
public class HelloWorld {
public static void method1(int num) {
num = num + 1;
Console.WriteLine(num);
}
public static void method2(ref int num) {
num = num + 1;
Console.WriteLine(num);
}
public static void Main(string[] args) {
int num = 10;
method1(num);
Console.WriteLine(num);
method2(ref num);
Console.WriteLine(num);
}
}
1.At first I passed the num value to the method1 which accepts the parameter of
value type int.Method1 does not affect the origal value of num declared in main
method.It passes the copy of the value num to the method1.
OUTPUT[METHOD 1]
11
10
2.Method 2 accepts the parameter as ref int type which we need to pass the reference of the variable.Reference holds the memory address of the variable.If we pass the reference of the variable num it will affect the origal value.
OUTPUT[METHOD 2]
11
11
3.Mostly value types comes under primitive data types and reference types will be arrays , objects , delegate etc..
On
Holds some value not memory addresses
Example:
Struct
Storage:
TL;DR: A variable's value is stored wherever it is decleared. Local variables live on the stack for example, but when declared inside a class as a member it lives on the heap tightly coupled with the class it is declared in.
Longer: Thus value types are stored wherever they are declared.
E.g.: an int's value inside a function as a local variable would be stored on the stack, whilst an in int's value declared as member in a class would be stored on the heap with the class it is declared in. A value type on a class has a lifetype that is exactly the same as the class it is declared in, requiring almost no work by the garbage collector. It's more complicated though, i'd refer to @JonSkeet's book "C# In Depth" or his article "Memory in .NET" for a more concise explenation.
Advantages:
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.
Drawbacks:
When large set of values are passed to a method the receiving variable actually copies so there are two redundant values in memory.
As classes are missed out.it losses all the oop benifits
Holds a memory address of a value not value
Example:
Class
Storage:
Stored on heap
Advantages:
When you pass a reference variable to a method and it changes it indeed changes the original value whereas in value types a copy of the given variable is taken and that's value is changed.
When the size of variable is bigger reference type is good
As classes come as a reference type variables, they give reusability, thus benefitting Object-oriented programming
Drawbacks:
More work referencing when allocating and dereferences when reading the value.extra overload for garbage collector
On
Before explaining the different data types available in C#, it's important to mention that C# is a strongly-typed language. This means that each variable, constant, input parameter, return type and in general every expression that evaluates to a value, has a type.
Each type contains information that will be embedded by the compiler into the executable file as metadata which will be used by the common language runtime (CLR) to guarantee type safety when it allocates and reclaims memory.
If you wanna know how much memory a specific type allocates, you can use the sizeof operator as follows:
static void Main()
{
var size = sizeof(int);
Console.WriteLine($"int size:{size}");
size = sizeof(bool);
Console.WriteLine($"bool size:{size}");
size = sizeof(double);
Console.WriteLine($"double size:{size}");
size = sizeof(char);
Console.WriteLine($"char size:{size}");
}
The output will show the number of bytes allocated by each variable.
int size:4
bool size:1
double size:8
char size:2
The information related to each type are:
The members (methods, fields, events, etc.) contained by the type. For example, if we check the definition of type int, we will find the following struct and members:
namespace System
{
[ComVisible(true)]
public struct Int32 : IComparable, IFormattable, IConvertible, IComparable<Int32>, IEquatable<Int32>
{
public const Int32 MaxValue = 2147483647;
public const Int32 MinValue = -2147483648;
public static Int32 Parse(string s, NumberStyles style, IFormatProvider provider);
...
}
}
Memory management When multiple processes are running on an operating system and the amount of RAM isn't enough to hold it all, the operating system maps parts of the hard disk with the RAM and starts storing data in the hard disk. The operating system will use than specific tables where virtual addresses are mapped to their correspondent physical addresses to perform the request. This capability to manage the memory is called virtual memory.
In each process, the virtual memory available is organized in the following 6 sections but for the relevance of this topic, we will focus only on the stack and the heap.
Stack The stack is a LIFO (last in, first out) data structure, with a size-dependent on the operating system (by default, for ARM, x86 and x64 machines Windows's reserve 1MB, while Linux reserve from 2MB to 8MB depending on the version).
This section of memory is automatically managed by the CPU. Every time a function declares a new variable, the compiler allocates a new memory block as big as its size on the stack, and when the function is over, the memory block for the variable is deallocated.
Heap This region of memory isn't managed automatically by the CPU and its size is bigger than the stack. When the new keyword is invoked, the compiler starts looking for the first free memory block that fits the size of the request. and when it finds it, it is marked as reserved by using the built-in C function malloc() and a return the pointer to that location. It's also possible to deallocate a block of memory by using the built-in C function free(). This mechanism causes memory fragmentation and has to use pointers to access the right block of memory, it's slower than the stack to perform the read/write operations.
Custom and Built-in types While C# provides a standard set of built-in types representing integers, boolean, text characters, and so on, You can use constructs like struct, class, interface, and enum to create your own types.
An example of custom type using the struct construct is:
struct Point
{
public int X;
public int Y;
};
Value and reference types We can categorize the C# type into the following categories:
Value types Value types derive from the System.ValueType class and variables of this type contain their values within their memory allocation in the stack. The two categories of value types are struct and enum.
The following example shows the member of the type boolean. As you can see there is no explicit reference to System.ValueType class, this happens because this class is inherited by the struct.
namespace System
{
[ComVisible(true)]
public struct Boolean : IComparable, IConvertible, IComparable<Boolean>, IEquatable<Boolean>
{
public static readonly string TrueString;
public static readonly string FalseString;
public static Boolean Parse(string value);
...
}
}
Reference types On the other hand, the reference types do not contain the actual data stored in a variable, but the memory address of the heap where the value is stored. The categories of reference types are classes, delegates, arrays, and interfaces.
At run time, when a reference type variable is declared, it contains the value null until an object that has been created using the keywords new is assigned to it.
The following example shows the members of the generic type List.
namespace System.Collections.Generic
{
[DebuggerDisplay("Count = {Count}")]
[DebuggerTypeProxy(typeof(Generic.Mscorlib_CollectionDebugView<>))]
[DefaultMember("Item")]
public class List<T> : IList<T>, ICollection<T>, IEnumerable<T>, IEnumerable, IList, ICollection, IReadOnlyList<T>, IReadOnlyCollection<T>
{
...
public T this[int index] { get; set; }
public int Count { get; }
public int Capacity { get; set; }
public void Add(T item);
public void AddRange(IEnumerable<T> collection);
...
}
}
In case you wanna find out the memory address of a specific object, the class System.Runtime.InteropServices provides a way to access to managed objects from unmanaged memory. In the following example, we are gonna use the static method GCHandle.Alloc() to allocate a handle to a string and then the method AddrOfPinnedObject to retrieve its address.
string s1 = "Hello World";
GCHandle gch = GCHandle.Alloc(s1, GCHandleType.Pinned);
IntPtr pObj = gch.AddrOfPinnedObject();
Console.WriteLine($"Memory address:{pObj.ToString()}");
The output will be
Memory address:39723832
References Official documentation: https://learn.microsoft.com/en-us/cpp/build/reference/stack-stack-allocations?view=vs-2019
On
I think these two pictures describe it the best. This is the case in languages like C#, Java, JavaScript, and Python. For C++ references mean different, and the equivalent of reference types are pointer types (That's why you see in various documents of different languages that they are used interchangeably). One of the important things is the meaning of "Pass by Value" and "Pass by Reference". I think there are other questions about them on StackOverflow you can seek for.
On
Suppose v is a value-type expression/variable, and r is a reference-type expression/variable
x = v
update(v) //x will not change value. x stores the old value of v
x = r
update(r) //x now refers to the updated r. x only stored a link to r,
//and r can change but the link to it doesn't .
So, a value-type variable stores the actual value (5, or "h"). A reference-type varaible only stores a link to a metaphorical box where the value is.
On
Sometimes explanations won't help especially for the beginners. You can imagine value type as data file and reference type as a shortcut to a file.
So if you copy a reference variable you only copy the link/pointer to a real data somewhere in memory. If you copy a value type, you really clone the data in memory.
Your examples are a little odd because while
int,boolandfloatare specific types, interfaces and delegates are kinds of type - just likestructandenumare kinds of value types.I've written an explanation of reference types and value types in this article. I'd be happy to expand on any bits which you find confusing.
The "TL;DR" version is to think of what the value of a variable/expression of a particular type is. For a value type, the value is the information itself. For a reference type, the value is a reference which may be null or may be a way of navigating to an object containing the information.
For example, think of a variable as like a piece of paper. It could have the value "5" or "false" written on it, but it couldn't have my house... it would have to have directions to my house. Those directions are the equivalent of a reference. In particular, two people could have different pieces of paper containing the same directions to my house - and if one person followed those directions and painted my house red, then the second person would see that change too. If they both just had separate pictures of my house on the paper, then one person colouring their paper wouldn't change the other person's paper at all.