i am a java programmer and i know in java objects are stored on heap. Just for curiosity wanted to where does objects reside in c#.
For reference types : on the heap
For value types : on the stack for local variables and method parameters, or on the heap for members of a reference type
The C# language doesn't specify where an object or a value should be stored. It simply defines the semantics of reference types and value types.
Microsoft .NET CLR stores values (instances of value types) contained of local variables on stack and instances of reference types (objects) and non-local value types on the heap. However, as stated previously, other implementations of the C# language are free to store things as they wish as long as they conform to value and reference semantics defined by the C# Language Specification.
detailed explaination,
C# Heap(ing) Vs Stack(ing) in .NET: Part I
By Matthew Cochran January 14, 2006 http://www.c-sharpcorner.com/UploadFile/rmcochran/csharp_memory01122006130034PM/csharp_memory.aspx
Objects are stored on heap in C# too.
Reference types stored on managed heap.
Value types by default stored on stack.
Value types also can stored on heap in several cases:
During boxing (casting value type to interface, downcasting value type to object etc).
If value type is a member of reference type
If value type uses in closure
Related
I think we can use a Struct type for that, but I don't know how can C# consider that as a value type, can anyone give a confirmation for that or an example
So is there a case where a value type is stored on the Heap and a reference type in the stack ?
Thank's
Yes, struct is a value type in C#.
A struct type is a value type that is typically used to encapsulate small groups of related variables, such as the coordinates of a rectangle or the characteristics of an item in an inventory
https://msdn.microsoft.com/en-us/library/ah19swz4.aspx
As an interesting aside, there is a common misconception that reference variables are always allocated on the heap and value types always on the stack. Eric Lippert points out that The Stack is an Implementation Detail.
Classes are objects that are stored by reference and structs are stored by value. The referenced classes are stored on the heap (the large pool of memory in your RAM) and the structs are stored on the stack, which is a small portion of memory reserved for your application. This resource is limited and you should not abuse of structs if they are not needed.
I have an exam question from a past paper that I'm trying to answer:
Discuss variables of type primitive, reference and static in the context of a programming language. Give suitable examples [8].
The answer I have so far is:
A primitive type is an object which the language has given a predefined value. These types include int, bool and float. Reference type objects refer to these primitive types in a particular sequence when instantiated. Examples of these are strings and arrays. The static keyword, when assigned to a variable, means that there is only one instance of this variable and the value assigned applies to all references of the variable.
I'm fairly new to programming so I don't know if this is exactly right, so if anyone could give me some tips on how to improve the mark I would get for this question I'd greatly appreciate it.
A primitive type is an object which the language has given a
predefined value
Why? Even references can have predefined values as noted. For primitive (built in) types you may want to say these are types that a language provides built in support for. What your instructor might be glad to hear about is if you say that most primitive types are also value types in C# and you might want to discuss value types semantics (e.g., value type variable directly contains value - whereas a reference variable just contains an address to some object in memory).
About reference types again you may say that a reference variable doesn't contain the value or object directly - rather just a reference to it. Now again you may want to discuss reference semantics. For example if you have two reference variables pointing to same object - and you change the object from one reference change will be visible from another reference too - because both references point to same object. This is not the case with value types. If you assign same value type object to two different value type variables and change one variable - this change will not be visible in the second value type variable because each of them holds the value directly (e.g. each will have its own copy of the value type variable it was assigned to).
Static types you have already described.
You are on the right track for sure, but you are missing some fundamental concepts about these. Also, the 3 are not mutually exclusive:
A primitive type is simply a syntax shortcut defined by the compiler for Framework Class Library or FCL types.
A reference type is a pointer that represents an instance of a class. The objects they point to are allocated on the heap and the value of the variable is the memory address of that object rather than the class itself.
Static is not a type at all, but really defines where and when fields, properties, methods, and classes can be used. A static variable lives on the class rather then an instance. A static constructor is called the first time you access the class. A static method can be called from the class definition. That explains the persistence you see on static variables as you create and destroy them.
The answer to that question, in my opinion -- has not a thing to do with OOP and everything to do with the compiler and microprocessor.
The simplest and most accurate definition of the term that subsumes all of the qualities of a primitive type -- as I understand it -- is:
A primitive type must fit into the register used for operations on it -- IOW, in an X86 system -- the Accumulator.
So, primitive types are limited to the size of the Accumulator and can be operated upon by native processor instructions. (Basic math and Boolean/bit-shifting operations). Yes, it fits into heap memory and on the stack, but those are still essentially 8-bit entities and the registers are not.
OOP languages do not use primitive types for their managed memory processes, they use structures that mimic primitive types. (Even in .NET, when you use the keyword int -- it uses System.Int32 to wrap that.)
This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
What are the differences between value types and reference types in C#?
what are the basics differences between values types and rereference types
Consider two variables:
SomeReferenceType x;
SomeValueType y;
The value of x is a reference - it will either be null or a reference to an object which is itself an instance of SomeReferenceType or a derived class. The value of x is not, in itself, the object.
The value of y is the data itself - if SomeValueType has three fields, the value of y will directly contain those fields.
That's a very brief summary - see Eric Lippert's blog post about value types and my article for more information. (You might also be interested in my article about parameter passing which is related, but not quite the same.)
Value types, as name tells, are values stored in memory; referencer types are (a kind of) pointer to an object (a class, an object, etc...)
From Microsoft:
A data type is a value type if it holds the data within its own memory allocation. A reference type contains a pointer to another memory location that holds the data.
Value Types
Value types include the following:
All numeric data types
Boolean, Char, and Date
All structures, even if their members are reference types
Enumerations, since their underlying type is always SByte, Short, Integer, Long, Byte, UShort, UInteger, or ULong
Reference Types
Reference types include the following:
String
All arrays, even if their elements are value types
Class types, such as Form
Delegates
Variables of reference types, referred to as objects, store references to the actual data, see here for details. They include classes, interfaces and delegates.
From MSDN:
Value Types are structs and
enumerations. Variables that are
based on value types directly contain
values. Assigning one value type
variable to another copies the
contained value. This differs from the
assignment of reference type
variables, which copies a reference to
the object but not the object itself.
All value types are derived implicitly
from the System.ValueType. Unlike
with reference types, you cannot
derive a new type from a value type.
However, like reference types, structs
can implement interfaces. Unlike
reference types, a value type cannot
contain the null value. However, the
nullable types feature does allow for
values types to be assigned to null
Read this: http://www.csharptocsharp.com/node/41
When you have a variable of a value type, that variable directly holds a value. If you assign it to another variable, the value is copied directly. When the variable is of a reference type, it does not hold the value directly, but rather a reference (a pointer) to the value. When you copy the variable, you don't copy the value that it points to, but the reference (pointer).
You can read more about in MSDN: http://msdn.microsoft.com/en-us/library/s1ax56ch.aspx and http://msdn.microsoft.com/en-us/library/490f96s2.aspx
Here you are: C# Concepts: Value vs Reference Types
This question already has answers here:
What is the difference between a reference type and value type in c#?
(15 answers)
Closed 9 years ago.
I know a few differences,
Value types are stored on the stack where as reference types are stored on the managed heap.
Value type variables directly contain their values where as reference variables holds only a reference to the location of the object that is created on the managed heap.
Is there any other difference i missed... If so,what are they?
Please read: The stack is an implementation detail, and don't ever again repeat the canard that stack allocation is what differentiates value types from reference types in .NET. The CLR may choose to allocate a variable anywhere it wants to.
The most important difference is in the assignment semantics. When you assign a value type to a variable (or pass it to a method as an argument), all of the data is copied. When you assign a reference type, only a reference is copied - both references point to the same object instance in memory.
Here are some additional differences:
Value types cannot be inherited, whereas reference types can
Value types are implemented as a struct, reference types as a class
Value types, by default, cannot be assigned real null values (the ? syntax is a workaround and still doesn't result in a true null value)
Assigning a value type to another variable, or passing it as a parameter in a method, makes a copy of it whereas with a reference type the variable represents the memory location of the object
It is a compile-time error for a struct to declare an explicit parameterless constructor, but the same does not apply to a class
It is a compile-time error to use the "this" object before all fields are assigned in a struct, but the same does not apply to a class
It is a compile-time error for a struct to not fully assign all properties in its constructor, but the same does not apply to a class
As mentioned by Aaronaught and Eric blog post:
Remember the rule, Reference types always goes to the Heap, whereas Value Types always go where they were declared? If a Value Type is declared outside of a method, but inside a Reference Type it will be placed within the Reference Type on the Heap.
i think values are implicitly assigned memory but not reference types they must be assigned memory explicitly
What is the definition of a value class and reference class in C#?
How does this differ from a value type and reference type?
I ask this question because I read this in the MCTS Self-Paced Training Kit (Exam 70-536). Chapter 1, Lesson 1, Lesson review 4 :
You need to create a simple class or
structure that contains only value
types. You must create the class or
structure so that it runs as
efficiently as possible. You must be
able to pass the class or structure to
a procedure without concern that the
procedure will modify it. Which of the
following should you create?
A reference class
B reference structure
C value class
D value structure
Correct Answer: D
A Incorrect: You could create a
reference class; however, it could be
modified when passed to a procedure.
B Incorrect: You cannot create a
reference structure.
C Incorrect: You could create a value
class; however, structures tend to be
more efficient.
D Correct: Value structures are
typically the most efficient.
You may be thinking of C++/CLI which, unlike C#, allows the user to declare a "value class" or a "ref class."
In C#, any class you declare will implicitly be a reference class - only built-in types, structs, and enums have value semantics.
To read about value class in C++/CLI, look here:
http://www.ddj.com/cpp/184401955
Value classes have very little functionality compared to ref classes, and are useful for "plain old data"; that is, data which has no identity. Since you're copying the data when you assign one to another, the system provides you with a default (and mandatory) copy constructor which simply copies the data over to the other object.
To convert a value class into a reference class (thereby putting it on the garbage-collected heap) you can "box" it.
To decide whether a class you are writing is one or the other, ask yourself whether it has an identity. That usually means that it has some state, or has an identifier or a name, or a notion of its own context (for example a node pointing to nearby nodes).
If it doesn't, it's probably a value class.
In C#, however, value classes are declared as "structs".
See the overview on the subject, but seriously follow the msnd links and read the full Common Type system chapter of it. (You could also have asked in a comment in the first, question)
Value types are passed by value, while reference types are passed by reference.
Edit: value/reference classes
There is no concept of a 'value class' or 'reference class' in C#, so asking for its definition is moot.
Value types store the actual data while reference types store references to the data. Reference types are stored dynamically on the heap while value types are stored on the stack.
Value Types: http://msdn.microsoft.com/en-us/library/s1ax56ch.aspx
Reference Types: http://msdn.microsoft.com/en-us/library/490f96s2.aspx
When you refer to a value type (that is, by using its name), you're talking about the place in memory where the data is. As such, value types can't be null because there's no way for the memory location to say "I don't represent anything." By default, you pass value types by value (that is, the object you pass in to methods doesn't change as a result of the method's execution).
When you use a reference type object, you're actually using a pointer in disguise. The name refers to a memory location, which then references a place in memory where the object actually lives. Hence you can assign null to a reference type, because they have a way of saying "I point to nowhere." Reference types also allow the object to be changed as a result of methods executing, so you can change myReferenceObject's properties by passing it into a method call.
Reference types are passed to methods by reference and value types by value; in the latter case a method receives a copy of the variable and in the former it receives a reference to the original data. If you change your copy, the original does not change. If you change the original data you have a reference to, the data changes everywhere a reference to the data is changed. If a similar program to your C# program was created in C, generally reference types would be like data using pointers and value types would be normal data on the stack.
Numeric types, char, date, enumerations, and structures are all value types. Strings, arrays, delegates and classes (i.e., most things, really) are reference types.
If my understanding is correct, you can accomplish a "value class", or immutable class, through the use of readonly member variables initialized through the constructor. Once created, these cannot be changed.