NP119 value types & reference types

Question

Hi Guys

http://www.c-sharpcorner.com/UploadFile/rmcochran/csharp_memory01122006130034PM/csharp_memory.aspx

Above website says that Value Types and Pointers (Reference) always go where they were declared.

http://www.expresscomputeronline.com/20021111/techspace2.shtml

This website says that variables of value type are allocated on stack.

I wish to know really what is happing. Please explain.

Thank you

Alan · Answer

In brief: 1. Reference types are always allocated on the heap. 2. Value types are allocated as follows: (i) On the stack if they represent local variables or parameters passed by value. (ii) On the stack if they are fields of value types allocated as in 2(i) above. (iii) On the heap otherwise. So if you have a value type which is a field of a reference type or you have an array of value types they are allocated on the heap. Also when a value type is assigned to a System.Object or an interface variable it is 'boxed' on the heap. This means that a wrapper is created on the heap and the value of the value type is copied into it.

Maha · Answer

Thank you for your explanation.

Alan · Answer

Suppose you have this class: public class MyClass { public int MyInt; public double MyDouble; public string MyString; public MyClass(int myInt double myDouble string myString) { MyInt = myInt; MyDouble = myDouble; MyString = myString; } } Now MyClass is of course a reference type and so when you execute this line: MyClass mc = new MyClass(3 4.5 hello); an instance of MyClass is created on the heap. This instance requires 24 bytes of storage broken down as follows: 8 bytes overhead (assuming a 32 bit system) 4 bytes for MyInt 8 bytes for MyDouble 4 bytes for a pointer to where MyString is stored elsewhere on the heap. So it will be seen that the value types MyInt and MyDouble are allocated on the heap because they are fields of a reference type. Similarly if you have this: int[] ia = new int[]{123}; the array is a reference type and so is allocated on the heap. It contains three integers stored one after the other and requiring 4 bytes of storage each plus the overhead required for the array itself (8 bytes plus 4 bytes for the length). Finally if you have this within a method: int a = 3; object o = a; then the integer is 'boxed' on the heap . The wrapper needs 8 bytes of overhead plus 4 further bytes to hold the copy of the integer. Notice that value types don't require any overhead themselves.

Maha · Answer

I mean in which specific circumstances Value types are allocated on the heap. It is much appreciated if you can demonstrate with example.

Alan · Answer

Otherwise means 'in other circumstances'. So in the context of my earlier post it means in circumstances other than those described in 2(i) and 2(ii).

Maha · Answer

Thank you for your explanation. It is much appreciated if you explain what is meant by word otherwise.

Arshad · Answer

Hi In the C# All the Built in type is Value type like Int float etc. except object and string and all the user defined type are reference type except struct and Enumeration (1. Value type is those(variables and all the builtin type) are going to create on to the Stackstack is a DataStructureits based on LAST IN FIRST OUTand stack is Memory occupied by the Processor. (2. Reference type is those(Objectsstring and Userdefined Type) are going to create on to the Heap but there Memory Reference will be there on to the Stack. Thats all Thanks