Differencebetween a C# Reference and a Pointer

What is the difference between a C# Reference and a Pointer?

C# references can, and will be relocated by garbage collector but normal pointers are static. This is why we use fixed keyword when acquiring a pointer to an array element, to prevent it from getting moved.

EDIT: Conceptually, yes. They are more or less the same.

C# ref is it like a pointer in C/C++ or a reference in C++?

In C#, when you see something referring to a reference type (that is, a type declared with class instead of struct), then you're essentially always dealing with the object through a pointer. In C++, everything is a value type by default, whereas in C# everything is a reference type by default.

When you say "ref" in the C# parameter list, what you're really saying is more like a "pointer to a pointer." You're saying that, in the method, that you want to replace not the contents of the object, but the reference to the object itself, in the code calling your method.

Unless that is your intent, then you should just pass the reference type directly; in C#, passing reference types around is cheap (akin to passing a reference in C++).

Learn/understand the difference between value types and reference types in C#. They're a major concept in that language and things are going to be really confusing if you try to think using the C++ object model in C# land.

The following are essentially semantically equivalent programs:

#include <iostream>

class AClass
{
    int anInteger;
public:
    AClass(int integer)
        : anInteger(integer)
    {  }

    int GetInteger() const
    {
        return anInteger;
    }

    void SetInteger(int toSet)
    {
        anInteger = toSet;
    }
};

struct StaticFunctions
{
    // C# doesn't have free functions, so I'll do similar in C++
    // Note that in real code you'd use a free function for this.

    static void FunctionTakingAReference(AClass *item)
    {
        item->SetInteger(4);
    }

    static void FunctionTakingAReferenceToAReference(AClass **item)
    {
        *item = new AClass(1729);
    }
};

int main()
{
    AClass* instanceOne = new AClass(6);
    StaticFunctions::FunctionTakingAReference(instanceOne);
    std::cout << instanceOne->GetInteger() << "\n";

    AClass* instanceTwo;
    StaticFunctions::FunctionTakingAReferenceToAReference(&instanceTwo);
    // Note that operator& behaves similar to the C# keyword "ref" at the call site.
    std::cout << instanceTwo->GetInteger() << "\n";

    // (Of course in real C++ you're using std::shared_ptr and std::unique_ptr instead,
    //  right? :) )
    delete instanceOne;
    delete instanceTwo;
}

And for C#:

using System;

internal class AClass
{
    public AClass(int integer)
        : Integer(integer)
    {  }

    int Integer { get; set; }
}

internal static class StaticFunctions
{
    public static void FunctionTakingAReference(AClass item)
    {
        item.Integer = 4;
    }

    public static void FunctionTakingAReferenceToAReference(ref AClass item)
    {
        item = new AClass(1729);
    }
}

public static class Program
{
    public static void main()
    {
        AClass instanceOne = new AClass(6);
        StaticFunctions.FunctionTakingAReference(instanceOne);
        Console.WriteLine(instanceOne.Integer);

        AClass instanceTwo  = new AClass(1234); // C# forces me to assign this before
                                                // it can be passed. Use "out" instead of
                                                // "ref" and that requirement goes away.
        StaticFunctions.FunctionTakingAReferenceToAReference(ref instanceTwo);
        Console.WriteLine(instanceTwo.Integer);
    }
}

What is the real difference between Pointers and References?

It's all just indirection: The ability to not deal with data, but say "I'll direct you to some data, over there". You have the same concept in Java and C#, but only in reference format.

The key differences are that references are effectively immutable signposts - they always point to something. This is useful, and easy to understand, but less flexible than the C pointer model. C pointers are signposts that you can happily rewrite. You know that the string you're looking for is next door to the string being pointed at? Well, just slightly alter the signpost.

This couples well with C's "close to the bone, low level knowledge required" approach. We know that a char* foo consists of a set of characters beginning at the location pointed to by the foo signpost. If we also know that the string is at least 10 characters long, we can change the signpost to (foo + 5) to point at then same string, but start half the length in.

This flexibility is useful when you know what you're doing, and death if you don't (where "know" is more than just "know the language", it's "know the exact state of the program"). Get it wrong, and your signpost is directing you off the edge of a cliff. References don't let you fiddle, so you're much more confident that you can follow them without risk (especially when coupled with rules like "A referenced object will never disappear", as in most Garbage collected languages).

Difference between pointer in C++ and reference type in C#

In C# the reference type will be automatically garbage collected when no longer needed.

What exactly is a reference in C#

From what I understand by now, I can say that a reference in C# is a kind of pointer to an object

If by "kind of" you mean "is conceptually similar to", yes. If you mean "could be implemented by", yes. If you mean "has the is-a-kind-of relationship to", as in "a string is a kind of object" then no. The C# type system does not have a subtyping relationship between reference types and pointer types.

which has reference count

Implementations of the CLR are permitted to use reference counting semantics but are not required to do so, and most do not.

and knows about the type compatibility.

I'm not sure what this means. Objects know their own actual type. References have a static type which is compatible with the actual type in verifiable code. Compatibility checking is implemented by the runtime's verifier when the IL is analyzed.

My question is not about how a value type is different than a
reference type, but more about how a reference is implemented.

How references are implemented is, not surprisingly, an implementation detail.

Can somebody provide me with a complete, but hopefully simple explanation about what references really are in C#

References are things that act as references are specified to act by the C# language specification. That is:

objects (of reference type) have identity independent from the values of their fields
any object may have a reference to it
such a reference is a value which may be passed around like any other value
equality comparison is implemented for those values
two references are equal if and only if they refer to the same object; that is, references reify object identity
there is a unique null reference which refers to no object and is unequal to any valid reference to an object
A static type is always known for any reference value, including the null reference
If the reference is non-null then the static type of the reference is always compatible with the actual type of the referent. So for example, if we have a reference to a string, the static type of the reference could be string or object or IEnumerable, but it cannot be Giraffe. (Obviously if the reference is null then there is no referent to have a type.)

There are probably a few rules that I've missed, but that gets across the idea. References are anything that behaves like a reference. That's what you should be concentrating on. References are a useful abstraction because they are the abstraction which enables object identity independent of object value.

and a bit about how they are implemented?

In practice, objects of reference type in C# are implemented as blocks of memory which begin with a small header that contains information about the object, and references are implemented as pointers to that block. This simple scheme is then made more complicated by the fact that we have a multigenerational mark-and-sweep compacting collector; it must somehow know the graph of references so that it can move objects around in memory when compacting the heap, without losing track of referential identity.

As an exercise you might consider how you would implement such a scheme. It builds character to try to figure out how you would build a system where references are pointers and objects can move in memory. How would you do it?

it is hard for me to understand what really is a reference when I have tried to explain to my colleagues why a parameter sent by reference can not be stored inside a closure

This is tricky. It is important to understand that conceptually, a reference to a variable -- a ref parameter in C# -- and a reference to an object of reference type are conceptually similar but actually different things.

In C# you can think of a reference to a variable as an alias. That is, when you say

void M() 
{
  int x = 123;
  N(ref x);
}
void N(ref int y)
{ 
    y = 456;

Essentially what we are saying is that x and y are different names for the same variable. The ref is an unfortunate choice of syntax because it emphasizes the implementation detail -- that behind the scenes, y is a special "reference to variable" type -- and not the semantics of the operation, which is that logically y is now just another name for x; we have two names for the same variable.

References to variables and references to objects are not the same thing in C#; you can see this in the fact that they have different semantics. You can compare two references to objects for equality. But there is no way in C# to say:

static bool EqualAliases(ref int y, ref int z)
{
  return true iff y and z are both aliases for the same variable
}

the way you can with references:

static bool EqualReferences(object x, object y)
{
  return x == y;
}

Behind the scenes both references to variables and references to objects are implemented by pointers. The difference is that a reference to a variable might refer to a variable on the short-term storage pool (aka "the stack"), whereas a reference to an object is a pointer to the heap-allocated object header. That's why the CLR restricts you from storing a reference to a variable into long-term storage; it does not know if you are keeping a long-term reference to something that will be dead soon.

Your best bet to understand how both kinds of references are implemented as pointers is to take a step down from the C# type system into the CLI type system which underlies it. Chapter 8 of the CLI specification should prove interesting reading; it describes different kinds of managed pointers and what each is used for.

C++ pointer vs C# pointer

I'll say that the weak point of C# is that, given a void* pointer, you can't always cast it to a MyStruct* pointer, and surely as hell you can't cast it to a MyStruct[] or a byte[] (and the array type is one of the basic types of .NET, and is used pretty much everywhere). This makes interop quite difficult and slow, because often you have to first copy from a void* to a newly created MyStruct[] just to be able to use the data in .NET.

The alternative clearly is working everywhere with pointers in C# (you can probably do it) and minimize the use of arrays [], but the languages isn't built for that. For example the generic subsystem (List<T>) doesn't accept pointers (you can't List<int*>). You can clearly use IntPtr... but then you have to cast it to int* when you need a int*... it is a pain.

Are ref and out in C# the same a pointers in C++?

They're more limited. You can say ++ on a pointer, but not on a ref or out.

EDIT Some confusion in the comments, so to be absolutely clear: the point here is to compare with the capabilities of pointers. You can't perform the same operation as ptr++ on a ref/out, i.e. make it address an adjacent location in memory. It's true (but irrelevant here) that you can perform the equivalent of (*ptr)++, but that would be to compare it with the capabilities of values, not pointers.

It's a safe bet that they are internally just pointers, because the stack doesn't get moved and C# is carefully organised so that ref and out always refer to an active region of the stack.

EDIT To be absolutely clear again (if it wasn't already clear from the example below), the point here is not that ref/out can only point to the stack. It's that when it points to the stack, it is guaranteed by the language rules not to become a dangling pointer. This guarantee is necessary (and relevant/interesting here) because the stack just discards information in accordance with method call exits, with no checks to ensure that any referrers still exist.

Conversely when ref/out refers to objects in the GC heap it's no surprise that those objects are able to be kept alive as long as necessary: the GC heap is designed precisely for the purpose of retaining objects for any length of time required by their referrers, and provides pinning (see example below) to support situations where the object must not be moved by GC compacting.

If you ever play with interop in unsafe code, you will find that ref is very closely related to pointers. For example, if a COM interface is declared like this:

HRESULT Write(BYTE *pBuffer, UINT size);

The interop assembly will turn it into this:

void Write(ref byte pBuffer, uint size);

And you can do this to call it (I believe the COM interop stuff takes care of pinning the array):

byte[] b = new byte[1000];
obj.Write(ref b[0], b.Length);

In other words, ref to the first byte gets you access to all of it; it's apparently a pointer to the first byte.

C# difference between ref and complex object as parameter

In your second example var context = new DataPackage(); will be ignored and this is intended behavior.

In both examples context.IsAValue is false unless its initialized as true

Your second example is an incorrect usage of ref

example as per the Reference

Don't confuse the concept of passing by reference with the concept of reference types. The two concepts are not the same. A method parameter can be modified by ref regardless of whether it is a value type or a reference type. There is no boxing of a value type when it is passed by reference.

void Method(ref int refArgument)
{
    refArgument = refArgument + 44;
}

int number = 1;
Method(ref number);
Console.WriteLine(number);
// Output: 45

Also look at: passing an argument by reference an example

The ref keyword indicates that a value is passed by reference. It is used in four different contexts:

In a method signature and in a method call, to pass an argument to a method by reference. For more information, see Passing an argument by reference.
In a method signature, to return a value to the caller by reference. For more information, see Reference return values.
In a member body, to indicate that a reference return value is stored locally as a reference that the caller intends to modify. Or to indicate that a local variable accesses another value by reference. For more information, see Ref locals.
In a struct declaration, to declare a ref struct or a readonly ref struct. For more information, see the ref struct section of the Structure types article.