What is the counterpart of this C pointer programme in C#?

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I'd like to know how the following programme on pointer written in C can be written in C#, especially how to use the keywords in the C programme like &i and *p in C#. Please clarify.

What I have tried:

#include <stdio.h>

void f(int *p, int *q)
{
	p = q;
	*p = 2;
}

int i = 0, j = 1;

int main()
{
	f(&i, &j);
	printf("%d %d \n", i, j);
	return 0;
}

Posted 18-Mar-21 4:27am

priyamtheone

Updated 18-Mar-21 8:15am

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BillWoodruff 18-Mar-21 11:06am

Are you asking how to use "unsafe" code in C# ... or, are you asking how to write a C# equivalent to the C function ?

priyamtheone 18-Mar-21 12:52pm

@BillWoodruff: I'm not asking to write unsafe code. I'm just asking how to write the C# equivalent to the C function, especially, how do we make use of pointer concepts like &i and *p in C#. For example, &i holds the memory address of the variable i. So how do we get the memory address of a variabe in C#? Similarly, if p = 100, then *p returns the value being held at the memory location of 100. So, how do we do this in C#?

BillWoodruff 18-Mar-21 13:29pm

see my post below

3 solutions

Solution 3

Consider a simple function:

public void f1(ref int iref)
{
    iref = iref * iref;
}

// calling the function:

int ival = 4;
f1(ref ival);  // ival is now #16

In C# we can use the 'ref parameter attribute to tell the compiler to pass the address of the variable rather than its value. In this case Type 'int is a ValueType (stored on the Stack).

To really get a foundation for understanding how Value/Reference Types work in C#, you need to ... as OriginalGriff wisely advises you to ... realize C# is a very different, and equally complex, reality than C: C# manages memory allocation for you using the stack and heap, and has quite complex garbage collection algorithms which are seldom directly modified by the programmer.

Suggested reading:

0 stack and heap: [^]

1 ValueType and Reference Type: [^]

2 use of 'ref, 'in, and 'out parameter attributes: [^]

3 Class and Struct: [^]

4 special case Arrays: [^]

5 special case String: [^], [^], [^]

6 Mutable and Immutable: [^]

7 Boxing and Unboxing: [^]

Posted 18-Mar-21 8:15am

BillWoodruff

Updated 18-Mar-21 12:18pm

v2

Comments

priyamtheone 18-Mar-21 15:19pm

Thanks for the tutorial references. They'll definitely be helpful in improving my knowledge base.

Maciej Los 19-Mar-21 3:09am

5ed!

Solution 2

Although Maciej is correct, you probably shouldn't be using pointers directly in C# - that's one of the reasons why it is difficult to do: to remind you that this is not a good idea unless you really, really, need to (and in the 14 years I've been using C#, I haven't needed a single pointer yet).

Instead, you need to stop assuming that C (or C++) and C# are the same language, and realize that they are very different languages which share some similar syntax. Trying to write C code in C# will end up with bad code that is hard to use, hard to maintain, and hard to read.
The C# equivalent of that code doesn't need pointers because C# is reference based - and you can consider references as "pointers on steroids"!

private static void f(int p, ref int q)
   {
   q = 2;
   }

private static int i = 0, j = 1;

public static int Main()
   {
   f(i, ref j);
   Console.WriteLine($"{i} {j}");
   return 0;
   }

But if you use more complex objects, you don't need to pass by reference at all.

Posted 18-Mar-21 5:08am

OriginalGriff

Comments

jsc42 18-Mar-21 13:06pm

Should 'f(i, ref j);' be 'f(i, out j);'?

OriginalGriff 18-Mar-21 13:33pm

It could be - but the C original doesn't allow for one-way references, so ref was closer to the actual pointer.

And that's ignoring that you wouldn't write code like that in C# anyway ... :laugh:

OriginalGriff 18-Mar-21 13:35pm

out would also complicate matters, as you must set a value to out parameters in all code paths or get a compiler error. Wouldn't be a problem in this trivial code, but ...

priyamtheone 18-Mar-21 14:37pm

OriginalGriff: Thanks for the explanation. After going through it, I'll make two assumptions based on the context of C# language: 1) If we write

int* p = &a;

, the pointer variable will hold the memory address of the int variable a (suppose 1001).

2) If p is currently holding 1001, then writing

return *p;

will return the value being held at the memory location of 1001.

Am I right to make the above two assumptions?

OriginalGriff 18-Mar-21 15:06pm

Yes and no: if you go assigning random numbers to a pointer, what will actually happen is your app will crash with a "memory access violation" or a "segmentation fault"
This is generally what you get when you forget to assign a value to a pointer before you use it!

Maciej Los 19-Mar-21 3:09am

5ed!

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Maciej Los · Accepted Answer · 2021-03-18T04:37:00

Start here: Pointer types - C# Programming Guide | Microsoft Docs[^]

MSDN wrote:

Pointer types (C# Programming Guide)

In an unsafe context, a type may be a pointer type, a value type, or a reference type. A pointer type declaration takes one of the following forms:

C#
type* identifier;
void* identifier; //allowed but not recommended
The type specified before the * in a pointer type is called the referent type. Only an unmanaged type can be a referent type.

Pointer types do not inherit from object and no conversions exist between pointer types and object. Also, boxing and unboxing do not support pointers. However, you can convert between different pointer types and between pointer types and integral types.

When you declare multiple pointers in the same declaration, the asterisk (*) is written together with the underlying type only; it is not used as a prefix to each pointer name. For example:

C#
int* p1, p2, p3;   // Ok
int *p1, *p2, *p3;   // Invalid in C#

What is the counterpart of this C pointer programme in C#?

3 solutions

Solution 3

Solution 2

Solution 1

Pointer types (C# Programming Guide)

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