Managed struct to/from pointer without boxing?

Question

Let's say I have a struct that's not one of the C# primitives.

I'd like to know if there's a way (including unsafe code) to get a pointer from an instance of the struct type (the pointer doesn't have to be type-safe, it could be void*) and get an instance of the struct type from a pointer, but without boxing involved.

(I know that the System.Runtime.InteropServices.Marshal class offers almost what I'm describing, but it involves passing and getting structs as object, which means that boxing is involved.)

NeddySpaghetti · Accepted Answer · 2014-12-29 11:16:21Z

4

You can use an unsafe region to get the pointer using the & operator and convert back to the type using the * operator.

public struct Foo
{
    public int bar;
};

void Main()
{
    var foo = new Foo();
    foo.bar = 5;

    unsafe
    {
        Foo * fooPtr = &foo;
        Console.WriteLine("Foo address {0:X}", *fooPtr);

        Foo anotherFoo = *fooPtr;
        Console.WriteLine("Bar= {0}", anotherFoo.bar);
    }
}

answered Dec 29, 2014 at 11:16

NeddySpaghetti

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3 Comments

Sriram Sakthivel Over a year ago

I guess 5 is the lucky number. We both use the same :p

NeddySpaghetti Over a year ago

I was just thinking that :)

Thomas Levesque Over a year ago

@SriramSakthivel, why 5? Everyone knows that the answer is 42 ;)

Sriram Sakthivel · Accepted Answer · 2014-12-29 11:12:11Z

3

I don't quite understand what is the problem with taking the address of the struct with & operator. If you're not aware of that, there exist address of operator.

For example:

struct Demo
{
    public int SomeField;
}

internal static class Program
{
    static Demo demo = new Demo { SomeField = 5 };

    private static void Main()
    {
        Print();
    }

    public static unsafe void Print()
    {
        fixed (Demo* demop = &demo)
        {
            Console.WriteLine(demop->SomeField);
        }
    }
}

This code doesn't boxes the struct, though it needs unsafe context to compile.

answered Dec 29, 2014 at 11:12

Sriram Sakthivel

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1 Comment

Theodoros Chatzigiannakis Over a year ago

Oh, this is very embarassing - I had tried doing this in a generic method using a T : struct and it failed with a "cannot take address of a managed type" error (while it worked on int, etc). So I (quite wrongly) assumed the problem was the type, not the fact that I was using generics. Considering this, my actual question should have been different, but since it is what it is, you are both correct of course.

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