CODEWITHSUNDEEP
c#c++unions
Viktor Elofsson
Viktor Elofsson

Reputation: 1601

C++ union in C#

I'm translating a library written in C++ to C#, and the keyword 'union' exists once. In a struct.

What's the correct way of translating it into C#? And what does it do? It looks something like this;

struct Foo {
    float bar;

    union {
        int killroy;
        float fubar;
    } as;
}

Upvotes: 87

Views: 75717

Answers (8)

abc2025
abc2025

Reputation: 1

https://learn.microsoft.com/en-us/dotnet/csharp/advanced-topics/reflection-and-attributes/how-to-create-a-c-cpp-union-by-using-attributes

[System.Runtime.InteropServices.StructLayout(LayoutKind.Explicit)]
struct TestUnion
{
    [System.Runtime.InteropServices.FieldOffset(0)]
    public int i;

    [System.Runtime.InteropServices.FieldOffset(0)]
    public double d;

    [System.Runtime.InteropServices.FieldOffset(0)]
    public char c;

    [System.Runtime.InteropServices.FieldOffset(0)]
    public byte b;
}

Upvotes: -2

Kenneth Parker
Kenneth Parker

Reputation: 101

You could write a simple wrapper but in most cases just use an object it is less confusing.

    public class MyUnion
    {
        private object _id;
        public T GetValue<T>() => (T)_id;
        public void SetValue<T>(T value) => _id = value;
    }

Upvotes: -1

Nir
Nir

Reputation: 29594

In C/C++ union is used to overlay different members in the same memory location, so if you have a union of an int and a float they both use the same 4 bytes of memory to store, obviously writing to one corrupts the other (since int and float have different bit layout).

In .Net Microsoft went with the safer choice and didn't include this feature.

EDIT: except for interop

Upvotes: 6

Yan Chen
Yan Chen

Reputation: 69

public class Foo
{
    public float bar;
    public int killroy;

    public float fubar
    {
        get{ return (float)killroy;}
        set{ killroy = (int)value;}
    }
}

Upvotes: -6

Steve Lillis
Steve Lillis

Reputation: 3256

If you're using the union to map the bytes of one of the types to the other then in C# you can use BitConverter instead.

float fubar = 125f; 
int killroy = BitConverter.ToInt32(BitConverter.GetBytes(fubar), 0);

or;

int killroy = 125;
float fubar = BitConverter.ToSingle(BitConverter.GetBytes(killroy), 0);

Upvotes: 4

Steve Fallows
Steve Fallows

Reputation: 6424

You can't really decide how to deal with this without knowing something about how it is used. If it is merely being used to save space, then you can ignore it and just use a struct.

However that is not usually why unions are used. There two common reasons to use them. One is to provide 2 or more ways to access the same data. For instance, a union of an int and an array of 4 bytes is one (of many) ways to separate out the bytes of a 32 bit integer.

The other is when the data in the struct came from an external source such as a network data packet. Usually one element of the struct enclosing the union is an ID that tells you which flavor of the union is in effect.

In neither of these cases can you blindly ignore the union and convert it to a struct where the two (or more) fields do not coincide.

Upvotes: 27

ckramer
ckramer

Reputation: 9443

Personally, I would ignore the UNION all together and implement Killroy and Fubar as separate fields

public struct Foo
{
    float bar;
    int Kilroy;
    float Fubar;
}

Using a UNION saves 32 bits of memory allocated by the int....not going to make or break an app these days.

Upvotes: -3

Armin Ronacher
Armin Ronacher

Reputation: 32533

You can use explicit field layouts for that:

[StructLayout(LayoutKind.Explicit)] 
public struct SampleUnion
{
    [FieldOffset(0)] public float bar;
    [FieldOffset(4)] public int killroy;
    [FieldOffset(4)] public float fubar;
}

Untested. The idea is that two variables have the same position in your struct. You can of course only use one of them.

More informations about unions in struct tutorial

Upvotes: 104

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