How to use C struct with 2D array in C# Unity

Question

So I have a C API with the following struct

typedef struct mat4f_ { float m[4][4]; } mat4f;

It gets passed as a parameter to one of my API functions:

void myFunction(const mat4f matrix);

I am exporting this function to C# in Unity using a dll:

[DllImport ("mylib")] 
private static extern void myFunction(mat4f matrix);

My question is, what should I make the corresponding C# struct be?

Right now I have the following:

[StructLayout(LayoutKind.Sequential)]
public struct mat4f
{
    public float[,] m;
}

and use try to use the function as follows:

//Just make an identity matrix
mat4f matrix; 
matrix.m = new float[4, 4] { { 1, 0, 0, 0 }, { 0, 1, 0, 0 }, { 0, 0, 1, 0 }, { 0, 0, 0, 1 } };

myFunction(matrix); //Call dll function

Is this the correct thing to do? Is there a better way to do this?

Community · Accepted Answer · 2017-05-23 10:27:27Z

For a 4×4 matrix, you can use UnityEngine.Matrix4x4. If you want to use your own struct, I recommend you implement it the same way UnityEngine.Matrix4x4 is implemented:

[StructLayout(LayoutKind.Sequential)]
public struct mat4f
{
    public float m00;
    public float m01;
    public float m02;
    public float m03;
    public float m10;
    public float m11;
    public float m12;
    public float m13;
    public float m20;
    public float m21;
    public float m22;
    public float m23;
    public float m30;
    public float m31;
    public float m32;
    public float m33;

    public static mat4f Identity = new mat4f
    {
        m11 = 1.0f,
        m22 = 1.0f,
        m33 = 1.0f,
        m44 = 1.0f
    };
}

This is a blittable type. Blittable types do not require conversion when they are passed between managed and unmanaged code.

Sample use:

mat4f matrix = mat4f.Identity;
myFunction(matrix);  // Call DLL function

Existing implementations are similar to the one I presented above.

Matrix implementation of Unity (decompiled; original source code is not available for free). See UnityEngine.Matrix4x4 docs.
Matrix implementation of SharpDX.
Matrix implementation of OpenTK. (Note: the Vector4 type is also a value type.)

js441 · Accepted Answer · 2016-08-28 19:30:20Z

1

Passing a C# struct to an imported function like you are doing is valid, but you should specify the length of the array in the struct, even if you later specify its size.

The c declaration basically specifies an array of length 16, so I would specify the c# struct as follows:

[StructLayout(LayoutKind.Sequential)]
public struct mat4f
{
[MarshalAs(UnmanagedType.ByValArray, SizeConst=16)] public float[,] m;
}

You can read more about how arrays are marshalled here.

answered Aug 28, 2016 at 19:30

js441

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