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?

Answer 1

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.)