Update Vector3 array from C++ native plugin

Question

Marshal.Copy() Method only supports a few array types. Now I only know how to copy from IntPtr(pointing to a float array from C++ code) to float[].

IntPtr pvertices = GetVerticesFromCPP();
float[] vertices = new float[nVertices * 3];
Marshal.Copy(pvertices, vertices, 0, nVertices * 3);

But what I really want is a UnityEngine.Vector3[].

Do I need to manually convert float[] to UnityEngine.Vector3[]? Or is there a simpler and faster way that directly do this?

Answer 1

Do I need to manually convert float[] to UnityEngine.Vector3[]? Or is there a simpler and faster way that directly do this?

Yes, you have to do that manually but there is a better way to do this.

I will assume that you need to modify a Vector3 on the native side then return the result. There is no need to use float array for this. Just create a Vector3 struct on the C++ side then use pointer to pass it between C++ and C#. Do not return the Vector3 array from C++, create it on the C# side, then pass it to the C++ function to modify and apply the changes to the argument.

C++:

This requires that you enable the unsafe keyword in Unity.

struct Vector3
{
     float x;
     float y;
     float z;
};

then your function:

extern "C" void UpdateVectorArray(Vector3* vecArray, int vecSize)
{
    for(int i = 0; i < vecSize; ++i)
    {
        //Simply modify each Vector reference
        Vector3 &vec = vecArray[i];
        vec.x = 11;
        vec.y = 20;
        vec.z = 55;
    }
}

C#:

[DllImport("Plugin Name")]
static unsafe extern void UpdateVectorArray(Vector3* vecArray, int vecSize);


void UpdateVectorArray(Vector3[] vecArray)
{
    unsafe
    {
        //Pin array then send to C++
        fixed (Vector3* vecPtr = vecArray)
        {
            UpdateVectorArray(vecPtr, vecArray.Length);
        }
    }
}

Usage:

Get vertices from model, send to C++ and modify it the re-assign the modified mesh:

void Start()
{
    Mesh mesh = GetComponent<MeshFilter>().mesh;

    Vector3[] vertices = mesh.vertices;
    UpdateVectorArray(vertices);

    //Re-assign the modified mesh
    mesh.vertices = vertices;
    mesh.RecalculateBounds();
}

---

To avoid using the unsafe keyword in Unity use the [In, Out] attribute.

[DllImport("Plugin Name")]
static extern void UpdateVectorArray([In, Out] Vector3[] vecArray, int vecSize);

void Start()
{
    Mesh mesh = GetComponent<MeshFilter>().mesh;

    Vector3[] vertices = mesh.vertices;
    UpdateVectorArray(vertices, vertices.Length);

    mesh.vertices = vertices;
    mesh.RecalculateBounds();
}

The C++ side remains the-same. You can also use the GCHandle to pin the array and avoid using the unsafe keyword but the unsafe keyword solution is better and faster.