Why doesn't Array.Copy to mesh.verticies work in Unity?

Question

I am rebuilding a mesh but something seems odd that I have to create a temp array just to hold the verticies rather than copy an array.

I'm not using list because this is used frequently and I do not want to resize list and instead just use height index boundary to prevent additional garbage collection from resizing lists.

Doesn't work and results in no mesh showing:

Mesh mesh = new Mesh();
//copy myVerticiesArray to mesh.verticies. myVerticiesArray
//length has null values above verticiesIndexHeight but array copy height set in Array.Copy()
Array.Copy(myVerticiesArray, mesh.vertices, verticiesIndexHeight);
[...] code to set UVs etc omitted
mesh.Optimize();
mesh.RecalculateNormals();
mesh.RecalculateBounds();
this.GetComponent<MeshFilter>().mesh = mesh;

Also doesn't work and results in no mesh showing:

Mesh mesh = new Mesh();
//loop through built verticies array to mesh.verticies of correct size because myVerticiesArray
//length has null values above verticiesIndexHeight
mesh.verticies = new Vector3[verticiesIndexHeight];
for(int i = 0; i < verticiesIndexHeight; i++)
{
    mesh.verticies[i] = myVerticiesArray[i];
}
[...] code to set UVs etc omitted
mesh.Optimize();
mesh.RecalculateNormals();
mesh.RecalculateBounds();
this.GetComponent<MeshFilter>().mesh = mesh;

Works and renders mesh:

Mesh mesh = new Mesh();
Vector3[] tempVerticiesArray = new Vector3[verticiesIndexHeight];
//copy built verticies array to temp array of correct size because myVerticiesArray
//length has null values above verticiesIndexHeight
Array.Copy(myVerticiesArray, tempVerticiesArray, verticiesIndexHeight);
mesh.verticies= tempVerticiesArray;
[...] code to set UVs etc omitted
mesh.Optimize();
mesh.RecalculateNormals();
mesh.RecalculateBounds();
this.GetComponent<MeshFilter>().mesh = mesh;

Answer 1

Mesh.vertices is a property, not a variable. According to docs, it:

Returns a copy of the vertex positions or assigns a new vertex positions array.

Which means, when you call mesh.vertices, you don't get the actual vertices, you get a copy of actual vertices. So when you copy something to this array, you actual modify the clone, not the actual data, hence it will have no effect. The third way is the correct way to modify mesh.

There is a reason it is done this way: the actual mesh data may be in GPU memory, so modifying it may be an expensive operation. You should first build you mesh locally, then update the actual data.