Variable as array size in struct

I implement a file's struct in my program but for some arrays in the struct I don't know the size. The size of the array is stored in another variable but it's unknown before the struct is filled in.

struct Vertex {
    float x;
    float y;
    float z;
};
struct myFile {
    ulong nVertices;
    Vertex vertices[nVertices];
};

That gives an error: "error C2065: 'nVertices' : undeclared identifier".