void Call(int *count, Rect **arr)
{
 //…..
 //重新Malloc一段内存，指针复制给入参,外部调用前并不知道长度，另外提供接口Free内存
 //….
}

Struct Rect
{
int x;
int y;
int width;
int height;
};

Struct Rect
{
public int x;
public int y;
public int width;
public int height;
}

 [DllImport("xxx.dll", EntryPoint = "Call", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
 public static extern void Call(
     ref int count,
     ref IntPtr pArray);

IntPtr pArray = IntPtr.Zero; //数组指针
int count = 0;
Call(ref count, ref pArray);
var rects = new Rect[count]; //结果数组
for (int i = 0; i < count; i++)
{
var itemptr = (IntPtr)((Int64)Rect + i * Marshal.SizeOf(typeof(Rect))); //这里有人用的UInt32，我用的时候溢出了，换成Int64
rects[i] = (Rect)Marshal.PtrToStructure(itemptr, typeof(Rect));
}