How to pass an array with unknown size in C# function from C++ dll?

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C#

Hi all,
I have a C++ dll file which contains an exported function named fn(double* p, int* pLength), where, p is a pointer (is an out array used in C#), pLength which is calculated in this function is the length (size) of p . Code here:

void _stdcall fn(double* p, int* pLength)
{
    int i=0;
    double p_=0;
    do
    {
        p[i]=p_;
        p_=an expression!!!!!;
        i++;
    }
    while (p_<20);

    *pLength=i;

    return;
}

I compile to dll file successfuly. This file is named "testFile.dll" and move it to System32 folder.
Now, I start C# console project and declare that exported function fn() from "testFile.dll", and this code is:

using System;
using System.Collections.Generic;
using System.Text;
using System.Runtime.InteropServices;

namespace ConsoleApplication1
{
    class Program
    {
        [DllImport("testFile.dll")]
        public static extern void fn(double[] p, ref int pLength);
        static void Main(string[] args)
        {
            // I want to declare an array with unknown length (size),
            // because the length is calculated from fn() function!!!
            // And pass to fn() function to get p[i].....
            double[] p;
            int pLength;
            fn(p, ref pLength);

            // Display pLength and p[i]
            Console.WriteLine(pLength);
            for (int i = 0; i < pLength; i++)
                Console.WriteLine(p[i]);
            Console.ReadKey();
        }
    }
}

I run and get two errors:
Error 1 Use of unassigned local variable 'p'
Error 2 Use of unassigned local variable 'pLength'

How to fix them?, and I want to get pLength and p[i] fully from fn() function in "testFile.dll". Thanks in advance.