Free unmanaged memory from C#

How do I free memory allocated on unmanaged heap from a C# App?
Below is the sample code which one can use.
There is a C# app calling a native c-style DLL's method which is allocating some memory.

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// MemoryDll.cpp : Defines the entry point for the DLL application.
//

#include "stdafx.h"
#include "MemoryDll.h"
#include <stdlib.h>

BOOL APIENTRY DllMain( HANDLE hModule,
                       DWORD  ul_reason_for_call,
                       LPVOID lpReserved
)
{
    switch (ul_reason_for_call)
{
case DLL_PROCESS_ATTACH:
case DLL_THREAD_ATTACH:
case DLL_THREAD_DETACH:
case DLL_PROCESS_DETACH:
break;
    }
    return TRUE;
}


// This is an example of an exported variable
MEMORYDLL_API int nMemoryDll=0;

// This is an example of an exported function.
MEMORYDLL_API int AllocateMemory(char ** achMemory)
{
*achMemory=(char*)calloc(10*1000*1000,1);
return 0;
}


MEMORYDLL_API int FreeMemory(char ** achMemory)
{
free((char*)*achMemory);
return 0;
}

// This is the constructor of a class that has been exported.
// see MemoryDll.h for the class definition
CMemoryDll::CMemoryDll()
{
return;
}

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using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
using System.Runtime.InteropServices;

namespace JeffSuggestion
{
/// <summary>
/// Summary description for Form1.
/// </summary>
public class Form1 : System.Windows.Forms.Form
{

private static string strInBuff = "";

// [DllImport("CryptoUtils.dll")]
// static extern int EncryptBufferUsingSymmetricKey(out IntPtr outBuffer, out Int32 outBufferLen, IntPtr inBuffer, Int32 inBufferLen, IntPtr SymmKey, Int32 symmKeyLen);


[DllImport("MemoryDll_debug.dll")]
static extern int AllocateMemory(out IntPtr outBuffer);

[DllImport("MemoryDll_debug.dll")]
static extern int FreeMemory(IntPtr outBuffer);

private System.Windows.Forms.Button button1;
/// <summary>
/// Required designer variable.
/// </summary>
private System.ComponentModel.Container components = null;

public Form1()
{
//
// Required for Windows Form Designer support
//
InitializeComponent();

//
// TODO: Add any constructor code after InitializeComponent call
//
}

/// <summary>
/// Clean up any resources being used.
/// </summary>
protected override void Dispose( bool disposing )
{
if( disposing )
{
if (components != null)
{
components.Dispose();
}
}
base.Dispose( disposing );
}

#region Windows Form Designer generated code
/// <summary>
/// Required method for Designer support - do not modify
/// the contents of this method with the code editor.
/// </summary>
private void InitializeComponent()
{
this.button1 = new System.Windows.Forms.Button();
this.SuspendLayout();
//
// button1
//
this.button1.Location = new System.Drawing.Point(140, 92);
this.button1.Name = "button1";
this.button1.TabIndex = 0;
this.button1.Text = "unsafe";
this.button1.Click += new System.EventHandler(this.button1_Click);
//
// Form1
//
this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
this.ClientSize = new System.Drawing.Size(292, 266);
this.Controls.Add(this.button1);
this.Name = "Form1";
this.Text = "Form1";
this.Load += new System.EventHandler(this.Form1_Load);
this.ResumeLayout(false);

}
#endregion

/// <summary>
/// The main entry point for the application.
/// </summary>
[STAThread]
static void Main()
{
Application.Run(new Form1());
}

private void Form1_Load(object sender, System.EventArgs e)
{

}

// [DllImport("CryptoUtils.dll")]
// static extern void Funx(out IntPtr outBuffer, out Int32 outBufferLen, IntPtr inBuffer, Int32 inBufferLen);


private void button1_Click(object sender, System.EventArgs e)
{
string strInBuff1 = "1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890";

for(int j=0; j < 10; j++)
strInBuff += strInBuff1;

for(int i=0; i < 10; i++)
x();

MessageBox.Show("DONE");
}

unsafe private void x()
{
Int32 outBufferLen;
IntPtr outBuffer;

int iRet = -1;

Byte[] inBuffer = null;//{ 1, 2, 3, 4 };
// Byte[] inSymm   = null;

inBuffer = System.Text.Encoding.UTF8.GetBytes(strInBuff);

// inSymm = Convert.FromBase64String("vbWMjXdNLL6rQbXzvKKTrg==");


fixed (Byte* p = (Byte *) &inBuffer[0])
{
iRet = AllocateMemory(out outBuffer);
}

FreeMemory(outBuffer);

// fixed (Byte* p1 = (Byte *) &inSymm[0])
// {
// fixed (Byte* p = (Byte *) &inBuffer[0])
// {
// iRet = EncryptBufferUsingSymmetricKey(out outBuffer, out outBufferLen, (IntPtr) p, inBuffer.Length,(IntPtr) p1, inSymm.Length );
// }
// }
// Byte *q = (Byte *) outBuffer;
// Access q[0], etc...
}

}
}

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