Call C function inside C# via function pointer

You'll want to use Marshal.GetDelegateForFunctionPointer. You won't even need to use unsafe code.

delegate void TestCallbackDelegate(); //must match the signature of test_callback()

public static void callCallback(int ptr)
{
    IntPtr nativePtr = new IntPtr( ptr );

    var callback = Marshal.GetDelegateForFunctionPointer<TestCallbackDelegate>( nativePtr );

    callback();
}

public static int test(string param)
{
    char[] ptrChar = param.ToCharArray();
    int ptrInt = 0;

    ptrInt = ( ((int)(0xFF00 & (int)ptrChar[1]) | (0x00FF & (int)ptrChar[1])) << 16 ) |
                (int)(0xFF00 & (int)ptrChar[0]) | (0x00FF & (int)ptrChar[0]);

    callCallback(ptrInt);
}

Although a much simpler way would be to just pass a void* to the C# method, and it will automatically get marshalled to IntPtr. Here's a minimal example of that:

C++

//invoke.cpp
//compile with: cl /EHsc /LD /nologo invoke.cpp
#include <stdio.h>

static void test_callback() 
{
    printf("test_callback called\n");
}

extern "C" __declspec( dllexport ) void* getPointer()
{
    return (void*)&test_callback; //Return a raw pointer to the test_callback function.
}

C#

//invoke.cs
//compile with: csc /nologo invoke.cs
using System;
using System.Runtime.InteropServices;

class Program
{
    [DllImport( "invoke.dll" )]
    private static extern IntPtr getPointer();

    private delegate void TestCallbackDelegate(); //Delegate that matches the signature of test_callback

    static void main()
    {
        IntPtr ptr = getPointer(); //Fetch the native void pointer.
        TestCallbackDelegate test_callback = Marshal.GetDelegateForFunctionPointer<TestCallbackDelegate>( ptr ); //Marshal the void pointer to a delegate.
        test_callback(); //Invoke the native C function.
    }
}

I used the DllImport attribute to avoid having to invoke the CLR as you're doing, but it's the same idea.

EDIT: Because I realized the above doesn't apply to what the OP was asking, I'll include an updated and proper sample. The above will remain for posterity.

C

#define COBJMACROS

#include <stdio.h>
#include <windows.h>
#include <mscoree.h>

static void test_callback()
{
    printf( "test_callback has been called.\n" );
}

int main( void )
{
    HRESULT status;
    ICLRRuntimeHost *Host;
    BOOL Started;
    DWORD Result;

    Host = NULL;
    Started = FALSE;

    status = CorBindToRuntimeEx( NULL, NULL, 0, CLSID_CLRRuntimeHost, IID_ICLRRuntimeHost, (void**)&Host );

    if( FAILED( status ) )
        goto cleanup;

    status = ICLRRuntimeHost_Start( Host );

    if( FAILED( status ) )
        goto cleanup;

    Started = TRUE;

    int ptr = (int)&test_callback;
    printf( "test_callback is at 0x%X\n", ptr );

    char param[5];
    param[0] = 0xFF & ( ptr >> 0 );
    param[1] = 0xFF & ( ptr >> 8 );
    param[2] = 0xFF & ( ptr >> 16 );
    param[3] = 0xFF & ( ptr >> 24 );
    param[4] = '\0';

    status = ICLRRuntimeHost_ExecuteInDefaultAppDomain( Host, L"invoke.dll", L"InteropTesting.Invoker", L"InvokeCallback", (LPCWSTR)param, &Result );

    if( FAILED( status ) )
        goto cleanup;

cleanup:
    if( Started )
        ICLRRuntimeHost_Stop( Host );

    if( Host != NULL )
        ICLRRuntimeHost_Release( Host );

    return SUCCEEDED( status ) ? 0 : 1;
}

C#

using System;
using System.Runtime.InteropServices;

namespace InteropTesting
{
    public static class Invoker
    {
        private delegate void TestCallbackDelegate();

        public static int InvokeCallback( string param )
        {
            //C# has a built-in means of turning byte arrays into integers
            //so we'll use BitConverter instead of using the bitwise operators.
            char[] chars = param.ToCharArray();
            int ptr = BitConverter.ToInt32( Array.ConvertAll( chars, c => (byte)c ), 0 );

            var test_callback = (TestCallbackDelegate)Marshal.GetDelegateForFunctionPointer( new IntPtr( ptr ), typeof( TestCallbackDelegate ) );
            test_callback();

            return 0;
        }
    }
}