ClrHacks. Kids, don't try this at home.

StreamTools.cpp

// StreamTools.h

#pragma once

#include "StdAfx.h"

using namespace System;
using namespace System::IO;
using namespace System::Runtime::InteropServices;

namespace ClrHacks {

	[StructLayout(LayoutKind::Sequential)]
	private value class SZARRAYHeader
	{
	public:
		IntPtr SyncBlock;
		IntPtr MethodTable;
		int32_t Length;
	};

	template<typename T>
	[StructLayout(LayoutKind::Sequential)]
	private value class AnyRef
	{
	public:
		T Ref;
	};

	template<typename T, typename U>
	T managed_cast(U ptr)
	{
#pragma warning( push )
#pragma warning( disable : 4669 )
		return reinterpret_cast<AnyRef<T>*>(&ptr)->Ref;
#pragma warning( pop )
	}

	public ref class StreamTools
	{
	public:
		static void WriteLargeBlock(Stream^ stream, uint8_t* buffer, int32_t offset, int32_t count)
		{
			 ;
		}

		static void WriteLargeBlockUnsafe(Stream^ stream, uint8_t* buffer, int32_t offset, int32_t count)
		{
			array<uint8_t>^ dummy_arr = gcnew array<uint8_t>(1);
			pin_ptr<uint8_t> dummy_arr_pin = &dummy_arr[0];
			SZARRAYHeader* dummy_arr_header = reinterpret_cast<SZARRAYHeader*>(dummy_arr_pin - sizeof(SZARRAYHeader));
			SZARRAYHeader* header = reinterpret_cast<SZARRAYHeader*>(buffer + offset - sizeof(SZARRAYHeader));
			header->SyncBlock = dummy_arr_header->SyncBlock;
			header->MethodTable = dummy_arr_header->MethodTable; //array<uint8_t>::typeid->TypeHandle.Value;
			Console::WriteLine(header->MethodTable);
			Console::WriteLine(array<uint8_t>::typeid->TypeHandle.Value);
			header->Length = count;

			array<uint8_t>^ tmp_arr = managed_cast<array<uint8_t>^>(&header->MethodTable);
			/*pin_ptr<uint8_t> t = &tmp_arr[0];
			Console::WriteLine(IntPtr(t));
			Console::WriteLine(IntPtr(buffer + offset));
			Console::WriteLine(IntPtr(t - (buffer + offset)));
			Console::WriteLine("Length: {0:X2}, Rank: {1:X2}, lower bound: {2:X2}", tmp_arr->Length, tmp_arr->Rank, tmp_arr->GetLowerBound(0));
			Console::WriteLine("Type: {0}", tmp_arr->GetType()->FullName);*/
			stream->Write(tmp_arr, 0, count);
		}
	};
}