Using Unmanaged code and assembler in C#

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Introduction

The first question is why we call unmanaged code before we discuss how to call unmanaged code.

There are possibly two reasons to call unmanaged code

1. You want to reuse your code which is already written in unmanaged environment e.g. VC 6.0
2. You want to Perform some low level work i.e. (need in line assembly in your program)

How to call unmanaged code

The first time I saw this topic in Tom Archer's "Inside C#" which explain how to call unmanaged DLL from the C#

Program 1

// Sample program to call unmanaged code

using System;
using System.Runtime.InteropServices;

class PInvoke1App
{
    [DllImport("user32.dll")]
    static extern int MessageBoxA(int hWnd, string strMsg, string strCaption, int iType);

    public static void Main() 
    {
        MessageBoxA(0, "Hello, World!", "This is called from a C# app!", 0);
    }
}

Then I tried to make my own DLL and call that DLL from my Application

Program 2

// Dll1.cpp

// Written by Zeeshan Amjad


#include <windows.h>


BOOL __stdcall DllMain(HINSTANCE hInst, DWORD dwReason, LPVOID lpReserved) {
	return TRUE;
}

__declspec(dllexport) void __stdcall Message(char* p_szMessage) {
	MessageBox(NULL, p_szMessage, "Message from DLL", MB_OK);
}

And my C# Program is:

Program 3

// Native2.cs

// Written by Zeeshan Amjad


using System;
using System.Runtime.InteropServices;

class MainClass {
	[DllImport("Dll1.dll")]
	static extern void Message(string msg);
	
	public static void Main() {
		Message("Hello world");
	}
};

Now I ran program and my program crashed because it can not find DLL1.dll - either in current directory nor in the path. It threw a DllNotFoundException. To handle this I had to catch this exception. So I change my program little bit to:

Program 4

// Native3.cs

// Written by Zeeshan Amjad


using System;
using System.Runtime.InteropServices;

class MainClass {
	[DllImport("Dll1.dll")]
	static extern void Message(string msg);
	
	public static void Main() {
		try {
			Message("Hello world");
		}
		catch(DllNotFoundException e) {
			Console.WriteLine(e.ToString());
		}
	}
};

And also copied the DLL1.dll to the current folder to avoid this exception.

Now again my program crashed when I tried to run it. This time it threw a EntryPointNotFoundException. To handle this more elegantly I should also catch this and display an error message for this exception rather than crash the program. This is new version of the program:

Program 5

// Native4.cs

// Written by Zeeshan Amjad


using System;
using System.Runtime.InteropServices;

class MainClass {
	[DllImport("Dll1.dll")]
	static extern void Message(string msg);
	
	public static void Main() {
		try {
			Message("Hello world");
		}
		catch(DllNotFoundException e) {
			Console.WriteLine(e.ToString());
		}
		catch(EntryPointNotFoundException e) {
			Console.WriteLine(e.ToString());
		}
	}
};

This program now give this error message

System.EntryPointNotFoundException: Unable to find an entry point named Message in DLL Dll1.dll.
   at MainClass.Message(String msg)
   at MainClass.Main()

The problem is not in this C# program. In fact when you write a function in C++ the compiler decorates the function name to enable function overloading. The function which exports by DLL is not Message. To get the exact name type 

dumpbin -exports dll1.dll  

at command prompt. The part of output of this utility is

    ordinal hint RVA      name

          1    0 00001005 ?Message@@YGXPAD@Z

There isn't any standard way of decorating function names, so we should tell the compiler not to decorate function name.

This is revised version of DLL code.

Program 6

// Dll1.cpp

// Written by Zeeshan Amjad


#include <windows.h>


BOOL __stdcall DllMain(HINSTANCE hInst, DWORD dwReason, LPVOID lpReserved) {
	return TRUE;
}

extern "C" __declspec(dllexport) void __stdcall Message(char* p_szMessage) {
	MessageBox(NULL, p_szMessage, "Message from DLL", MB_OK);
}

extern "C" is used to tell the compiler not to decorate the function name.

Now if we see the function name from the dumpbin utility its output looks like this.

    ordinal hint RVA      name

          1    0 0000100A _Message@4

Here @ shows the function uses the standard calling convention and 4 shows the number of bytes pushed on the stack for parameters. In a 32 bit environment like windows 9x and NT/2000 the address is stored in 32 bits i.e. 4 bytes. It means there is only one parameter in the stack  - in other words this function takes only one parameter.

Now the above C# Program works fine without any change and displays a message box with Text "Hello world" with the caption "Message from DLL"

How to call assembly in C#

Let's do an experiment with inline assembly in a DLL. I can not call assembly language from C# but I know I can call unmanaged DLLs from C#. I'll make a DLL which calculates the speed of CPU, vendor name, Family, Model and Stepping of CPU using in line assembly language.

Program 7

// SysInfo.cpp

// written by Zeeshan Amjad


#include "SysInfo.h"


BOOL __stdcall DllMain(HINSTANCE hInst, DWORD dwReason, LPVOID lpReserved) {
	return  TRUE;
}

extern "C" __declspec(dllexport) int __stdcall getCPUSpeed() {
	LARGE_INTEGER ulFreq, ulTicks, ulValue, ulStartCounter, ulEAX_EDX, ulResult;

	// it is number of ticks per seconds

	QueryPerformanceFrequency(&ulFreq);

	// current valueofthe performance counter

	QueryPerformanceCounter(&ulTicks);

	// calculate one second interval

	ulValue.QuadPart = ulTicks.QuadPart + ulFreq.QuadPart;

	// read time stamp counter

	// this asm instruction load the highorder 32 bit of the register into EDX

	// and the lower order 32 bits into EAX

	_asm {
		rdtsc
		mov ulEAX_EDX.LowPart, EAX
		mov ulEAX_EDX.HighPart, EDX
	}

	// start no of ticks

	ulStartCounter.QuadPart = ulEAX_EDX.QuadPart;

	// loop for 1 second

	do {
		QueryPerformanceCounter(&ulTicks);
	} while (ulTicks.QuadPart <= ulValue.QuadPart);

	// get the actual no of ticks

	_asm {
		rdtsc
		mov ulEAX_EDX.LowPart, EAX
		mov ulEAX_EDX.HighPart, EDX
	}

	// calculate result

	ulResult.QuadPart = ulEAX_EDX.QuadPart - ulStartCounter.QuadPart;

	return (int)ulResult.QuadPart / 1000000;
}

extern "C" __declspec(dllexport) char* __stdcall getCPUType() {
	static char pszCPUType[13];
	memset(pszCPUType, 0, 13);

	_asm {
		mov eax, 0
		cpuid

		// getting information from EBX

		mov pszCPUType[0], bl
		mov pszCPUType[1], bh

		ror  ebx, 16
		mov pszCPUType[2], bl
		mov pszCPUType[3], bh

		// getting information from EDX

		mov pszCPUType[4], dl
		mov pszCPUType[5], dh

		ror  edx, 16
		mov pszCPUType[6], dl
		mov pszCPUType[7], dh

		// getting information from ECX

		mov pszCPUType[8], cl
		mov pszCPUType[9], ch

		ror  ecx, 16
		mov pszCPUType[10], cl
		mov pszCPUType[11], ch
	}

	pszCPUType[12] = '\0';

	return pszCPUType;
}

extern "C" __declspec(dllexport) int __stdcall getCPUFamily() {
	int retVal;

	_asm {
		mov eax, 1
		cpuid
		mov retVal, eax
	}

	return (retVal >> 8);
}

extern "C" __declspec(dllexport) int __stdcall getCPUModel() {
	int retVal;

	_asm {
		mov eax, 1
		cpuid
		mov retVal, eax
	}

	return ((retVal >> 4 ) & 0x0000000f);
}

extern "C" __declspec(dllexport) int __stdcall getCPUStepping() {
	int retVal;

	_asm {
		mov eax, 1
		cpuid
		mov retVal, eax
	}

	return (retVal & 0x0000000f);
}

Here is a simple client of this DLL which is written in VC++ to check the functionality of this.

Program 8

// Client1.cpp

// Written by Zeeshan Amjad


#include 

#include "SysInfo.h"


#pragma comment(lib, "SysInfo.lib")

int main() {
	cout << "CPU Speed = " << getCPUSpeed() << endl;
	cout << "CPU Type = " << getCPUType() << endl;
	cout << "CPU Family = " << getCPUFamily() << endl;
	cout << "CPU Model = " << getCPUModel() << endl;
	cout << "CPU Stepping = " << getCPUStepping() << endl;
	return 0;
}

Now I m going to write the same client in C#.

Program 9

// Native5.cs

// Written by Zeeshan Amjad


using System;
using System.Runtime.InteropServices;

class MainClass {
	[DllImport("SysInfo.dll")]
	static extern int getCPUSpeed();
	
	[DllImport("SysInfo.dll")]
	static extern string getCPUType();
	
	[DllImport("SysInfo.dll")]
	static extern int getCPUFamily();
	
	[DllImport("SysInfo.dll")]
	static extern int getCPUModel();

	[DllImport("SysInfo.dll")]
	static extern int getCPUStepping();
	
	// main program

	public static void Main() {
		
		// get CPU Speed

		try {
			int iCPUSpeed = getCPUSpeed();
			Console.WriteLine("CPU Speed = {0}", iCPUSpeed.ToString());
		}
		catch (DllNotFoundException e) {
			Console.WriteLine(e.ToString());
		}
		catch (EntryPointNotFoundException e) {
			Console.WriteLine(e.ToString());
		}
		
		// get CPU Type

		try {
			string strType = getCPUType();
			Console.WriteLine("CPU Type = {0}", strType);
		}
		catch (DllNotFoundException e) {
			Console.WriteLine(e.ToString());
		}
		catch (EntryPointNotFoundException e) {
			Console.WriteLine(e.ToString());
		}
		
		// get CPU Family

		try {
			int iFamily = getCPUFamily();
			Console.WriteLine("CPU Family = {0}", iFamily.ToString());
		}
		catch (DllNotFoundException e) {
			Console.WriteLine(e.ToString());
		}
		catch (EntryPointNotFoundException e) {
			Console.WriteLine(e.ToString());
		}
		
		// get CPU Model

		try {
			int iModel = getCPUModel();
			Console.WriteLine("CPU Model = {0}", iModel.ToString());
		}
		catch (DllNotFoundException e) {
			Console.WriteLine(e.ToString());
		}
		catch (EntryPointNotFoundException e) {
			Console.WriteLine(e.ToString());
		}

		// get CPU Stepping

		try {
			int iStepping = getCPUStepping();
			Console.WriteLine("CPU Stepping = {0}", iStepping.ToString());
		}
		catch (DllNotFoundException e) {
			Console.WriteLine(e.ToString());
		}
		catch (EntryPointNotFoundException e) {
			Console.WriteLine(e.ToString());
		}
	}
};

I write GUI for this program. The image below shows the output.