Tuesday, October 03, 2006

Dynamically calling an unmanaged dll from .NET (C#)

I like to keep collecting small snippets of code in my blog, which i know will be useful later on.
Below is the code snippet for "Dynamically calling an unmanaged dll from .NET (C#)" from Jonathans blog on MSDN blogs:

To start and to refresh our memories, let's create a very basic C++ dll that does very little..... your code should resemble the following (check out my previous post for more info on this):

Header file

extern "C" __declspec(dllexport) int MultiplyByTen(int numberToMultiply);

Source code file

#include "DynamicDLLToCall.h"

int MultiplyByTen(int numberToMultiply)
int returnValue = numberToMultiply * 10;
return returnValue;

As you can probably infer from the function name, an int is passed into this function and it will return the number passed in multiplied by ten. Told you it would be simple.

Now comes the more interesting part, actually calling this dll dynamically from your C# source code. There are two Win32 functions that are going to help us do this:

1) LoadLibrary - returns a handle to the dll in question
2) GetProcAddress - obtain the address of an exported function within the previously loaded dll

The rest is rather simple. We use LoadLibrary and GetProcAddress to get the address of the function within the dll we want to call, and then we use the GetDelegateForFunctionPointer static method within the Marshal class to assign this address to a C# delegate that we define. Take a look at the following C# code:

static class NativeMethods
public static extern IntPtr LoadLibrary(string dllToLoad);

public static extern IntPtr GetProcAddress(IntPtr hModule, string procedureName);

class Program
private delegate int MultiplyByTen(int numberToMultiply);

static void Main(string[] args)
IntPtr pDll = NativeMethods.LoadLibrary(@"PathToYourDll.DLL");
//oh dear, error handling here
//if (pDll == IntPtr.Zero)

IntPtr pAddressOfFunctionToCall = NativeMethods.GetProcAddress(pDll, "MultiplyByTen");
//oh dear, error handling here
//if(pAddressOfFunctionToCall == IntPtr.Zero)

MultiplyByTen multiplyByTen = (MultiplyByTen)Marshal.GetDelegateForFunctionPointer(

int theResult = multiplyByTen(10);

The only item worthy of note is the UnmanagedFunctionPointer attribute, which was introduced to version 2.0 of the .NET framework,

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