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

3 Oct 2006 8:06 AM

This sample is in response to a question left on my previous post, namely how to call an unmanaged dll from managed code when the dll in question isn't known until runtime (for instance, the path is stored in the registry, or an xml file, etc etc).

Apologies if this sample seems a little hurried, but I have another presentation to write and so time is short!

So let's begin.

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
{
        [DllImport("kernel32.dll")]
        public static extern IntPtr LoadLibrary(string dllToLoad);

        [DllImport("kernel32.dll")]
        public static extern IntPtr GetProcAddress(IntPtr hModule, string procedureName);

[DllImport("kernel32.dll")]
public static extern bool FreeLibrary(IntPtr hModule);
}

class Program
{
        [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
        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(
                                                                                        pAddressOfFunctionToCall,
                                                                                        typeof(MultiplyByTen));

                int theResult = multiplyByTen(10);

                bool result = NativeMethods.FreeLibrary(pDll);
                //remaining code here

                Console.WriteLine(theResult);
        }
}

The only item worthy of note is the UnmanagedFunctionPointer attribute, which was introduced to version 2.0 of the .NET framework, check out the docs online for more information.

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