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Dynamic Invoke C++ DLL function in C#

, 26 Jun 2008 CPOL
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This article introduces why I use dynamic invoke C++ DLL function in C# and how to call it.


This article introduces why I use dynamic invoke C++ DLL function in C# and how to call it.

Why I Use Dynamic Invoke Instead of Static Invoke?

Sometimes when I want to write and call some unmanaged function (C++ DLL function), I do the following:

In the *.cpp file...

int __declspec(dllexport) a(int b)
      return b;

... compile the *.cpp file to a *.dll file and in the C# file:

class Program
   private static extern int a(int b);
   static void Main(string[] args)

In this method, you see that I have to declare a static DLL filename, and when I compile to an excutable file (*.exe), I cannot change the DLL filename.

The second case is, I want to write some C++ DLL function for an ASP.NET/IIS (Internet Information Services) Web site. If I static invoke unmanaged, when I want to update the DLL file, I stop the Web site in IIS and replace the old file with the new file, but .. IIS still keeps the old DLL file. It does not release the file, and I have to stop the entire Web site, all services running in IIS for replacing the file.

So, I want to dynamically invoke an unmanaged file. I can load, invoke and free unmanaged DLL function initiatively.

How to Dynamic Invoke Unmanaged DLL Function?

The first way is by referring to dynamicinvokedll.aspx.

The second way that I want to introduce is how to use the WindowAPI function. We use three functions in kernel32.dll (this is a kernel DLL file and appears in [WindowRoot]\System32) LoadLibrary, GetProcess, and FreeLibrary. You can find the C++ declare type functions on MSDN ( I only show the declare in C#:

class Program
        [DllImport("kernel32.dll", EntryPoint = "LoadLibrary")]
        static extern int LoadLibrary(
            [MarshalAs(UnmanagedType.LPStr)] string lpLibFileName);

        [DllImport("kernel32.dll", EntryPoint = "GetProcAddress")]
        static extern IntPtr GetProcAddress( int hModule,
            [MarshalAs(UnmanagedType.LPStr)] string lpProcName);

                [DllImport("kernel32.dll", EntryPoint = "FreeLibrary")]
        static extern bool FreeLibrary(int hModule);

The LoadLibrary function loads an unmanaged file, the GetProcAddress function gets the function pointer of an unmanaged file and the FreeLibrary function frees the unmanaged file. Now I can get the function pointer in a.dll very easily:

int hModule = LoadLibrary(@"c:\a.dll");
if (hModule == 0) return false;
IntPtr intPtr = GetProcAddress(hModule, "a");

And when I want to free the a.dll file, I can call:


But C# doesn’t suport C++ function pointer, so we cannot invoke a C++ function pointer here. C# only has Delegate objects and we have to convert the function pointer to Delegate by Marshal.GetDelegateForFunctionPointer. It is declared in System.Runtime.InteropServices as follows:

public static Delegate GetDelegateForFunctionPointer (
      IntPtr ptr,
      Type t

(You can find more support for this function in MSDN.)

We first declare the Delegate for a function:

delegate int A(int b);

And get the delegate object as follows:

A a = (A)Marshal.GetDelegateForFunctionPointer(intPtr, typeof(A));

Now we can invoke a function as follows:

static void Main(string[] args)

Hope you found this article helpful.


  • 26th June, 2008: Initial post


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Engineer NaisCorp -
Vietnam Vietnam
No Biography provided

Comments and Discussions

QuestionWhat if I need to marshal parameters and return value? PinmemberMember 789751523-Sep-13 8:14 
GeneralThis helps me a lot!!!! PinmemberKydz_Leo19-May-12 6:01 
QuestionIt's very good PinmemberAndrewpeter22-Mar-12 22:47 
GeneralMy vote of 5 PinmemberAndrewpeter22-Mar-12 22:46 
GeneralMy vote of 5 Pinmembercmma8624-Feb-12 5:48 
GeneralMy vote of 5 PinmemberMorries13-Dec-11 16:38 
QuestionPInvoke function unbalanced the stack PinmemberSilaghi20-Nov-11 2:29 
Hi, I'm just starting to use DLLs. I found your tutorial very helpful , however whenever I try debugging the code in visual studio 2010 I keep getting
A call to PInvoke function 'ConsoleApplication1!ConsoleApplication1.Program+func::Invoke' has unbalanced the stack. This is likely because the managed PInvoke signature does not match the unmanaged target signature. Check that the calling convention and parameters of the PIn
voke signature match the target unmanaged signature.

Running it without debugging outputs the correct answer ,however I would like to know what is causing this Smile | :)
GeneralMy vote of 5 PinmemberXanblax11-Oct-11 7:01 
GeneralMy vote of 4 PinmemberJimmyGong24-May-11 17:00 
GeneralMy vote of 5 PinmemberJangjeet Singh25-Apr-11 5:26 
GeneralMy vote of 5 PinmemberLeKoosy31-Mar-11 4:27 
GeneralI have a problem Pinmemberhenry369521-Nov-10 1:30 
GeneralRe: I have a problem [modified] PinmemberAli Fakoor1-Oct-11 23:16 
GeneralRe: I have a problem Pinmemberpurgatory28-May-12 22:40 
GeneralA note about FreeLibrary() Pinmembermaeloc26-Jun-08 22:26 
GeneralRe: A note about FreeLibrary() Pinmembertwaindev1-Jul-08 6:18 

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