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)
... compile the *.cpp file to a *.dll file and in the C# file:
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
FreeLibrary. You can find the C++ declare type functions on MSDN (msdn.com). I only show the declare in C#:
[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);
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
Marshal.GetDelegateForFunctionPointer. It is declared in
System.Runtime.InteropServices as follows:
public static Delegate GetDelegateForFunctionPointer (
(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