P/Invoke in ASP.NET (Dll not found exception)

Question

I have a .dll that I generated thorugh a C++ project. I have to use this dll in my ASP.NET project and I have written DllImport functions for the same in my project.

The static class inside App_Code has some DllImport functions

public static class Functions
{
    [DllImport("MyFav.dll", EntryPoint = "fnmain", 
     CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
    public static extern StringBuilder fnmain();
}

Since I could not add the C++ dll directly as a reference in my ASP.NET project (because it is not a .NET assembly), I just copied into the top level directory. ( Name of ASP.NET Project-> Right Click -> Add Existing Item )

Now, when I try to run the project, I get the following error:

Exception:

Unable to load DLL 'MyFav.dll': The specified module could not be found. (Exception from HRESULT: 0x8007007E)

Any suggestions? Where has the .dll to be kept?

Maybe the dll is not found, maybe the dll found but you have use some libraries (like mfc) and not found on the directories that used by the asp.net account — Aristos

gideon gideon · Accepted Answer · 2012-03-27T08:42:38

The .dll should be in the \bin folder of your asp.net application.

Apparently there are rules for how dlls are resolved. See Dynamic-Link Library Search Order on msdn

You could also try:

[DllImport("C:\path_to_dll\MyFav.dll", EntryPoint = "fnmain", 
 CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
public static extern StringBuilder fnmain();

If all this doesn't work, maybe your MyFav.dll has additional unresolved dependencies. You could use Dependency Walker to check for these.

P/Invoke in ASP.NET (Dll not found exception)

3 Answers