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Convert C++ .lib file to .NET C# .dll file

Posted on 2006-06-26
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Last Modified: 2012-05-05
Hello everyone,

I have a complete C++ application that I need to convert entirely to C#. The C++ application has .lib files which are the class libraries. In converting the entire project to C#, how would I convert those .lib files to .dll files, in order to reference them in my C# project? I am using Visual Studio 2003 .NET

Thanks,

Tone
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Question by:worldknown
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Expert Comment

by:dstanley9
ID: 16985283
it isn't possible (that I know of) to convert compiled C++ (unmanaged) code to managed DLLs.  Best bet bay be to create a C++.NET app that links in the .LIB files and defines wrapper classes that can be called from C#.
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Author Comment

by:worldknown
ID: 16985753
How would I take your approach? Don't know much about C++, all I know is that I have to convert the project to C#. Thanks
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Author Comment

by:worldknown
ID: 16985770
I also thought I could convert the .libs to .dll by COM interop, no?
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Expert Comment

by:Bob Learned
ID: 16986146
How big and complex is the DLL?  C++ and C# are significantly different--in fact C# is more like VB.NET than C#.

Bob
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Author Comment

by:worldknown
ID: 16986185
It's not a .dll, It's a C++ filter library. I would like to convert that filter library into a .net assembly( .dll) that I can reference into my c# application. I found this online http://www.codeproject.com/cs/media/directxcapture.asp, but am looking to convert the filter library( .lib) into a .net .dll assembly. It's a pretty large filter library. Has 5 different filters in the library.

Thanks
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Expert Comment

by:dstanley9
ID: 16986216
No, C++ LIBs are nut runnable.  They can be "linked" into a C++ project to create something that is runnable (an EXE or a DLL).  To interop between C# and raw unmanaged C++, you need to link the LIB into a C++ project, preferebly a managed DLL so that interop with C# is easier.
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Author Comment

by:worldknown
ID: 16986309
Cool. Is there any simple tutorials on how to accomplish this? I could email my project, so you can get a better feel if you like. I would really appreciate it.
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Expert Comment

by:dstanley9
ID: 16986581
To link in a LIB to your C++.NET project:

* create a Class Library in C++
* add the header file(s) for the LIB
* Open up Project-->Properties and go to the Configuration Properties-->Linker-->Input section.
* Click the ... button in the Additional Dependencies box and add your LIB
* Resolve any linker errors by adding additional LIBs if necessary (you'll get errors when you build)
* Write the C++ wrapper to expose the LIB classes and functions to .NET (this is NOT done automatically!!!)

Once the C++ DLL is build, it can be added as a reference to your C# project.

Good Luck!
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Expert Comment

by:dstanley9
ID: 16986640
I feel that I should warn you that this is NOT a "wizard" or "flip the switch" type of project...  I have had projects that took WEEKS to build a C++ wrapper to an unmanaged library.  The coding itself isn't that complex, it just takes a lot of time to create each method, which usually is just a pass-through to the unmanaged library.

Bascailly what you need to do is take ALL of the public classes and methods from the header files and generate managed wrappers for them.
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Author Comment

by:worldknown
ID: 16987279
Let me give your solution a try and I'll let you know how things go.
Thanks,
T
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by:worldknown
ID: 16987582
* Open up Project-->Properties and go to the Configuration Properties-->Linker-->Input section.

 I don't see anything that says linker. Am I missing something? Please help.

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Author Comment

by:worldknown
ID: 16987618
Never mind, found it under Project --> New Class Properties :)
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by:dstanley9
ID: 16987849
Make sure you set that in all configurations (debug & release) or you'll get build errors when building in release mode.
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by:worldknown
ID: 16988026
When I build the project, I get a prompt called "Executable for Debugging file to be used" and it gives me a "Please specify the name of the executable file to be used for the debug session". What executable is it referring to?
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Expert Comment

by:dstanley9
ID: 16988090
You can't debug a DLL file directly, you have to have an executable (or web app) to run.  Create a simple C# console app in the same solution, add a project reference to your C++ DLL, and call some methods from it.  thet set the C# EXE as your startup project.
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Author Comment

by:worldknown
ID: 17001612
Hi Dstanley,

Sorry for late response. I found this link http://www.codeguru.com/cpp/cpp/cpp_managed/interop/article.php/c6867/ based on the guidance you have provided and i'm hoping that this is my answer. I will give you the points based on your guidance and patience alone. However, I ask you to do one more thing for me, could you confirm that this link is what I need?

Thanks,
T
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Expert Comment

by:dstanley9
ID: 17001719
In a way, yes.  That example is using a C++.NET wrapper to interop with an existing DLL.  My suggestion (since you have LIBs and not DLLs is to link the LIB files into the C++.NET DLL.  So you have one DLL instead of two.

Good Luck!
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Author Comment

by:worldknown
ID: 17001912
I'm confused again. Could you be so kind in throwing together a very basic project showing simply just the essence of the logic?
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Accepted Solution

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dstanley9 earned 2000 total points
ID: 17002037
The difference is between _static_ linking and _dynamic_ linking.  LIBs are linked statically, meaning that the compiled code (machine code, not C++ code) is actually embedded in the resulting file (either a DLL or an EXE).  The LIB file is not deployed with the application.  Dynamic linking using DLLs keeps the compiled code inside the DLL, and the linking is done dynamically at run time.

The link you posted is using C++ to interop with a DLL, which cannot be statically linked into the new DLL, so both DLLs must be deployted together.  Since you have LIB files, you can statically link them into the new C++.NET DLL - in which case the code within the LIB is embedded in the DLL, so you don't need to deploy the LIBs.  You can then reference the C++.NET DLL from your C# projects.

So you've got two files:  The C# EXE (presumably) and the C++.NET DLL which includes all of the unmanaged C++ LIBs.  

If you want, you _could_ create separate C++.NET DLLs for each LIB and wrap them separately.  This may be a good option for flexibility since you could deploy only the C++.NET DLLs that the C# project references.

So the C++.NET project is a Class Library (DLL) project that includes .NET classes to wrap the classes in the LIB and statically links in the
LIB files.  The C# project simple references the C++.NET projet, and knows nothing about the LIB files.


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Author Comment

by:worldknown
ID: 17002348
Hey Dstanley,

Could I send you my project so you could at least compile it as a .dll? I'm having difficulties with this initial part. I understand the logic however, build my C++ app as a .dll, reference that .dll in my C# project, then use marshalling to communicate with that .dll through interop. I can't seem to get my project to compile into a .dll. If you could just do that and send it back, that would be awesome.
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Author Comment

by:worldknown
ID: 17002478
Or better yet, look what I found. I think this may be it. http://www.thecodeproject.com/csharp/UseCDLLlibinCS.asp
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Author Comment

by:worldknown
ID: 17002563
I got this buddy. Thanks for all your help and patience.

T
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Expert Comment

by:dstanley9
ID: 17002565
What compiler or linker errors are you getting?

You can use DllImport as well.  You'd basically create a C++ DLL that exports a bunch of C-style functions.  However it will limit the types of data that yo can pass back and forth.  Creating Managed C++ classes will give you more flexibility.
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Author Comment

by:worldknown
ID: 17002590
Is there an article for creating managed C++ classes that you would recommend?
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Author Comment

by:worldknown
ID: 17003759
I've created my .dll file from C++, not when trying to reference it in my C# app, it tells me that my .dll is not  a valid assembly. My hell continues! Any suggestions?

T
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Expert Comment

by:dstanley9
ID: 17003956
Open the project properties, and check the "Use Managed Extensions" setting in the "General Section".  It should be set to "Yes".


To test, add a "Hello World" class:

HelloWorld.cpp:

// This is the main DLL file.

#include "stdafx.h"
#include "HelloWorld.h"

String* TestProject::HelloWorldClass::HelloWorld()
{
      return S"Hello from managed C++!";
}


HelloWorld.h:


#pragma once

using namespace System;

namespace TestProject
{
      public __gc class HelloWorldClass
      {
            // TODO: Add your methods for this class here.
      public:
            String* HelloWorld();
      };
}

Then add a new C# colsole app to the solution and add a reference the C++ project, then add the following code:

static void Main(string[] args)
{
      //
      // TODO: Add code to start application here
      //
      TestProject.HelloWorldClass o = new TestProject.HelloWorldClass();
      Console.WriteLine(o.HelloWorld());
}
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Author Comment

by:worldknown
ID: 17004161
Ok I can import now. Thanks.
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