Using C++ DLL in C# project

Question

I got a C++ dll which has to be integrated in a C# project.

I think I found the correct way to do it, but calling the dll gives me this error: System.BadImageFormatException: An attempt was made to load a program with an incorrect format. (Exception from HRESULT: 0x8007000B)

This is the function in the dll:

extern long FAR PASCAL convert (LPSTR filename);

And this is the code I'm using in C#

namespace Test{
public partial class Form1 : Form
{
    [DllImport("convert.dll", SetLastError = true)]
    static extern Int32 convert([MarshalAs(UnmanagedType.LPStr)] string filename);

    private void button1_Click(object sender, EventArgs e)
    {
        // generate textfile
        string filename = "testfile.txt";

        StreamWriter sw = new StreamWriter(filename);
        sw.WriteLine("line1");
        sw.WriteLine("line2");
        sw.Close();

        // add checksum
        Int32 ret = 0;
        try
        {
            ret = convert(filename);

            Console.WriteLine("Result of DLL:  {0}", ret.ToString());
        }
        catch (Exception ex)
        {
            lbl.Text = ex.ToString();
        }
    }
}}

Any ideas on how to proceed with this?

Thanks a lot, Frank

Oleg · Accepted Answer · 2010-06-08 07:51:53Z

4

try to use __stdcall (or WINAPI or APIENTRY) in the function exported from the DLL.

answered Jun 8, 2010 at 7:51

Oleg

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11 Comments

Frank Over a year ago

I have to use the dll "as is" ... no changes allowed

Oleg Over a year ago

There are different declaration of PASCAL in different headers (also in WinDef.h), but dierct usage of __stdcall will be allways interpreted in the same way

Oleg Over a year ago

@Frank: Ohhh! The DLL is not from your code, than probably the DLL has really another format like 64-bit etc?

Oleg Over a year ago

You can use dumpbin.exe with /headers parameter from Visual Studio or dependency walker www.dependencywalker.com to see more information about dll

Frank Over a year ago

pffff... dependency walker shows: Error: at least one file was not a 32-bit or 64-bit Windows module No PE signature found. This file appears to be a 16-bit Windows module. I could be wrong, but this doesn't seem to look good...

|

Josip Medved · Accepted Answer · 2010-06-08 07:48:42Z

4

Try to switch your C# code from AnyCPU to x86 (in Properties dialog).

answered Jun 8, 2010 at 7:48

Josip Medved

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2 Comments

Frank Over a year ago

same result ... System.BadImageFormatException: An attempt was made to load a program with an incorrect format. (Exception from HRESULT: 0x8007000B)

Josip Medved Over a year ago

@Frank: Exception points to x86/x64 mismatch in DLL and C# code. If C++ code is compiled as x64, try setting C# code also to x64 (and vice-versa if code is x86). When dealing with DllImport, that is only cause for that exception (as per Microsoft documentation - msdn.microsoft.com/en-us/library/…).

Frank Bollack · Accepted Answer · 2010-06-08 08:01:48Z

4

Your exported function uses the PASCAL calling convention, which in Windows is the same as stdcall. The .Net runtime needs to know about that, so modify your C# method signature as follows:

[DllImport("convert.dll", SetLastError = true, CallingConvention=CallingConvention.StdCall)]
static extern Int32 convert([MarshalAs(UnmanagedType.LPStr)] string filename);

edited Jun 8, 2010 at 8:01

answered Jun 8, 2010 at 7:51

Frank Bollack

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2 Comments

Frank Over a year ago

there was an ) too much at the first line, so removed this: [DllImport("convert.dll", SetLastError = true, CallingConvention = CallingConvention.StdCall)] static extern Int32 convert([MarshalAs(UnmanagedType.LPStr)] string filename); but still the same error System.BadImageFormatException: An attempt was made to load a program with an incorrect format. (Exception from HRESULT: 0x8007000B)

Frank Bollack Over a year ago

Can you please give some more details about your platform, project settings and the DLL used. (Architecture x86/x64 etc)

Croko · Accepted Answer · 2015-12-31 04:47:32Z

Two main steps involved are

1- Creating a C++ dll

In visual studio

**New->Project->Class Library** in c++ template Name of project here is first_dll in visual studio 2010. Now **declare your function as public** in first_dll.h file and write the code in first_dll.cpp file as shown below.

Header File

Cpp File

Check **Project-> Properties -> Configuration/General -> Configuration Type** 
this option should be **Dynamic Library(.dll)** and build the solution/project now.

first_dll.dll file is created in Debug folder

2- Linking it in C# project

Open C# project

Rightclick on project name in solution explorer -> Add -> References -> Browse to path
where first_dll.dll is created and add the file

Add this line at top in C# project

Using first_dll;

Now file can be accessed using below statement in some function

double var = Class1.sum(4,5);

I linked the C++ project .dll created in VS2010 to C# project created in VS2013. It works well.

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