Access C++ DLL from C# program

Question

I'm very new to interop and I am having trouble defining the dll imports from a C++ DLL. The documentation for the DLL is as follows:

bool __stdcall __declspec(dllexport) InitHW(char *name, char *model, int& type)

So the code I tried is as follows and it gives a system.AccessViolation exception:

[DllImport("extIO_IC7610.dll", CallingConvention = CallingConvention.Cdecl)]
public unsafe static extern bool InitHW(string name, string model, int type);

private unsafe void Initialize()
{
    try
    {
        bool result;

        string name = "Test";
        string model = "Model";
        int type = 3;

        result = InitHW(name, model, type);
    }
    catch (Exception ex)
    {

    }
}

I just realized this is supposed to return data. Could someone please show me the errors in my understanding here? Thanks, Tom

Based on the comments I changed things to look like this:

[DllImport("extIO_IC7610.dll", CallingConvention = CallingConvention.Cdecl)]
[return: MarshalAs(UnmanagedType.I1)] 
public unsafe static extern bool InitHW(string name, string model, ref int type);

unsafe private void Initialize())
{

    try
    {
        bool result;

        string name = ""; 
        string model = ""; 
        int type = 3;
        result = InitHW(name, model, ref type);

    }
    catch (Exception ex)
    {

    }
}

This still does not work. I now get an error that the stack is unbalanced due to the signatures not matching. I think the strings are done correctly but the &int parameter may still be an issue. Tom

I think you need ref int type... Not sure about the rest though I see no need for unsafe — pinkfloydx33
– pinkfloydx33, Commented Sep 23, 2018 at 19:08
Aside from the ref int already mentioned, note that the return type of your native C++ function is the C++ data type bool. Your DLLImport has to specifically account for that (the default marshalling for the C# data type bool is the native WinAPI type BOOL, which is different from the C++ data type bool). Why and how to do this exactly, see here: blogs.msdn.microsoft.com/jaredpar/2008/10/14/…; or see here: stackoverflow.com/questions/4608876/… — user2819245
– user2819245, Commented Sep 23, 2018 at 19:53
also make sure you specify the string encoding that your dll expects! — Daniel A. White
– Daniel A. White, Commented Sep 23, 2018 at 20:03
Getting closer. I need to use CallingConvention.StdCall. That seems to work and I have no errors, but all returned parameters are null. So something is still wrong with the parameters. — Tom
– Tom, Commented Sep 23, 2018 at 23:28

Tom · Accepted Answer · 2018-09-24 01:48:22Z

0

Well I solved it. There were a couple of hints here but nothing really correct. After looking around for the last several hours this is the solution:

    [DllImport("extIO_IC7610.dll", CallingConvention = CallingConvention.StdCall, CharSet = CharSet.Ansi)]
    [return: MarshalAs(UnmanagedType.I1)] 
    public unsafe static extern bool InitHW(StringBuilder name,  StringBuilder model, int* type);

unsafe void Initialize()
{
           bool result;

            int type = 0;
            var name = new StringBuilder(250);
            var model = new StringBuilder(250);

            result = InitHW(name, model, &type);

            string _name = name.ToString();
            string _model = model.ToString();

}

answered Sep 24, 2018 at 1:48

Tom

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2 Comments

David Heffernan Over a year ago

No need for unsafe. Use out int or ref int.

Tom Over a year ago

You are right. I do have other functions that require unsafe. I just didn't put them in. Thanks

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